Corrective responses to perturbation applied during walking in humans

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.

Pneumatic AFO Powered by a Miniature Custom Compressor for Drop Foot Correction

For active AFO applications, pneumatic remote transmission has advantages in minimizing the mass and complexity of the system due to the flexibility in placing pneumatic components and providing high back-drivability via simple valve control. However, pneumatic systems are generally tethered to large stationary air compressors, which greatly limit the practical daily usage. In this study, we implemented a wearable custom compressor that can be worn at the trunk of the body and can generate up to 1050 kPa of pressurized air to power an unilateral active AFO for dorsiflexion (DF) assistance of drop-foot patients. In order to minimize the size and weight of the custom compressor, the compression rate of the custom compressor was optimized to the rate of consumption required to power the active AFO. The finalized system can provide a maximum assistive torque of 9.8 Nm at a functional frequency of 1 Hz and the average resistive torque during free movement was 0.03 Nm. The system was tested for five hemiplegic drop-foot patients. The proposed system showed an average improvement of 12.3° of ankle peak dorsiflexion angle during the mid to late swing phase.

Contribution of Phase Resetting to Adaptive Rhythm Control in Human Walking Based on the Phase Response Curves of a Neuromusculoskeletal Model

Humans walk adaptively in varying environments by manipulating their complicated and redundant musculoskeletal system. Although the central pattern generators in the spinal cord are largely responsible for adaptive walking through sensory-motor coordination, it remains unclear what neural mechanisms determine walking adaptability. It has been reported that locomotor rhythm and phase are regulated by the production of phase shift and rhythm resetting (phase resetting) for periodic motor commands in response to sensory feedback and perturbation. While the phase resetting has been suggested to make a large contribution to adaptive walking, it has only been investigated based on fictive locomotion in decerebrate cats, and thus it remains unclear if human motor control has such a rhythm regulation mechanism during walking. In our previous work, we incorporated a phase resetting mechanism into a motor control model and demonstrated that it improves the stability and robustness of walking through forward dynamic simulations of a human musculoskeletal model. However, this did not necessarily verify that phase resetting plays a role in human motor control. In our other previous work, we used kinematic measurements of human walking to identify the phase response curve (PRC), which explains phase-dependent responses of a limit cycle oscillator to a perturbation. This revealed how human walking rhythm is regulated by perturbations. In this study, we integrated these two approaches using a physical model and identification of the PRC to examine the hypothesis that phase resetting plays a role in the control of walking rhythm in humans. More specifically, we calculated the PRC using our neuromusculoskeletal model in the same way as our previous human experiment. In particular, we compared the PRCs calculated from two different models with and without phase resetting while referring to the PRC for humans. As a result, although the PRC for the model without phase resetting did not show any characteristic shape, the PRC for the model with phase resetting showed a characteristic phase-dependent shape with trends similar to those of the PRC for humans. These results support our hypothesis and will improve our understanding of adaptive rhythm control in human walking.

Biarticular elements as a contributor to energy efficiency: biomechanical review and application in bio-inspired robotics

Despite the increased interest in exoskeleton research in the last decades, not much progress has been made on the successful reduction of user effort. In humans, biarticular elements have been identified as one of the reasons for the energy economy of locomotion. This document gives an extensive literature overview concerning the function of biarticular muscles in human beings. The exact role of these muscles in the efficiency of human locomotion is reduced to three elementary functions: energy transfer towards distal joints, efficient control of output force direction and double joint actuation. This information is used to give an insight in the application of biarticular elements in bio-inspired robotics, i.e. bipedal robots, exoskeletons, robotic manipulators and prostheses. Additionally, an attempt is made to find an answer on the question whether the biarticular property leads to a unique contribution to energy efficiency of locomotion, unachievable by mono-articular alternatives. This knowledge is then further utilised to indicate how biarticular actuation of exoskeletons can contribute to an increased performance in reducing user effort.

