What functions do reflexes serve during human locomotion?

Reflexive and Perceptual Characteristics as Functional Outcome Measures of Chronic Ankle Instability

BACKGROUND

Cutaneous reflexes serve an integral role in maintaining efficient movement patterns and may be linked to sensory-perceptual deficits experienced by those with chronic ankle instability (CAI), namely, during the stance and transitional phases of the gait cycle. Abnormal reflex characteristics and perceptions of instability following unexpected perturbation have the potential to serve as potential patient-specific biomarkers for neuromuscular recovery following a lateral ankle sprain (LAS), however, these outcomes are scarcely explored.

METHODS

The purpose of this retrospective case control study (level III) was to observe lower limb cutaneous reflex patterns and variability as well as perceived instability following sural nerve stimulation in those with CAI. These outcomes were measured via electromyography of 6 lower limb muscles and self-reported perceived instability as participants received random, non-noxious stimulations while walking on a treadmill.

RESULTS

Those with CAI exhibited elevated peroneus longus facilitation and gastrocnemius reflex variability during midstance that were correlated with greater levels of perceived instability following sural stimulation.

CONCLUSION

These findings indicate reflex alterations may contribute to perceived instability during functional activity, a hallmark symptom of CAI.

Evaluation of spasticity: IFCN Handbook Chapter

There is no generally accepted definition of spasticity, but hyperexcitable stretch reflexes, exaggerated tendon jerks, clonus, spasms, cramps, increased resistance to passive joint movement, sustained involuntary muscle activity and aberrant muscle activation, including co-contraction of antagonist muscles are all signs and symptoms which are usually associated clinically to the term spasticity. This review describes how biomechanical and electrophysiological techniques may be used to provide quantitative and objective measures of each of these signs and symptoms. The review further describes how neurophysiological techniques may be used to evaluate pathophysiological changes in spinal motor control mechanisms. It is emphasized that understanding the pathophysiology and distinguishing the specific signs and symptoms associated with spasticity, using objective, valid, and reproducible measurements, is essential for providing optimal therapy.

Soleus H-reflex size versus stimulation rate in the presence of background muscle activity: A methodological study

Introduction

Hoffmann reflex (HR) operant conditioning has emerged as an important intervention in neurorehabilitation. During conditioning, the HR is elicited at low rates (∼0.2 Hz) to avoid the initial reduction in HR size that can occur over repeated stimulation, i.e., rate-dependent depression (RDD), thereby maintaining reflex size. This study investigated the impact of higher stimulation rates on HR size, where a stable, low-level, background electromyographic (EMG) signal is maintained over 225 conditioning trials in each of 30 sessions. A higher rate could shorten session length and/or number.

Methods

Fifteen healthy participants maintained low background soleus EMG (5-18 µV, ∼1-3% of the maximum stimulation evoked direct muscle (M-wave) EMG response (M max ) while standing. Soleus HR and M-wave recruitment curves were obtained at rates of 0.2, 1, and 2 Hz, from which M max and H max were calculated. Seventy-five HR trials (HRT) were collected for each stimulation rate at a target M-wave size (∼10-20% of M max ).

Results

There was no evidence of RDD at higher stimulation rates. In addition, the mean HR over trials was reliable across participants and rates. The Intraclass Correlation Coefficient (ICC) was 0.965 (95%CI:0.915, 0.987).

Discussion

This study shows that H-reflex conditioning might be performed at rates up to 2 Hz with no RDD and with consistent HR values. A faster rate could increase the number of conditioning trials per session, reduce session duration, and/or reduce the number of sessions. It could thereby accelerate the conditioning process and make the process less demanding for participants.

Support

NIH Grant P41 EB018783 (Wolpaw), NYS Spinal Cord Injury Research Board C37714GG (Gupta) and C38338GG (Wolpaw), VA SPiRE NCT05880251 (Brangaccio), Stratton Veterans Affairs Medical Center.

Adaptive locomotion of active solids

Active systems composed of energy-generating microscopic constituents are a promising platform to create autonomous functional materials1-16 that can, for example, locomote through complex and unpredictable environments. Yet coaxing these energy sources into useful mechanical work has proved challenging. Here we engineer active solids based on centimetre-scale building blocks that perform adaptive locomotion. These prototypes exhibit a non-variational form of elasticity characterized by odd moduli8,12,17, whose magnitude we predict from microscopics using coarse-grained theories and which we validate experimentally. When interacting with an external environment, these active solids spontaneously undergo limit cycles of shape changes, which naturally lead to locomotion such as rolling and crawling. The robustness of the locomotion is rooted in an emergent feedback loop between the active solid and the environment, which is mediated by elastic deformations and stresses. As a result, our active solids are able to accelerate, adjust their gaits and locomote through a variety of terrains with a similar performance to more complex control strategies implemented by neural networks. Our work establishes active solids as a bridge between materials and robots and suggests decentralized strategies to control the nonlinear dynamics of biological systems8,18-22, soft materials5,6,9,11,12,23-25 and driven nanomechanical devices7,26-30.

Reliability of reflex measurements and perceived instability following cutaneous stimulation during gait

Individuals with chronic ankle instability (CAI) exhibit a variety of sensorimotor deficits which contribute to long-term health risks and lower overall health-related quality of life. Recent literature finds abnormal cutaneous reflex characteristics and perceptions of instability during gait among those with CAI. These may serve as important patient-specific outcome measures in diagnosing and monitoring the condition, however, the test-retest reliability of these measurements is still undetermined. Therefore, the purpose of this study is to assess reliability of cutaneous reflex amplitudes and variability and perceived instability following perturbation in those with CAI and healthy controls during gait. Subjects walked on a treadmill while receiving random, non-noxious sural nerve stimulations throughout the stance phases of gait. Muscle activity was measured via electromyography for the peroneus longus, and medial and lateral gastrocnemius. Subjects reported their perceived instability following each stimulation. Cutaneous reflex amplitudes and perceived instability throughout the stance phases of gait can be reliably measured within the same day and over a 1-week period while reflex variability may be a less reliable measure. Our findings support the use of these variables as clinical outcome measures to identify and monitor neuromuscular recovery.

Establishing Personalized and Sparse Spinal Reflex Circuitry from Locomotion Data

Spinal reflex circuitry-based controllers have shown promising capabilities in controlling the biomechanics of locomotion in simulation. Studies have been done to tune or identify reflex circuitry from imposed objectives, such as metabolic energy efficiency, maintaining balance or mimicking experimental data. However, in those works, the reflex structure was predefined by the authors. This may limit the generalizing capabilities of such circuits and prevent a better understanding of the dominant reflex loops in human locomotion. In this work we propose an identification framework that can identify personalized sparse-structure reflex circuitry directly from experimental data. This is achieved by first performing muscle model personalization via optimization and consequently extensive model regularization when optimizing a densely connected neural reflex network. Trajectory optimization is used in both steps, namely direct collocation formulated as a non-linear program. Multi-speed walking and running data of five subjects was used to test the framework. Results show that personalized sparse reflex circuitry was identified that show exceptional torque and muscle activation reproduction for multiple walking speeds with a speed-independent, but phase-dependent, reflex network. Furthermore, similar sparse structures were found between subjects, but different structures between walking and running gait. However, due to the use of data of unperturbed locomotion, the actual plausibility of the found circuitry remains an open question. The controllers use slightly more reflex gain parameters than those chosen and designed in other studies, but the benefit of our method is that the structures were found automatically.

Middle and Long Latency Cutaneous Reflexes During the Stance Phase of Gait in Individuals with and Without Chronic Ankle Instability

BACKGROUND/OBJECTIVES

Lower limb cutaneous reflex amplitudes can modulate across gait, which helps humans adjust rhythmic motor outputs to maintain balance in an ever-changing environment. Preliminary evidence suggests people who suffer from repetitive ankle sprains and residual feelings of giving way demonstrate altered cutaneous reflex patterns in the gastrocnemius. However, before cutaneous reflex assessment can be implemented as a clinical outcome measure, there is a need to substantiate these early findings by measuring reflex amplitudes across longer latency periods and exploring the variability of reflexes within each subject.

METHODS

Forty-eight subjects with and without chronic ankle instability (CAI) walked on a treadmill at 4 km/h while activity of the lateral gastrocnemius (LG) was measured via surface electromyography. Non-noxious stimulations were elicited randomly to the ipsilateral sural nerve at the mid-stance phase of gait, and reflex amplitudes were calculated offline by comparing muscle activity during unstimulated and stimulated gait cycles. Two primary outcome measures were compared between groups at the middle latency (MLR: 80-120 ms) and late latency (LLR: 120-150 ms) time windows: (1) average reflex amplitudes and (2) standard deviation of reflex amplitudes for each subject across 10 trials.

RESULTS

Both groups demonstrated an equal amount of LG inhibition at the MLR and LG facilitation at the LLR. However, subjects with CAI showed significantly higher variability in LLR amplitude across trials than healthy controls.

CONCLUSIONS

Increased variability of cutaneous reflex amplitudes may relate to symptoms associated with CAI. These findings suggest that reflex variability following sural nerve stimulation could serve as an objective measure to track treatment progress in patients with CAI, offering clinicians a new tool for conducting rehabilitation assessments in a controlled environment.