Evaluation of the Phase-Dependent Rhythm Control of Human Walking Using Phase Response Curves

Humans and animals control their walking rhythms to maintain motion in a variable environment. The neural mechanism for controlling rhythm has been investigated in many studies using mechanical and electrical stimulation. However, quantitative evaluation of rhythm variation in response to perturbation at various timings has rarely been investigated. Such a characteristic of rhythm is described by the phase response curve (PRC). Dynamical simulations of human skeletal models with changing walking rhythms (phase reset) described a relation between the effective phase reset on stability and PRC, and phase reset around touch-down was shown to improve stability. A PRC of human walking was estimated by pulling the swing leg, but such perturbations hardly influenced the stance leg, so the relation between the PRC and walking events was difficult to discuss. This research thus examines human response to variations in floor velocity. Such perturbation yields another problem, in that the swing leg is indirectly (and weakly) perturbed, so the precision of PRC decreases. To solve this problem, this research adopts the weighted spike-triggered average (WSTA) method. In the WSTA method, a sequential pulsed perturbation is used for stimulation. This is in contrast with the conventional impulse method, which applies an intermittent impulsive perturbation. The WSTA method can be used to analyze responses to a large number of perturbations for each sequence. In the experiment, perturbations are applied to walking subjects by rapidly accelerating and decelerating a treadmill belt, and measured data are analyzed by the WSTA and impulse methods. The PRC obtained by the WSTA method had clear and stable waveforms with a higher temporal resolution than those obtained by the impulse method. By investigation of the rhythm transition for each phase of walking using the obtained PRC, a rhythm change that extends the touch-down and mid-single support phases is found to occur.

Humans and animals tune their walking rhythms when motion is disturbed, such that they hesitate before making the transition from stance to swing phase. The effectiveness of rhythm control for stability has also been shown, and thus the elucidation of rhythm responses is important to understanding human strategies for walking control. In this research, how and when humans change their walking rhythm in response to disturbance is analyzed over the complete walking cycle. Phase response of human walking has previously been estimated by pulling the swing leg. The problem with this perturbation is that it hardly disturbs the stance leg, so here we apply the perturbation by changing floor velocity. However, perturbation from the floor yields another problem in that it weakly influences the swing leg, decreasing the precision of the PRC. The present research tackles this problem by introducing a new method for identifying rhythm characteristics by use of high-frequency perturbation, which allows us to obtain results with clear temporal resolution. We found that the human walking rhythm changes by lengthening the touch-down and mid-single support phases. These phase responses are compared with neural mechanisms for rhythm control, and relevance to the cutaneous and proprioceptive originated responses is shown.

A biologically inspired soft exosuit for walking assistance

We present the design and evaluation of a multi-articular soft exosuit that is portable, fully autonomous, and provides assistive torques to the wearer at the ankle and hip during walking. Traditional rigid exoskeletons can be challenging to perfectly align with a wearer’s biological joints and can have large inertias, which can lead to the wearer altering their natural motion patterns. Exosuits, in comparison, use textiles to create tensile forces over the body in parallel with the muscles, enabling them to be light and not restrict the wearer’s kinematics. We describe the biologically inspired design and function of our exosuit, including a simplified model of the suit’s architecture and its interaction with the body. A key feature of the exosuit is that it can generate forces passively due to the body’s motion, similar to the body’s ligaments and tendons. These passively generated forces can be supplemented by actively contracting Bowden cables using geared electric motors, to create peak forces in the suit of up to 200 N. We define the suit–human series stiffness as an important parameter in the design of the exosuit and measure it on several subjects, and we perform human subjects testing to determine the biomechanical and physiological effects of the suit. Results from a five-subject study showed a minimal effect on gait kinematics and an average best-case metabolic reduction of 6.4%, comparing suit worn unpowered versus powered, during loaded walking with 34.6 kg of carried mass including the exosuit and actuators (2.0 kg on both legs, 10.1 kg total).

Modulation of flexor reflexes by static and dynamic hip proprioceptors in chronic human spinal cord injury

The aim of this study was to investigate the influence of hip proprioceptors on the organisation of the flexor reflex elicited by nociceptive stimulation in individuals with spinal cord injury. The influence of hip position and passive movement were tested in 10 subjects with chronic spinal cord injury. Stimuli were tested isometrically with the hip in three positions. Additionally, the response was also measured to stimuli applied with the hip at midposition during imposed hip flexion and extension movement. The torque and EMG responses were compared in order to identify the postural and movement-dependent modulation of the withdrawal reflex. Ankle and hip torques were significantly modulated by hip position (ANOVA, p<0.05), with the largest torque response obtained in the hip extended position, compared with the flexed position. We also observed a significant difference between the flexor reflex during movement and with the leg isometric. Ankle and hip torque and tibialis anterior electromyograms were significantly higher in the movement conditions than the isometric condition (Tukey test, p<0.05). We postulate that inputs from hip proprioceptors enhance the withdrawal reflex response. Movement appears to increase the response, regardless of movement direction, suggesting a novel role for the dynamic components of hip afferents.