Neonatal high-dose riboflavin treatment channels energy expenditure towards sensorimotor and somatic development and reduces rodent growth and weight gain by modulating NRF-1 in the hypothalamus

Metabolic adaptations early in life can drive energy expenditure towards brain and physical development, with less emphasis on body mass gain and somatic growth. Dietary or pharmacological manipulations can influence these processes, but to date, the effects provided by riboflavin have not been studied. The study aimed to evaluate the effects of neonatal treatment with different doses of riboflavin on sensorimotor and somatic development in rodents. Based on this, the following experimental groups were formed: Control (C, 0 mg/kg), Riboflavin 1 (R1, 1 mg/kg), Riboflavin 2 (R2, 10 mg/kg) and Riboflavin 3 (R3, 100 mg/kg). Treatment with 100 mg/kg riboflavin anticipated the reflex ontogeny of righting, cliff aversion, negative geotaxis, and free-fall righting. Intervention with 10 and 100 mg/kg of riboflavin anticipated the reflex maturation of vibrissae placement. Eye-opening, upper incisor eruption, and lower incisor eruption reached maturational age more quickly for animals treated with 100 mg/kg, while caudal growth and body weight gain were reduced from the second week of treatment, for groups R2 and R3. Pearson's correlation analysis indicated a positive association between the administration of high doses of riboflavin and murine growth in the first week of treatment. There was, however, a negative association between treatment with a high dose of riboflavin and growth in the second week of administration, coinciding with a reduction in body weight gain in the R3 group. Treatment with 100 mg/kg of riboflavin also reduced energy expenditure parameters in the open field and catwalk. Although high-dose treatment stimulates the physiological plasticity of the CNS and reduces weight gain, hepatic parameters were preserved, highlighting the participation of the liver in the supply of fatty acids for neural maturation. Furthermore, hypothalamic NRF-1 expression was increased in the R3 group inversely to the reduction in weight gain. Our results suggest that high-dose riboflavin stimulates sensorimotor and somatic development and reduces the energy invested in growth, body weight gain, and locomotor activity, possibly involving NRF-1 gene modulation in the hypothalamus.

Automatic gait evoking in healthy adults through Vojta's peripheric somatosensory stimulation: a double-blind randomized controlled trial

BACKGROUND

To study the effects of different interventions on automatic gait processing in contrast with voluntary gait processing in healthy subjects.

METHODS

A double-blind randomised controlled trial was designed (120 able-body persons between 18 and 65 years old entered and completed the study), with pre-intervention and post-intervention assessments using the 6-Minute Walk Test (6MWT). The participants were randomly distributed into four groups. Prior to intervention, all participants performed voluntary gait on the ground (VoG) in a calibrated circuit following the 6MWT. The presence of automatic gait (AG) was explored post-intervention without a voluntary demand in the same circuit following the 6MWT. Each group received a different intervention for 30 min: Vojta stimulation, MOTOMED® at no less than 60 revolutions/minute, treadmill walking at 3 km/h, and resting in a chair (control). The main assessment, conducted by a blinded rater, was the difference in distance covered (in meters) during the 6MWT between pre- and post-intervention. Surface electromyography (sEMG) average root mean square (RMS) signals in the right tibialis anterior, right soleus, right rectus femoris, and right biceps femoris were also considered outcome measures.

RESULTS

The Vojta group was the only one that initiated AG after the intervention (476.4 m ± 57.1 in VoG versus 9.0 m ± 8.9 in AG, p < 0.001) with comparable kinematics and EMG parameters during voluntary gait, except for ankle dorsal flexion. Within the Vojta group, high variability in kinematics, sEMG activity, and distance covered was observed.

CONCLUSIONS

AG isolation is approachable through Vojta at only one session measurable with the 6MWT without any voluntary gait demand. No automatic gait effects were observed post-intervention in the other groups.

TRIAL REGISTRATION

NCT04689841 (ClinicalTrials.gov).

The NewGait Rehabilitative Device Corrects Gait Deviations in Individuals With Foot Drop

The purpose of this quasiexperimental study was to test the effects of wearing the NewGait rehabilitative device on walking abilities in individuals with foot drop. The study involved 16 participants with foot drops caused by stroke (11 participants), multiple sclerosis (one participant), and peripheral neuropathies (four individuals). During a single testing session, participants walked 12 m at their self-selected speed in four experimental conditions: walking without any orthotic device; walking while wearing a regular plastic posterior leaf ankle foot orthosis (AFO); walking with the NewGait device assisting ankle dorsiflexion only; and walking with the NewGait device assisting the hip, knee, and ankle joint motions. Body motions during walking were recorded using a 3D system for motion analysis and analyzed with a set of spatiotemporal and kinematic parameters and a gait decomposition index. The gait decomposition index indicated sagittal interjoint coordination in the three joint pairs (hip-knee, knee-ankle, and hip-ankle) of the paretic (foot drop) leg during walking and was validated in a previous study. Overall, wearing all three orthotic devices improved the gait velocity, ankle dorsiflexion, and foot clearance compared to gait trials in which no assistive devices were used. However, wearing the AFO significantly restricted the plantarflexion range of motion and decreased interjoint coordination as measured by joint decomposition. In contrast, the NewGait device altered the ankle plantarflexion motions but also increased coordinated movement (reduced the decomposition) in most lower-extremity joint pairs and conditions. Therefore, the NewGait rehabilitative device can be considered superior to a regular AFO in correcting gait deviations caused by foot drop.

Contralateral cutaneous reflex modulation during gait in individuals with and without chronic ankle instability

INTRODUCTION

Chronic ankle instability (CAI), a common seqeula to ankle injury is characterized by a variety of sensorimotor deficits extending beyond the previously injured limb. Cutaneous reflexes have been identified as a potential contributor to these functional limitations with recent studies identifying alterations in reflex patterns following sural nerve stimulation among those with CAI. To date, no studies have measured cutaneous reflexes of the unaffected limb in this population, therefore, the objective of this study was to measure contralateral cutaneous reflexes during gait in individuals with unilateral CAI and healthy controls.

METHODS

Muscle activity of 6 lower limb muscles was measured in nineteen participants while receiving random, non-noxious sural nerve stimulations during a walking task.

RESULTS

Control reflex patterns were generally well-aligned with previous literature while CAI patterns varied from controls in several muscles throughout the gait cycle. Namely, a lack of lateral gastrocnemius facilitation during late stance and medial gastrocnemius inhibition at midstance. Additionally, a lack of significant BF facilitation throughout contralateral swing was noted. These results indicate reflex alterations extend beyond the affected limb in those with unilateral CAI indicating changes at the spinal level following lateral ankle sprains (LAS). Considering the symptom variability in CAI, the lack of significant reflexes exhibited by the CAI group may be due to increased variability in motor output between subjects or between stimulation trials.

CONCLUSIONS

These findings highlight the importance of identifying reflex alterations arising from LAS and subsequently treating these limitations through rehabilitation targeting systemic neural pathways rather than local deficits.

Mobile neuroimaging: What we have learned about the neural control of human walking, with an emphasis on EEG-based research

Our understanding of the neural control of human walking has changed significantly over the last twenty years and mobile brain imaging methods have contributed substantially to current knowledge. High-density electroencephalography (EEG) has the advantages of being lightweight and mobile while providing temporal resolution of brain changes within a gait cycle. Advances in EEG hardware and processing methods have led to a proliferation of research on the neural control of locomotion in neurologically intact adults. We provide a narrative review of the advantages and disadvantages of different mobile brain imaging methods, then summarize findings from mobile EEG studies quantifying electrocortical activity during human walking. Contrary to historical views on the neural control of locomotion, recent studies highlight the widespread involvement of many areas, such as the anterior cingulate, posterior parietal, prefrontal, premotor, sensorimotor, supplementary motor, and occipital cortices, that show active fluctuations in electrical power during walking. The electrocortical activity changes with speed, stability, perturbations, and gait adaptation. We end with a discussion on the next steps in mobile EEG research.

Impact of visual rotations on heading direction and center of mass control during steady-state gait

Visual information is essential to navigate the environment and maintain postural stability during gait. Visual field rotations alter the perceived heading direction, resulting in gait trajectory deviations, known as visual coupling. It is unclear how center of mass (CoM) control relative to a continuously changing base of support (BoS) is adapted to facilitate visual coupling. This study aimed to characterize mediolateral (ML) balance control during visual coupling in steady-state gait. Sixteen healthy participants walked on an instrumented treadmill, naive to sinusoidal low-frequency (0.1 Hz) rotations of the virtual environment around the vertical axis. Rotations were continuous with 1) high or 2) low amplitude or were 3) periodic with 10-s intervals. Visual coupling was characterized with cross-correlations between CoM trajectory and visual rotations. Balance control was characterized with the ML margin of stability (MoSML) and by quantifying foot placement control as the relation between CoM dynamics and lateral foot placement. Visual coupling was strong on a group level (continuous low: 0.88, continuous high: 0.91, periodic: 0.95) and moderate to strong on an individual level. Higher rotation amplitudes induced stronger gait trajectory deviations. The MoSML decreased toward the deviation direction and increased at the opposite side. Foot placement control was similar compared with regular gait. Furthermore, pelvis and foot reorientation toward the rotation direction was observed. We concluded that visual coupling was facilitated by reorientating the body and shifting the extrapolated CoMML closer to the lateral BoS boundary toward the adjusted heading direction while preserving CoM excursion and foot placement control.NEW & NOTEWORTHY Healthy, naive participants were unaware of subtle, low-frequency rotations of the visual field but still coupled their gait trajectory to a rotating virtual environment. In response, participants decreased their margin of stability toward the new heading direction, without changing the center of mass excursion magnitude and foot placement strategy.

Forelimb movements contribute to hindlimb cutaneous reflexes during locomotion in cats

During quadrupedal locomotion, interactions between spinal and supraspinal circuits and somatosensory feedback coordinate forelimb and hindlimb movements. How this is achieved is not clear. To determine whether forelimb movements modulate hindlimb cutaneous reflexes involved in responding to an external perturbation, we stimulated the superficial peroneal nerve in six intact cats during quadrupedal locomotion and during hindlimb-only locomotion (with forelimbs standing on stationary platform) and in two cats with a low spinal transection (T12-T13) during hindlimb-only locomotion. We compared cutaneous reflexes evoked in six ipsilateral and four contralateral hindlimb muscles. Results showed similar occurrence and phase-dependent modulation of short-latency inhibitory and excitatory responses during quadrupedal and hindlimb-only locomotion in intact cats. However, the depth of modulation was reduced in the ipsilateral semitendinosus during hindlimb-only locomotion. Additionally, longer-latency responses occurred less frequently in extensor muscles bilaterally during hindlimb-only locomotion, whereas short-latency inhibitory and longer-latency excitatory responses occurred more frequently in the ipsilateral and contralateral sartorius anterior, respectively. After spinal transection, short-latency inhibitory and excitatory responses were similar to both intact conditions, whereas mid- or longer-latency excitatory responses were reduced or abolished. Our results in intact cats and the comparison with spinal-transected cats suggest that the absence of forelimb movements suppresses inputs from supraspinal structures and/or cervical cord that normally contribute to longer-latency reflex responses in hindlimb extensor muscles.NEW & NOTEWORTHY During quadrupedal locomotion, the coordination of forelimb and hindlimb movements involves central circuits and somatosensory feedback. To demonstrate how forelimb movement affects hindlimb cutaneous reflexes during locomotion, we stimulated the superficial peroneal nerve in intact cats during quadrupedal and hindlimb-only locomotion as well as in spinal-transected cats during hindlimb-only locomotion. We show that forelimb movement influences the modulation of hindlimb cutaneous reflexes, particularly the occurrence of long-latency reflex responses.

Soleus H-reflex amplitude modulation during walking remains physiological during transspinal stimulation in humans

The soleus H-reflex modulation pattern was investigated during stepping following transspinal stimulation over the thoracolumbar region at 15, 30, and 50 Hz with 10 kHz carry-over frequency above and below the paresthesia threshold. The soleus H-reflex was elicited by posterior tibial nerve stimulation with a single 1 ms pulse at an intensity that the M-wave amplitudes ranged from 0 to 15% of the maximal M-wave evoked 80 ms after the test stimulus, and the soleus H-reflex was half the size of the maximal H-reflex evoked on the ascending portion of the recruitment curve. During treadmill walking, the soleus H-reflex was elicited every 2 or 3 steps, and stimuli were randomly dispersed across the step cycle which was divided in 16 equal bins. For each subject and condition, the soleus M-wave and H-reflex were normalized to the maximal M-wave. The soleus background electromyographic (EMG) activity was estimated as the linear envelope for 50 ms duration starting at 100 ms before posterior tibial nerve stimulation for each bin. The gain was determined as the slope of the relationship between the soleus H-reflex and the soleus background EMG activity. The soleus H-reflex phase-dependent amplitude modulation remained unaltered during transspinal stimulation, regardless frequency, or intensity. Similarly, the H-reflex slope and intercept remained the same for all transspinal stimulation conditions tested. Locomotor EMG activity was increased in knee extensor muscles during transspinal stimulation at 30 and 50 Hz throughout the step cycle while no effects were observed in flexor muscles. These findings suggest that transspinal stimulation above and below the paresthesia threshold at 15, 30, and 50 Hz does not block or impair spinal integration of proprioceptive inputs and increases activity of thigh muscles that affect both hip and knee joint movement. Transspinal stimulation may serve as a neurorecovery strategy to augment standing or walking ability in upper motoneuron lesions.

Erosion of Stumble Correction Evoked with Superficial Peroneal Nerve Stimulation in Older Adults during Walking

In healthy young adults, electrical stimulation of the superficial peroneal cutaneous nerve (SPn) innervating the dorsum of the foot has been shown to elicit functionally relevant reflexes during walking that are similar to those evoked by mechanical perturbation to the dorsum of the foot during walking and are referred to as stumble corrective (obstacle avoidance) responses. Though age-related differences in reflexes induced by mechanical perturbation have been studied, toe clearance has not been measured. Further, age-related differences in reflexes evoked by electrical stimulation of SPn have yet to be determined. Thus, the purpose of this study was to characterize age-related differences between healthy young adults and older adults with no history of falls in stumble correction responses evoked by electrical stimulation of the SPn at the ankle during walking. Toe clearance relative to the walking surface along with joint displacement and angular velocity at the ankle and knee and EMG of the tibialis anterior, medial gastrocnemius, biceps femoris and vastus lateralis were measured. The combined background and reflex toe clearance was reduced in the older adults compared with the young in mid-early swing (p = 0.011). These age-related differences likely increase fall risk in the older adult cohort. Further, age-related changes were seen in joint kinematics and EMG in older adults compared with the young such as decreased amplitude of the plantarflexion reflex in early swing in older adults (p < 0.05). These altered reflexes reflect the degradation of the stumble corrective response in older adults.

A comprehensive dataset on biomechanics and motor control during human walking with discrete mechanical perturbations

Background

Humans have a remarkable capability to maintain balance while walking. There is, however, a lack of publicly available research data on reactive responses to destabilizing perturbations during gait.

Methods

Here, we share a comprehensive dataset collected from 10 participants who experienced random perturbations while walking on an instrumented treadmill. Each participant performed six 5-min walking trials at a rate of 1.2 m/s, during which rapid belt speed perturbations could occur during the participant's stance phase. Each gait cycle had a 17% probability of being perturbed. The perturbations consisted of an increase of belt speed by 0.75 m/s, delivered with equal probability at 10%, 20%, 30%, 40%, 50%, 60%, 70%, or 80% of the stance phase. Data were recorded using motion capture with 25 markers, eight inertial measurement units (IMUs), and electromyography (EMG) from the tibialis anterior (TA), soleus (SOL), lateral gastrocnemius (LG), rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), and gluteus maximus (GM). The full protocol is described in detail.

Results

We provide marker trajectories, force plate data, EMG data, and belt speed information for all trials and participants. IMU data is provided for most participants. This data can be useful for identifying neural feedback control in human gait, biologically inspired control systems for robots, and the development of clinical applications.

Capitalizing on skin in orthotics design: the effects of texture on plantar intrinsic foot muscles during locomotion

Foot orthoses (FO) are a commonly prescribed intervention to alter foot function during walking although their effects have been primarily studied in the extrinsic muscles of the foot. Furthermore, enhancing sensory feedback under the foot sole has been recently shown to alter extrinsic muscle activity during gait; however, the effects of FOs with enhanced sensory feedback on plantar intrinsic foot muscles (PIFMs) remain unknown. Thus, the purpose of this study was to investigate the effect of FOs with and without sensory facilitation on PIFM activity during locomotion. Forty healthy adults completed a series of gait trials in non-textured and textured FOs when walking over hard and soft flooring. Outcome measures included bilateral joint kinematics and electromyography (EMG) of four PIFMs. Results of this study highlight the distinct onset and cessations of each PIFM throughout the stance phase of gait. PIFMs remained active during mid-stance when wearing FOs and textured FOs facilitated muscle activity across the stance phase of gait. Increasing cutaneous input from foot sole skin, via the addition of texture under the foot sole, appears to alter motor-neuron pool excitation of PIFMs. Future academics are encouraged to increase our understanding on which pathologies, diseases, and/or medical conditions would best benefit from textured FOs.

The topographical attenuation of cutaneous input is modulated at the ankle joint during gait

The attenuation of sensory inputs via various methods has been demonstrated to impair balance control and alter locomotor behavior during human walking; however, the effects of attenuating foot sole sensation under distinct areas of the foot sole on lower extremity motor output remains poorly understood. Thus, the purpose of this study was to attenuate cutaneous feedback via regional hypothermia under five different areas of the foot sole and investigate the resultant modulation of kinematic and muscle activity during level walking. Electromyography from eight lower leg muscles, kinematics, and location of center of pressure was recorded from 48 healthy young adults completing walking trials with normal and reduced cutaneous sensation from bilateral foot soles. The results of this study highlight the modulatory response of the tibialis anterior in terminal stance (propulsion and toe-off) and medial gastrocnemius muscle throughout the entire stance phase of gait. The topographical organization of foot sole skin in response to the attenuation of cutaneous feedback from different areas of the foot sole significantly modified locomotor activity. Furthermore, the locomotor response to cutaneous attenuation under the same regions that we previously facilitated with tactile feedback do not oppose each other, suggesting different physiological changes to foot sole skin generate unique gait behaviors.