Location specificity of plantar cutaneous reflexes involving lower limb muscles in humans

It is known that cutaneous reflexes in human hand muscles show strong location-specificity dependent on the digit stimulated. We hypothesized that in lower leg muscles the cutaneous reflex following tactile sensation of the plantar surface of the foot is also organized in a location-specific manner. The purpose of the present study was to test this hypothesis. Middle latency reflexes (approximately 70-110 ms, MLR) following non-noxious electrical stimulation to different locations on the plantar foot were recorded from 16 neurologically intact volunteers (15 males, 1 female). Electrical stimulation was given to the fore-medial (f-M), fore-lateral (f-L) and heel (HL) regions of the plantar surface of the right foot while the subjects performed isometric dorsiflexion (tibialis anterior, TA), plantarflexion (soleus, Sol and medial gastrocnemius, MG), eversion (peroneus longus, PL) and knee extension (vastus lateralis, VL) while sitting and standing. In the Sol and MG, an excitatory response was observed following HL stimulation, which was switched to an inhibitory response following f-M or f-L stimulation (P < 0.001). A reciprocal pattern in contrast to Sol was observed in the TA. In the PL, MLR exhibited significant excitation following both f-L and HL stimulation, which, however, was switched to an inhibitory response following f-M stimulation (P < 0.001). Moderate inhibition of the MLR was seen in the VL for all stimulated positions. Systematic stimulation along the lateral side of the plantar foot demonstrated that the reflex reversal occurred around the middle of the plantar foot in the Sol and TA. In all muscles tested, the slope of the regression line between the magnitude of the MLR and background electromyographic activity significantly decreased during standing compared with sitting except for the PL following f-L simulation. These results suggest that reflex effects from cutaneous nerves in the plantar foot onto the motoneurons innervating the lower leg muscles are organized in a highly topographic manner in humans. The organization of these reflexes may play an important role in the alteration of limb loading and/or ground contact in response to tactile sensation of the plantar foot while sitting and standing.

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

Changing the texture of footwear can alter gait patterns

The foot provides an important source of afferent feedback for balance and locomotion. Sensory feedback from the feet can be altered by standing or walking on different surfaces. The purpose was to determine the effects of textured footwear on lower extremity muscle activity, limb kinematics, and joint kinetics while walking. Three-dimensional kinematics and kinetics, as well as muscle EMG, were collected as subjects walked with a smooth and textured shoe insert. Muscle activity was analyzed using a wavelet technique. The textured shoe insert caused a significant reduction in both soleus and tibialis anterior intensity during periods when these muscles are most active. Furthermore, the changes in muscle activity were only seen in the low frequency content of the EMG signal. The foot was significantly more plantar flexed at heel strike with the textured inserts. Small changes were also seen in vertical ground reaction forces and joint moments. It was assumed that the changes in gait patterns were due to a change in sensory feedback caused by the textured shoe insert. The possibilities of altered sensory feedback with footwear are discussed. Sensory feedback from the feet may affect specific motor unit pools during different activities. Changing the texture, without changing the geometry, of a shoe insert can alter muscle activity during walking. This may be useful in the prescription of footwear interventions and suggests that footwear may have sensory as well as mechanical effects.

The role of selected extrinsic foot muscles during running

OBJECTIVE

To determine the kinematic, kinetic and EMG responses to perturbations of the foot by running in varus, neutral, and valgus-wedged shoes.

DESIGN

Within-subjects study comparing kinematics, kinetics and EMG while running in three different shoe conditions.

BACKGROUND

Excessive pronation has been cited as a key contributor to many types of running injuries. However, the roles of the extrinsic foot muscles (those that control motion of the foot) during the stance phase of running have not been adequately identified, which is critical to determining the relationship between pronation and injury.

METHODS

Ten males ran in varus, valgus, and neutral-wedged shoes while three-dimensional kinematic and kinetic data and EMG data were collected. Surface EMG data were collected from the tibialis anterior, peroneus longus, medial and lateral gastrocnemius, and soleus. Indwelling EMG was obtained from the tibialis posterior. The net joint moment, power, and total positive and negative work was calculated in the frontal plane. EMG onset, offset, and integrated values were reported.