Investigating the roles of reflexes and central pattern generators in the control and modulation of human locomotion using a physiologically plausible neuromechanical model

Objective.Studying the neural components regulating movement in human locomotion is obstructed by the inability to perform invasive experimental recording in the human neural circuits. Neuromechanical simulations can provide insights by modeling the locomotor circuits. Past neuromechanical models proposed control of locomotion either driven by central pattern generators (CPGs) with simple sensory commands or by a purely reflex-based network regulated by state-machine mechanisms, which activate and deactivate reflexes depending on the detected gait cycle phases. However, the physiological interpretation of these state machines remains unclear. Here, we present a physiologically plausible model to investigate spinal control and modulation of human locomotion.Approach.We propose a bio-inspired controller composed of two coupled CPGs that produce the rhythm and pattern, and a reflex-based network simulating low-level reflex pathways and Renshaw cells. This reflex network is based on leaky-integration neurons, and the whole system does not rely on changing reflex gains according to the gait cycle state. The musculoskeletal model is composed of a skeletal structure and nine muscles per leg generating movement in sagittal plane.Main results.Optimizing the open parameters for effort minimization and stability, human kinematics and muscle activation naturally emerged. Furthermore, when CPGs were not activated, periodic motion could not be achieved through optimization, suggesting the necessity of this component to generate rhythmic behavior without a state machine mechanism regulating reflex activation. The controller could reproduce ranges of speeds from 0.3 to 1.9 m s-1. The results showed that the net influence of feedback on motoneurons (MNs) during perturbed locomotion is predominantly inhibitory and that the CPGs provide the timing of MNs' activation by exciting or inhibiting muscles in specific gait phases.Significance.The proposed bio-inspired controller could contribute to our understanding of locomotor circuits of the intact spinal cord and could be used to study neuromotor disorders.

Heating the skin on the foot sole enhances cutaneous reflexes in the lower limb

Cutaneous input is important in postural control and balance. Aging and diabetes impair skin sensitivity and motor control. Heat application can improve skin sensation, but its influence on motor control remains unknown. This study investigated the effects of heating the skin of the foot sole on lower limb cutaneous reflexes. Reflexes were evoked in the tibialis anterior muscle of 20 young, healthy adults before and after heating the foot sole to a maximum of 42°C. While holding a 15% maximum root mean square EMG generated during maximum isometric dorsiflexion, a filtered white noise (0-50 Hz) vibration at 10 times the perceptual threshold was applied to the heel to stimulate cutaneous mechanoreceptors. Reflexes were analyzed in both the time (cumulant density) and frequency (coherence, gain) domains. Heat increased foot skin temperature ∼15.4°C (P < 0.001). Cumulant density peak to peak amplitude significantly increased by 44% after heating (P = 0.01) while latencies did not vary (P = 0.46). Coherence and gain were significantly greater in the 30- to 40-Hz range following heating (P = 0.048; P = 0.02). Heating significantly enhances lower limb cutaneous reflexes. This may be due to the increased ability of cutaneous mechanoreceptors to encode in the 30- to 40-Hz range.NEW & NOTEWORTHY Cutaneous input is a known modulator of muscle activity. Targeting skin to intentionally enhance motor output has received little attention. We explored local skin heating to enhance skin sensitivity and found a significant increase in the amplitude, coherence, and gain of cutaneous reflexes in the tibialis anterior. Our current findings provide the first support for the use of heat as a viable and easily integrated modality in rehabilitation technology to improve balance and postural control.

The History and Future of Neuromusculoskeletal Biomechanics

The Executive Council of the International Society of Biomechanics has initiated and overseen the commemorations of the Society's 50th Anniversary in 2023. This included multiple series of lectures at the ninth World Congress of Biomechanics in 2022 and XXIXth Congress of the International Society of Biomechanics in 2023, all linked to special issues of International Society of Biomechanics' affiliated journals. This special issue of the Journal of Applied Biomechanics is dedicated to the biomechanics of the neuromusculoskeletal system. The reader is encouraged to explore this special issue which comprises 6 papers exploring the current state-of the-art, and future directions and roles for neuromusculoskeletal biomechanics. This editorial presents a very brief history of the science of the neuromusculoskeletal system's 4 main components: the central nervous system, musculotendon units, the musculoskeletal system, and joints, and how they biomechanically integrate to enable an understanding of the generation and control of human movement. This also entails a quick exploration of contemporary neuromusculoskeletal biomechanics and its future with new fields of application.

Predicting Gait Patterns of Children With Spasticity by Simulating Hyperreflexia

Spasticity is a common impairment within pediatric neuromusculoskeletal disorders. How spasticity contributes to gait deviations is important for treatment selection. Our aim was to evaluate the pathophysiological mechanisms underlying gait deviations seen in children with spasticity, using predictive simulations. A cluster analysis was performed to extract distinct gait patterns from experimental gait data of 17 children with spasticity to be used as comparative validation data. A forward dynamic simulation framework was employed to predict gait with either velocity- or force-based hyperreflexia. This framework entailed a generic musculoskeletal model controlled by reflexes and supraspinal drive, governed by a multiobjective cost function. Hyperreflexia values were optimized to enable the simulated gait to best match experimental gait patterns. Three experimental gait patterns were extracted: (1) increased knee flexion, (2) increased ankle plantar flexion, and (3) increased knee flexion and ankle plantar flexion when compared with typical gait. Overall, velocity-based hyperreflexia outperformed force-based hyperreflexia. The first gait pattern could mostly be explained by rectus femoris and hamstrings velocity-based hyperreflexia, the second by gastrocnemius velocity-based hyperreflexia, and the third by gastrocnemius, soleus, and hamstrings velocity-based hyperreflexia. This study shows how velocity-based hyperreflexia from specific muscles contributes to different spastic gait patterns, which may help in providing targeted treatment.

Uncovering and leveraging the return of voluntary motor programs after paralysis using a bi-cortical neuroprosthesis

Rehabilitative and neuroprosthetic approaches after spinal cord injury (SCI) aim to reestablish voluntary control of movement. Promoting recovery requires a mechanistic understanding of the return of volition over action, but the relationship between re-emerging cortical commands and the return of locomotion is not well established. We introduced a neuroprosthesis delivering targeted bi-cortical stimulation in a clinically relevant contusive SCI model. In healthy and SCI cats, we controlled hindlimb locomotor output by tuning stimulation timing, duration, amplitude, and site. In intact cats, we unveiled a large repertoire of motor programs. After SCI, the evoked hindlimb lifts were highly stereotyped, yet effective in modulating gait and alleviating bilateral foot drag. Results suggest that the neural substrate underpinning motor recovery had traded-off selectivity for efficacy. Longitudinal tests revealed that the return of locomotion after SCI was correlated with recovery of the descending drive, which advocates for rehabilitation interventions directed at the cortical target.

Effects of active and sham tDCS on the soleus H-reflex during standing

Weak transcranial direct current stimulation (tDCS) is known to affect corticospinal excitability and enhance motor skill acquisition, whereas its effects on spinal reflexes in actively contracting muscles are yet to be established. Thus, in this study, we examined the acute effects of Active and Sham tDCS on the soleus H-reflex during standing. In fourteen adults without known neurological conditions, the soleus H-reflex was repeatedly elicited at just above M-wave threshold throughout 30 min of Active (N = 7) or Sham (N = 7) 2-mA tDCS over the primary motor cortex in standing. The maximum H-reflex (Hmax) and M-wave (Mmax) were also measured before and immediately after 30 min of tDCS. The soleus H-reflex amplitudes became significantly larger (by 6%) ≈1 min into Active or Sham tDCS and gradually returned toward the pre-tDCS values, on average, within 15 min. With Active tDCS, the amplitude reduction from the initial increase appeared to occur more swiftly than with Sham tDCS. An acute temporary increase in the soleus H-reflex amplitude within the first minute of Active and Sham tDCS found in this study indicates a previously unreported effect of tDCS on the H-reflex excitability. The present study suggests that neurophysiological characterization of Sham tDCS effects is just as important as investigating Active tDCS effects in understanding and defining acute effects of tDCS on the excitability of spinal reflex pathways.

Does ankle push-off correct for errors in anterior-posterior foot placement relative to center-of-mass states?

Understanding the mechanisms humans use to stabilize walking is vital for predicting falls in elderly. Modeling studies identified two potential mechanisms to stabilize gait in the anterior-posterior direction: foot placement control and ankle push-off control: foot placement depends on position and velocity of the center-of-mass (CoM) and push-off covaries with deviations between actual and predicted CoM trajectories. While both control mechanisms have been reported in humans, it is unknown whether especially the latter one is employed in unperturbed steady-state walking. Based on the finding of Wang and Srinivasan that foot placement deviates in the same direction as the CoM states in the preceding swing phase, and assuming that this covariance serves the role of stabilizing gait, the covariance between the CoM states and foot placement can be seen as a measure of foot placement accuracy. We subsequently interpreted the residual variance in foot placement from a linear regression model as "errors" that must be compensated, and investigated whether these foot placement errors were correlated to push-off kinetic time series of the subsequent double stance phase. We found ankle push-off torque to be correlated to the foot placement errors in 30 participants when walking at normal and slow speeds, with peak correlations over the double stance phase up to 0.39. Our study suggests that humans use a push-off strategy for correcting foot placement errors in steady-state walking.

Priming locomotor training with transspinal stimulation in people with spinal cord injury: study protocol of a randomized clinical trial

BACKGROUND

The seemingly simple tasks of standing and walking require continuous integration of complex spinal reflex circuits between descending motor commands and ascending sensory inputs. Spinal cord injury greatly impairs standing and walking ability, but both improve with locomotor training. However, even after multiple locomotor training sessions, abnormal muscle activity and coordination persist. Thus, locomotor training alone cannot fully optimize the neuronal plasticity required to strengthen the synapses connecting the brain, spinal cord, and local circuits and potentiate neuronal activity based on need. Transcutaneous spinal cord (transspinal) stimulation alters motoneuron excitability over multiple segments by bringing motoneurons closer to threshold, a prerequisite for effectively promoting spinal locomotor network neuromodulation and strengthening neural connectivity of the injured human spinal cord. Importantly, whether concurrent treatment with transspinal stimulation and locomotor training maximizes motor recovery after spinal cord injury is unknown.

METHODS

Forty-five individuals with chronic spinal cord injury are receiving 40 sessions of robotic gait training primed with 30 Hz transspinal stimulation at the Thoracic 10 vertebral level. Participants are randomized to receive 30 min of active or sham transspinal stimulation during standing or active transspinal stimulation while supine followed by 30 min of robotic gait training. Over the course of locomotor training, the body weight support, treadmill speed, and leg guidance force are adjusted as needed for each participant based on absence of knee buckling during the stance phase and toe dragging during the swing phase. At baseline and after completion of all therapeutic sessions, neurophysiological recordings registering corticospinal and spinal neural excitability changes along with clinical assessment measures of standing and walking, and autonomic function via questionnaires regarding bowel, bladder, and sexual function are taken.

DISCUSSION

The results of this mechanistic randomized clinical trial will demonstrate that tonic transspinal stimulation strengthens corticomotoneuronal connectivity and dynamic neuromodulation through posture-dependent corticospinal and spinal neuroplasticity. We anticipate that this mechanistic clinical trial will greatly impact clinical practice because, in real-world clinical settings, noninvasive transspinal stimulation can be more easily and widely implemented than invasive epidural stimulation. Additionally, by applying multiple interventions to accelerate motor recovery, we are employing a treatment regimen that reflects a true clinical approach.

TRIAL REGISTRATION

ClinicalTrials.gov NCT04807764 . Registered on March 19, 2021.

Priming locomotor training with transspinal stimulation in people with spinal cord injury: study protocol of a randomized clinical trial

Background

The seemingly simple tasks of standing and walking require continuous integration of complex spinal reflex circuits between descending motor commands and ascending sensory inputs. Spinal cord injury greatly impairs standing and walking ability, but both improve with locomotor training. However, even after multiple locomotor training sessions, abnormal muscle activity and coordination persist. Thus, locomotor training alone cannot fully optimize the neuronal plasticity required to strengthen the synapses connecting the brain, spinal cord, and local circuits and potentiate neuronal activity based on need. Transcutaneous spinal cord (transspinal) stimulation alters motoneuron excitability over multiple segments by bringing motoneurons closer to threshold, a prerequisite for effectively promoting spinal locomotor network neuromodulation and strengthening neural connectivity of the injured human spinal cord. Importantly, whether concurrent treatment with transspinal stimulation and locomotor training maximizes motor recovery after spinal cord injury is unknown.

Methods

Forty-five individuals with chronic spinal cord injury are receiving 40 sessions of robotic gait training primed with 30 Hz transspinal stimulation at the Thoracic 10 vertebral level. Participants are randomized to receive 30-minutes of active or sham transspinal stimulation during standing or active transspinal stimulation while supine followed by 30-minutes of robotic gait training. Over the course of locomotor training, the body weight support, treadmill speed, and leg guidance force are adjusted as needed for each participant based on absence of knee buckling during the stance phase and toe dragging during the swing phase. At baseline and after completion of all therapeutic sessions, neurophysiological recordings registering corticospinal and spinal neural excitability changes along with clinical assessment measures of standing and walking, and autonomic function via questionnaires regarding bowel, bladder and sexual function are taken.

Discussion

The results of this mechanistic randomized clinical trial will demonstrate that tonic transspinal stimulation strengthens corticomotoneuronal connectivity and dynamic neuromodulation through posture-dependent corticospinal and spinal neuroplasticity. We anticipate that this mechanistic clinical trial will greatly impact clinical practice because in real-world clinical settings, noninvasive transspinal stimulation can be more easily and widely implemented than invasive epidural stimulation. Additionally, by applying multiple interventions to accelerate motor recovery, we are employing a treatment regimen that reflects a true clinical approach.

Trial registration

ClinicalTrials.gov: NCT04807764; Registered on March 19, 2021.

Altered cutaneous reflexes to non-noxious stimuli in the triceps surae of people with chronic incomplete spinal cord injury

Following spinal cord injury (SCI) task-dependent modulation of spinal reflexes are often impaired. To gain insight into the state of the spinal interneuronal pathways following injury, we studied the amplitude modulation of triceps surae cutaneous reflexes to non-noxious stimuli during standing and early-to-mid stance phase of walking in participants with and without chronic incomplete SCI. Reflex eliciting nerve stimulation was delivered to the superficial peroneal, sural, and distal tibial nerves about the ankle. Reflexes were analyzed in the short (SLR, 50-80 ms post stimulation onset) and the medium (MLR, 80-120 ms) latency response windows. Further, the relation between cutaneous and H-reflexes was also examined during standing. In participants without injuries the soleus SLR was modulated task-dependently with nerve specificity, and the soleus and medial gastrocnemius MLRs were modulated task-dependently. In contrast, participants with SCI, no task-dependent or nerve-specific modulation of triceps cutaneous reflexes was observed. The triceps surae cutaneous and H-reflexes were not correlated in either group (r = 0.01-0.37). The presence of cutaneous reflexes but the absence of significant amplitude modulation may suggest impaired function of spinal interneuronal pathways in this population. The lack of correlation between the cutaneous and H-reflexes may suggest that interneurons that are involved in H-reflex modulation and cutaneous reflex modulation do not receive common input, or the impact of the common input is outweighed by other input. Present findings highlight the importance of examining multiple spinal reflexes to better understanding spinal interneuronal pathways that affect motor control in people after SCI.

Cutaneous information processing differs with load type during isometric finger abduction

During submaximal isometric contraction, there are two different load types: maintenance of a constant limb angle while supporting an inertial load (position task) and maintenance of a constant force by pushing against a rigid restraint (force task). Previous studies demonstrated that performing the position task requires more proprioceptive information. The purpose of this study was to investigate whether there would be a difference in cutaneous information processing between the position and force tasks by assessing the gating effect, which is reduction of amplitude of somatosensory evoked potentials (SEPs), and cutaneomuscular reflex (CMR). Eighteen healthy adults participated in this study. They contracted their right first dorsal interosseous muscle by abducting their index finger to produce a constant force against a rigid restraint that was 20% maximum voluntary contraction (force task), or to maintain a target position corresponding to 10° abduction of the metacarpophalangeal joint while supporting a load equivalent to 20% maximum voluntary contraction (position task). During each task, electrical stimulation was applied to the digital nerves of the right index finger, and SEPs and CMR were recorded from C3' of the International 10-20 system and the right first dorsal interosseous muscle, respectively. Reduction of the amplitude of N33 component of SEPs was significantly larger during the force than position task. In addition, the E2 amplitude of CMR was significantly greater for the force than position task. These findings suggest that cutaneous information processing differs with load type during static muscle contraction.

The effects of foot orthoses and sensory facilitation on lower limb electromyography: A scoping review

Foot orthoses (FO) are used as a treatment for biomechanical abnormalities, overuse injuries, and neuropathologies, but study of their mechanism remains inconclusive. The neuromotor paradigm has proposed that FOs may manipulate sensory input from foot sole skin to reduce muscle activity for movement optimization. This review argues that a FO likely alters the incoming mechanical stimuli transmitted via cutaneous mechanoreceptors and nociceptors as the foot sole interfaces with the surface of the orthotic. Thus, all FOs with or without intentional sensory facilitation, likely changes sensory information from foot sole cutaneous afferents. Additionally, in light of understanding and applying knowledge pertaining to the cutaneous reflex loop circuitry, FO's increasing sensory input to the motorneuron pool can change EMG to either reflex sign (increase or decrease). The purpose of this scoping review was to synthesize FO and sensory augmentation literature and summarize how FO designs can capitalize on foot sole skin to modulate lower limb electromyography (EMG). Six database searches resulted in 30 FO studies and 22 sensory studies that included EMG as an outcome measure. Results revealed task and phase specific responses with some consistencies in EMG outcomes between testing modalities, however many inconsistencies remain. Electrical stimulation reflex research provides support for a likely sensory-to-motor factor contributing to muscle activity modulation when wearing FOs. The discussion divides trends in FO treatment modalities by desired increase or decrease in each compartment musculature. The results of this review provides a benchmark for future academics and clinicians to advance literature in support of a revised neuromotor paradigm while highlighting the importance of foot sole skin in FO design.

Reduced wrist flexor H-reflex excitability is linked with increased wrist proprioceptive error in adults with cerebral palsy

Although most neurophysiological studies of persons with cerebral palsy (CP) have been focused on supraspinal networks, recent evidence points toward the spinal cord as a central contributor to their motor impairments. However, it is unclear if alterations in the spinal pathways are also linked to deficits in the sensory processing observed clinically. This investigation aimed to begin to address this knowledge gap by evaluating the flexor carpi radialis (FCR) H-reflex in adults with CP and neurotypical (NT) controls while at rest and during an isometric wrist flexion task. The maximal H-wave (Hmax) and M-wave (Mmax) at rest were calculated and utilized to compute Hmax/Mmax ratios (H:M ratios). Secondarily, the facilitation of the H-wave was measured while producing an isometric, voluntary wrist flexion contraction (i.e., active condition). Finally, a wrist position sense test was used to quantify the level of joint position sense. These results revealed that the adults with CP had a lower H:M ratio compared with the NT controls while at rest. The adults with CP were also unable to facilitate their H-reflexes with voluntary contraction and had greater position sense errors compared with the controls. Further, these results showed that the adults with CP that had greater wrist position sense errors tended to have a lower H:M ratio at rest. Overall, these findings highlight that aberration in the spinal cord pathways of adults with CP might play a role in the sensory processing deficiencies observed in adults with CP.

The effect of texture under distinct regions of the foot sole on human locomotion

Sensory feedback from the foot sole plays an important role in shaping human locomotion. While net muscle activity and kinematic changes have been correlated with electrical stimulation to five topographical regions of the foot, it remains unknown if these responses are similar with tactile stimulation. The purpose of this study was to use texture in foot orthosis design, applied to five distinct regions under the foot sole, and measure joint kinematics, location of center of pressure, and muscle activity of eight lower leg muscles during level and incline walking. Fifty-five healthy adults completed 48 walking trials in textured and non-textured foot orthoses. Study results confirm that tactile stimulation is stimulation-site and gait-phase specific in modulating lower leg muscle activity during walking. For example, texture under the lateral forefoot consistently generated a suppression of EMG and texture under the lateral midfoot always generated a facilitation. In early stance, adding texture under the medial midfoot or calcaneus facilitated extensor muscle activity and suppressed flexor muscle activity. Texture under the lateral midfoot or medial forefoot facilitated tibialis posterior activation. These results support the topographical organization of cutaneous mechanoreceptors in foot sole skin while considering how texture can be used in foot orthosis design to target lower leg muscular changes during locomotion.

Misencoding of ankle joint angle control system via cutaneous afferents reflex pathway in chronic ankle instability

This study aimed to investigate how the cutaneous reflexes in the peroneus longus (PL) muscle are affected by changing the ankle joint position in patients with chronic ankle instability (CAI). We also investigated the correlation between the degree of reflex modulation and angle position sense of the ankle joint. The participants were 19 patients with CAI and 20 age-matched controls. Cutaneous reflexes were elicited by applying non-noxious electrical stimulation to the sural nerve at the ankle joint in the neutral standing and eversion/inversion standing positions. The suppressive middle latency cutaneous reflex (MLR; ~ 70-120 ms) and angle position sense of the ankle joint were assessed. During neutral standing, the gain of the suppressive MLR was more prominent in the CAI patients than in controls, although no significant difference was seen during 30° inversion standing. In addition, the ratios of the suppressive MLR and background electromyography in a neutral position were significantly larger than those at the 15°, 25°, and 30° inversion positions in CAI patients. No such difference was seen in control individuals. Furthermore, the correlations between reflex modulation degree and position sense error were quite different in CAI patients compared to controls. These findings suggest that the sensory-motor system was deteriorated in CAI patients due to changes in the PL cutaneous reflex pathway excitability and position sense of the ankle joint.

Kinematic trajectories in response to speed perturbations in walking suggest modular task-level control of leg angle and length

Navigating complex terrains requires dynamic interactions between the substrate, musculoskeletal and sensorimotor systems. Current perturbation studies have mostly used visible terrain height perturbations, which do not allow us to distinguish among the neuromechanical contributions of feedforward control, feedback-mediated and mechanical perturbation responses. Here, we use treadmill belt speed perturbations to induce a targeted perturbation to foot speed only, and without terrain-induced changes in joint posture and leg loading at stance onset. Based on previous studies suggesting a proximo-distal gradient in neuromechanical control, we hypothesized that distal joints would exhibit larger changes in joint kinematics, compared to proximal joints. Additionally, we expected birds to use feedforward strategies to increase the intrinsic stability of gait. To test these hypotheses, seven adult guinea fowl were video recorded while walking on a motorized treadmill, during both steady and perturbed trials. Perturbations consisted of repeated exposures to a deceleration and acceleration of the treadmill belt speed. Surprisingly, we found that joint angular trajectories and center of mass fluctuations remain very similar, despite substantial perturbation of foot velocity by the treadmill belt. Hip joint angular trajectories exhibit the largest changes, with the birds adopting a slightly more flexed position across all perturbed strides. Additionally, we observed increased stride duration across all strides, consistent with feedforward changes in the control strategy. The speed perturbations mainly influenced the timing of stance and swing, with the largest kinematic changes in the strides directly following a deceleration. Our findings do not support the general hypothesis of a proximo-distal gradient in joint control, as distal joint kinematics remain largely unchanged. Instead, we find that leg angular trajectory and the timing of stance and swing are most sensitive to this specific perturbation, and leg length actuation remains largely unchanged. Our results are consistent with modular task-level control of leg length and leg angle actuation, with different neuromechanical control and perturbation sensitivity in each actuation mode. Distal joints appear to be sensitive to changes in vertical loading but not foot fore-aft velocity. Future directions should include in vivo studies of muscle activation and force-length dynamics to provide more direct evidence of the sensorimotor control strategies for stability in response to belt speed perturbations.

A neuromuscular model of human locomotion combines spinal reflex circuits with voluntary movements

Existing models of human walking use low-level reflexes or neural oscillators to generate movement. While appropriate to generate the stable, rhythmic movement patterns of steady-state walking, these models lack the ability to change their movement patterns or spontaneously generate new movements in the specific, goal-directed way characteristic of voluntary movements. Here we present a neuromuscular model of human locomotion that bridges this gap and combines the ability to execute goal directed movements with the generation of stable, rhythmic movement patterns that are required for robust locomotion. The model represents goals for voluntary movements of the swing leg on the task level of swing leg joint kinematics. Smooth movements plans towards the goal configuration are generated on the task level and transformed into descending motor commands that execute the planned movements, using internal models. The movement goals and plans are updated in real time based on sensory feedback and task constraints. On the spinal level, the descending commands during the swing phase are integrated with a generic stretch reflex for each muscle. Stance leg control solely relies on dedicated spinal reflex pathways. Spinal reflexes stimulate Hill-type muscles that actuate a biomechanical model with eight internal joints and six free-body degrees of freedom. The model is able to generate voluntary, goal-directed reaching movements with the swing leg and combine multiple movements in a rhythmic sequence. During walking, the swing leg is moved in a goal-directed manner to a target that is updated in real-time based on sensory feedback to maintain upright balance, while the stance leg is stabilized by low-level reflexes and a behavioral organization switching between swing and stance control for each leg. With this combination of reflex-based stance leg and voluntary, goal-directed control of the swing leg, the model controller generates rhythmic, stable walking patterns in which the swing leg movement can be flexibly updated in real-time to step over or around obstacles.

Investigation of neural and biomechanical impairments leading to pathological toe and heel gaits using neuromusculoskeletal modelling

KEY POINTS

Pathological toe and heel gaits are commonly present in various conditions such as spinal cord injury, stroke or cerebral palsy. These conditions present various neural and biomechanical impairments and the cause-effect relationships between these impairments and pathological gaits are hard to establish clinically. Based on neuromechanical simulation, this study focuses on the plantarflexor muscles and builds a new reflex circuit controller to model and evaluate the potential effect of both neural and biomechanical impairments on gait. Our results suggest an important contribution of active reflex mechanisms in pathological toe gait. This "what if" based on neuromechanical modelling is thus deemed of great interest to target potential pathological gait causes.

ABSTRACT

This study investigates the pathological toe and heel gaits in human locomotion using neuromusculoskeletal modelling and simulation. In particular, it aims at investigating potential cause-effect relationships between biomechanical or neural impairments and pathological gaits. Toe and heel gaits are commonly present in spinal cord injury, stroke or cerebral palsy. Toe walking is mainly attributed to spasticity and contracture at plantarflexor muscles, whereas heel walking can be attributed to muscle weakness from biomechanical or neural origin. To investigate the effect of these impairments on gait, this study focuses on the soleus and gastrocnemius muscles as they contribute to ankle plantarflexion. We built a reflex circuit model on top of Geyer and Herr's work (2010) with additional pathways affecting the plantarflexor muscles. The SCONE software, which provides optimisation tools for 2D neuromechanical simulation of human locomotion, is used to optimise the corresponding reflex parameters and simulate healthy gait. We then modelled various bilateral plantarflexors biomechanical and neural impairments, and individually introduced them in the healthy model. We characterised the resulting simulated gaits as pathological or not by comparing ankle kinematics and ankle moment with the healthy optimised gait based on metrics used in clinical studies. Our simulations suggest that toe walking can be generated by hyperreflexia, whereas muscle and neural weaknesses induce partially heel gait. Thus, this "what if" approach is deemed of great interest as it allows the investigation of the effect of various impairments on gait and suggests an important contribution of active reflex mechanisms in pathological toe gait. Abstract figure legend Various biomechanical and neural impairments are individually modelled at the level of the plantarflexor muscles in a musculoskeletal model and a complex reflex circuit-based gait controller. For instance, as shown on the left, the plantarflexors spindle reflex gain (KS) is increased to mimic hyperreflexia. The gait controller is then optimised for each of the impaired condition and the resulting gaits are characterised as pathological gait based on ankle kinematics and ankle moment metrics used in clinical studies. Thus, this "what if" approach allows the investigation of the effect of various impairments on gait presented in the table on the right. This article is protected by copyright. All rights reserved.

Soleus H-reflex modulation during a double-legged drop landing task

Muscle spindle afferent feedback is modulated during different phases of locomotor tasks in a way that facilitates task goals. However, only a few studies have studied H-reflex modulation during landing. This study aimed to characterize soleus (SOL) H-reflex modulation during the flight and early landing period of drop landings. Since landing presumably involves a massive increase in spindle afferent firing due to rapid SOL muscle stretching, we hypothesized H-reflex size would decrease near landing reflecting neural modulation to prevent excessive motoneuron excitation. The soleus H-reflex was recorded during drop landings from a 30 cm height in nine healthy adults. Electromyography (SOL, tibialis anterior (TA), medial gastrocnemius, and vastus lateralis), ankle and knee joint motion and ground reaction force were recorded during landings. Tibial nerve stimulation was timed to elicit H-reflexes during the flight and early ground contact period (five 30 ms Bins from 90 ms before to 60 ms after landing). The H-reflexes recorded after landing (0-30 and 30-60 ms) were significantly smaller (21-36% less) than that recorded during the flight periods (90-0 ms before ground contact; P ≤ 0.004). The decrease in H-reflex size not occurring until after ground contact indicates a time-critical modulation of reflex gain during the last 30 ms of flight (i.e., time of tibial nerve stimulation). H-reflex size reduction after ground contact supports a probable neural strategy to prevent excessive reflex-mediated muscle activation and thereby facilitates appropriate musculotendon and joint stiffness.

Secondary auditory cortex mediates a sensorimotor mechanism for action timing

The ability to accurately determine when to perform an action is a fundamental brain function and vital to adaptive behavior. The behavioral mechanism and neural circuit for action timing, however, remain largely unknown. Using a new, self-paced action timing task in mice, we found that deprivation of auditory, but not somatosensory or visual input, disrupts learned action timing. The hearing effect was dependent on the auditory feedback derived from the animal's own actions, rather than passive environmental cues. Neuronal activity in the secondary auditory cortex was found to be both correlated with and necessary for the proper execution of learned action timing. Closed-loop, action-dependent optogenetic stimulation of the specific task-related neuronal population within the secondary auditory cortex rescued the key features of learned action timing under auditory deprivation. These results unveil a previously underappreciated sensorimotor mechanism in which the secondary auditory cortex transduces self-generated audiomotor feedback to control action timing.

Enhancing Locomotor Learning With Transcutaneous Spinal Electrical Stimulation and Somatosensory Augmentation: A Pilot Randomized Controlled Trial in Older Adults

This study investigated locomotor learning of a complex terrain walking task in older adults, when combined with two adjuvant interventions: transcutaneous spinal direct current stimulation (tsDCS) to increase lumbar spinal cord excitability, and textured shoe insoles to increase somatosensory feedback to the spinal cord. The spinal cord has a crucial contribution to control of walking, and is a novel therapeutic target for rehabilitation of older adults. The complex terrain task involved walking a 10-meter course consisting of nine obstacles and three sections of compliant (soft) walking surface. Twenty-three participants were randomly assigned to one of the following groups: sham tsDCS and smooth insoles (sham/smooth; control group), sham tsDCS and textured insoles (sham/textured), active tsDCS and smooth insoles (active/smooth), and active tsDCS and textured insoles (active/textured). The first objective was to assess the feasibility, tolerability, and safety of the interventions. The second objective was to assess preliminary efficacy for increasing locomotor learning, as defined by retention of gains in walking speed between a baseline visit of task practice, and a subsequent follow-up visit. Variability of the center of mass while walking over the course was also evaluated. The change in executive control of walking (prefrontal cortical activity) between the baseline and follow-up visits was measured with functional near infrared spectroscopy. The study results demonstrated feasibility based on enrollment and retention of participants, tolerability based on self-report, and safety based on absence of adverse events. Preliminary efficacy was supported based on trends showing larger gains in walking speed and more pronounced reductions in mediolateral center of mass variability at the follow-up visit in the groups randomized to active tsDCS or textured insoles. These data justify future larger studies to further assess dosing and efficacy of these intervention approaches. In conclusion, rehabilitation interventions that target spinal control of walking present a potential opportunity for enhancing walking function in older adults.

Neural Substrates of Transcutaneous Spinal Cord Stimulation: Neuromodulation across Multiple Segments of the Spinal Cord

Transcutaneous spinal cord stimulation (tSCS) has the potential to promote improved sensorimotor rehabilitation by modulating the circuitry of the spinal cord non-invasively. Little is currently known about how cervical or lumbar tSCS influences the excitability of spinal and corticospinal networks, or whether the synergistic effects of multi-segmental tSCS occur between remote segments of the spinal cord. The aim of this review is to describe the emergence and development of tSCS as a novel method to modulate the spinal cord, while highlighting the effectiveness of tSCS in improving sensorimotor recovery after spinal cord injury. This review underscores the ability of single-site tSCS to alter excitability across multiple segments of the spinal cord, while multiple sites of tSCS converge to facilitate spinal reflex and corticospinal networks. Finally, the potential and current limitations for engaging cervical and lumbar spinal cord networks through tSCS to enhance the effectiveness of rehabilitation interventions are discussed. Further mechanistic work is needed in order to optimize targeted rehabilitation strategies and improve clinical outcomes.

Remote Subthreshold Stimulation Enhances Skin Sensitivity in the Lower Extremity

Foot sole skin interfaces with the ground and contributes to successful balance. In situations with reduced sensitivity in the glabrous foot skin, stochastic resonance (SR) improves skin sensitivity by adding tactile noise. Some situations, however, involve an interface comprised of hairy skin, which has higher thresholds for sensitivity. For example, in lower extremity amputation the residual limb is comprised of hairy leg skin. The main objective of this study was to determine if SR improves skin sensitivity in hairy skin, and whether a specific intensity of noise is most effective. Secondary objectives were to compare the effect between locations, ages and modalities. In 60 healthy participants a vibrotactile (test) input was delivered at the lower extremity concurrently with a second, noisy stimulus applied more proximally. The presence of a remote SR effect was tested in 15 young participants using electrotactile noise at the calf. Secondary objectives were tested in separate groups of 15 subjects and differed by substituting for one of the three variables: vibrotactile noise, heel site, and with older participants. A forced-choice protocol was used to determine detection ability of the subthreshold vibration test input with varying noise levels applied simultaneously (0, 20, 40, 60, 80, and 100% of perceptual threshold). An SR effect was identified when increased detection of the input was obtained at any level of noise versus no noise. It was found that all four test groups demonstrated evidence of SR: 33–47% of individuals showed better detection of the input with added noise. The SR effect did not appear consistently at any specific noise level for any of the groups, and none of the variables showed a superior ability to evoke SR. Interestingly, in approximately 33% of cases, threshold values fluctuated throughout testing. While this work has provided evidence that SR can enhance the perception of a vibrotactile input in hairy skin, these data suggest that the ability to repeatably show an SR effect relies on maintaining a consistent threshold.

Control of Mammalian Locomotion by Somatosensory Feedback

When animals walk overground, mechanical stimuli activate various receptors located in muscles, joints, and skin. Afferents from these mechanoreceptors project to neuronal networks controlling locomotion in the spinal cord and brain. The dynamic interactions between the control systems at different levels of the neuraxis ensure that locomotion adjusts to its environment and meets task demands. In this article, we describe and discuss the essential contribution of somatosensory feedback to locomotion. We start with a discussion of how biomechanical properties of the body affect somatosensory feedback. We follow with the different types of mechanoreceptors and somatosensory afferents and their activity during locomotion. We then describe central projections to locomotor networks and the modulation of somatosensory feedback during locomotion and its mechanisms. We then discuss experimental approaches and animal models used to investigate the control of locomotion by somatosensory feedback before providing an overview of the different functional roles of somatosensory feedback for locomotion. Lastly, we briefly describe the role of somatosensory feedback in the recovery of locomotion after neurological injury. We highlight the fact that somatosensory feedback is an essential component of a highly integrated system for locomotor control. © 2021 American Physiological Society. Compr Physiol 11:1-71, 2021.

Bidirectional communication between neural and immune systems

The immune and nervous systems share many features, including receptor and ligand expression, enabling efficient communication between the two. Accumulating evidence suggests that the communication is bidirectional, with the neural system regulating immune cell functions and vice versa. Steroid hormones from the hypothalamus-pituitary-adrenal gland axis are examples of systemic regulators for this communication. Neural reflexes describe regional regulation mechanisms that are a historically new concept that helps to explain how the neural and body systems including immune system communicate. Several recently identified neural reflexes, including the inflammatory reflex and gateway reflex, significantly impact the activation status of the immune system and are associated with inflammatory diseases and disorders. Either pro-inflammatory or anti-inflammatory effects can be elicited by these neural reflexes. On the other hand, the activities of immune cells during inflammation, for example the secretion of inflammatory mediators, can affect the functions of neuronal systems via neural reflexes and modulate biological outputs via specific neural pathways. In this review article, we discuss recent advances in the understanding of bidirectional neuro-immune interactions, with a particular focus on neural reflexes.

Inhibition and Facilitation of the Spinal Locomotor Central Pattern Generator and Reflex Circuits by Somatosensory Feedback From the Lumbar and Perineal Regions After Spinal Cord Injury

Somatosensory feedback from peripheral receptors dynamically interacts with networks located in the spinal cord and brain to control mammalian locomotion. Although somatosensory feedback from the limbs plays a major role in regulating locomotor output, those from other regions, such as lumbar and perineal areas also shape locomotor activity. In mammals with a complete spinal cord injury, inputs from the lumbar region powerfully inhibit hindlimb locomotion, while those from the perineal region facilitate it. Our recent work in cats with a complete spinal cord injury shows that they also have opposite effects on cutaneous reflexes from the foot. Lumbar inputs increase the gain of reflexes while those from the perineal region decrease it. The purpose of this review is to discuss how somatosensory feedback from the lumbar and perineal regions modulate the spinal locomotor central pattern generator and reflex circuits after spinal cord injury and the possible mechanisms involved. We also discuss how spinal cord injury can lead to a loss of functional specificity through the abnormal activation of functions by somatosensory feedback, such as the concurrent activation of locomotion and micturition. Lastly, we discuss the potential functions of somatosensory feedback from the lumbar and perineal regions and their potential for promoting motor recovery after spinal cord injury.

Sensory modulation of gait characteristics in human locomotion: A neuromusculoskeletal modeling study

The central nervous system of humans and other animals modulates spinal cord activity to achieve several locomotion behaviors. Previous neuromechanical models investigated the modulation of human gait changing selected parameters belonging to CPGs (Central Pattern Generators) feedforward oscillatory structures or to feedback reflex circuits. CPG-based models could replicate slow and fast walking by changing only the oscillation’s properties. On the other hand, reflex-based models could achieve different behaviors through optimizations of large dimensional parameter spaces. However, they could not effectively identify individual key reflex parameters responsible for gait characteristics’ modulation. This study investigates which reflex parameters modulate the gait characteristics through neuromechanical simulations. A recently developed reflex-based model is used to perform optimizations with different target behaviors on speed, step length, and step duration to analyze the correlation between reflex parameters and their influence on these gait characteristics. We identified nine key parameters that may affect the target speed ranging from slow to fast walking (0.48 and 1.71 m/s) as well as a large range of step lengths (0.43 and 0.88 m) and step duration (0.51, 0.98 s). The findings show that specific reflexes during stance significantly affect step length regulation, mainly given by positive force feedback of the ankle plantarflexors’ group. On the other hand, stretch reflexes active during swing of iliopsoas and gluteus maximus regulate all the gait characteristics under analysis. Additionally, the results show that the hamstrings’ group’s stretch reflex during the landing phase is responsible for modulating the step length and step duration. Additional validation studies in simulations demonstrated that the modulation of identified reflexes is sufficient to regulate the investigated gait characteristics. Thus, this study provides an overview of possible reflexes involved in modulating speed, step length, and step duration of human gaits.

This study investigates the possible reflex parameters that the central nervous system could use to modulate human locomotion. Specifically, we target the modulation of three gait characteristics: speed, step length, and step duration. We utilize human locomotion simulations with a previously developed reflex-based model and perform multiple optimizations ranging targeting low to high values of the three gait characteristics investigated. From the data acquired in optimizations, we investigate which reflex parameter correlates most with the gait characteristics changes. We identified nine key reflex parameters affecting gait modulation, performed validation experiments, and verified that the optimization of key reflex parameters alone could generate modulation in the studied locomotion behaviors. Kinematics, ground reaction forces, and muscle activity obtained in simulations show similarities with past experimental studies on gait modulation. Therefore, the identified parameters could potentially be used by the nervous system to regulate locomotion behaviors in a task-dependent manner. Other circuits not modeled in this study could play a crucial role in gait modulation, and further investigations should be done in the co-optimization of feedforward and feedback circuits.

Indirect Vibration of the Upper Limbs Alters Transmission Along Spinal but Not Corticospinal Pathways

The use of upper limb vibration (ULV) during exercise and rehabilitation continues to gain popularity as a modality to improve function and performance. Currently, a lack of knowledge of the pathways being altered during ULV limits its effective implementation. Therefore, the aim of this study was to investigate whether indirect ULV modulates transmission along spinal and corticospinal pathways that control the human forearm. All measures were assessed under CONTROL (no vibration) and ULV (30 Hz; 0.4 mm displacement) conditions while participants maintained a small contraction of the right flexor carpi radialis (FCR) muscle. To assess spinal pathways, Hoffmann reflexes (H-reflexes) elicited by stimulation of the median nerve were recorded from FCR with motor response (M-wave) amplitudes matched between conditions. An H-reflex conditioning paradigm was also used to assess changes in presynaptic inhibition by stimulating the superficial radial (SR) nerve (5 pulses at 300Hz) 37 ms prior to median nerve stimulation. Cutaneous reflexes in FCR elicited by stimulation of the SR nerve at the wrist were also recorded. To assess corticospinal pathways, motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation of the contralateral motor cortex were recorded from the right FCR and biceps brachii (BB). ULV significantly reduced H-reflex amplitude by 15.7% for both conditioned and unconditioned reflexes (24.0 ± 15.7 vs. 18.4 ± 11.2% M max ; p < 0.05). Middle latency cutaneous reflexes were also significantly reduced by 20.0% from CONTROL (-1.50 ± 2.1% Mmax) to ULV (-1.73 ± 2.2% Mmax; p < 0.05). There was no significant effect of ULV on MEP amplitude (p > 0.05). Therefore, ULV inhibits cutaneous and H-reflex transmission without influencing corticospinal excitability of the forearm flexors suggesting increased presynaptic inhibition of afferent transmission as a likely mechanism. A general increase in inhibition of spinal pathways with ULV may have important implications for improving rehabilitation for individuals with spasticity (SCI, stroke, MS, etc.).

H-reflex conditioning during locomotion in people with spinal cord injury

KEY POINTS

In people or animals with incomplete spinal cord injury (SCI), changing a spinal reflex through an operant conditioning protocol can improve locomotion. All previous studies conditioned the reflex during steady-state maintenance of a specific posture. By contrast, the present study down-conditioned the reflex during the swing-phase of locomotion in people with hyperreflexia as a result of chronic incomplete SCI. The aim was to modify the functioning of the reflex in a specific phase of a dynamic movement. This novel swing-phase conditioning protocol decreased the reflex much faster and farther than did the steady-state protocol in people or animals with or without SCI, and it also improved locomotion. The reflex decrease persisted for at least 6 months after conditioning ended. The results suggest that conditioning reflex function in a specific phase of a dynamic movement offers a new approach to enhancing and/or accelerating recovery after SCI or in other disorders.

ABSTRACT

In animals and people with incomplete spinal cord injury, appropriate operant conditioning of a spinal reflex can improve impaired locomotion. In all previous conditioning studies, the reflex was conditioned during steady-state maintenance of a stable posture; this steady-state protocol aimed to change the excitability of the targeted reflex pathway; reflex size gradually changed over 8-10 weeks. The present study introduces a new protocol, comprising a dynamic protocol that aims to change the functioning of the reflex pathway during a specific phase of a complex movement. Specifically, we down-conditioned the soleus H-reflex during the swing-phase of locomotion in people with hyperreflexia as a result of chronic incomplete SCI. The swing-phase H-reflex, which is absent or very small in neurologically normal individuals, is abnormally large in this patient population. The results were clear. With swing-phase down-conditioning, the H-reflex decreased much faster and farther than did the H-reflex in all previous animal or human studies with the steady-state protocol, and the decrease persisted for at least 6 months after conditioning ended. The H-reflex decrease was accompanied by improvements in walking speed and in the modulation of locomotor electromyograph activity in proximal and distal muscles of both legs. These results provide new insight into the factors controlling spinal reflex conditioning; they suggest that the conditioning protocols targeting reflex function in a specific movement phase provide a promising new opportunity to enhance functional recovery after SCI or in other disorders.