RESULTS

The maximum eversion angle, maximum inversion moment and total negative work done in the frontal plane were greatest while running in the valgus shoe and least in the varus shoe. The greater joint moment was not accompanied by changes in muscle activation patterns, although the tibialis posterior data were inconclusive in this respect.

CONCLUSIONS

Greater pronation leads to greater energy absorption in the foot invertor muscles and tendons. While not conclusive, the EMG data suggest that for these muscles there was not a neuromuscular adaptation to the perturbation.

Relationship with dynamic balance function during standing and walking

OBJECTIVE

To investigate the relationship between dynamic balance functions in young adults and elderly adults while standing and walking.

DESIGN

In standing balance tests, the Sensory Organization Test (SOT) of six combinations of three visual and two support-surface conditions was used to measure standing balance, and the Motor Coordination Test (MCT) was used to provoke automatic postural reactions through a series of sudden translations of support surface. The gait test measured maximum anterior acceleration (MAA) and maximum posterior acceleration (MPA) of the trunk during perturbed walking using a bilaterally separated treadmill, and calculated latency until MAA and latency until MPA.

RESULTS

The elderly adults showed more significant functional decline than young adults in SOT1, SOT4, SOT6, medium intensity MCT, large intensity MCT, and MPA. In the correlation analysis of the outcome from the standing examinations, close correlations among SOT4, SOT5, and SOT6 conditions were observed in both groups of young adults and elderly adults. In the MCT, there was very close correlation between varied translation intensity in two groups. On the other hand, the only weak correlation between SOT and MCT findings was between SOT4 and large intensity MCT in elderly adults (r = -0.471, P = 0.049). In the gait test, although correlation was not significant in young adults, the significant correlations between MAA and latency until MAA (r = 0.705, P = 0.001) and latency until MAA and latency until MPA (r = 0.497, P = 0.036) were recognized in elderly adults. In the balance function findings of the standing examinations and the gait examinations, there was significant correlation between medium intensity MCT and latency until MAA (r = -0.552, P = 0.018) in young adults, and SOT6 and latency until MPA (r = -0.473, P = 0.047) in elderly adults. However, no relationship was observed in most of other factors.

CONCLUSIONS

Most falls experienced by elderly people are caused by tripping or slipping during walking. The fact that walking balance function did not correlate with standing balance function indicates that multifaceted evaluation is important to comprehend dynamic balance function while standing and walking.

A test of a dual central pattern generator hypothesis for subcortical control of locomotion

This study was designed to examine the nature of neural circuits involved in subcortical inter-limb coordination and reflex modulation mechanisms of locomotion. These circuits, called central pattern generators (CPGs), are believed to receive tonic input and generate rhythmically alternating sets of commands. Although CPGs have been theorized to exist in humans, their potential dual role in inter-limb coordination and reflex modulation is unclear. In the present study, nine participants walked on a treadmill, timing their heel-strikes to a metronome which varied the phase lag from 0.5 to 1.0 pi radians (0.1 pi intervals). A stimulus was delivered to the sural nerve and reflexes were measured in the ipsilateral and contralateral lower extremities through electromyography. The similarity between phase lag conditions for both temporal coordination (i.e., relative timing aspects between muscles and/or limbs) and reflex intensities suggested that they may be controlled by the same subcortical circuitry. Two plausible explanations exist: (1) a single CPG coordinates muscular contractions and phasically alters proprioceptive reflex modulation, as well as cutaneous input, using feed-forward control; (2) two separate circuits are strongly entrained, producing synchronous outputs for inter-limb coordination and reflex modulation. The out-of-phase task used in this study was limited in discerning such a difference, if it exists.

Stumbling corrective responses in healthy human subjects to rapid reversal of treadmill direction

The kinematics of stumbling and recovery induced by a rapidly reversing treadmill is described for eight healthy adults. Stability was achieved in approximately 400 ms following treadmill reversal (initiated at heel-strike) and the ensuing stumble. It appeared to be accomplished primarily by rapid flexion of the thigh and knee of the stance limb, which prevented damage to the knee joint and lowered the trunk, and by extension of the contralateral joints (swing limb), which contacted the ground presumably to deliver an impulsive thrust to counter the backward lean of the trunk. The movements of the ankle also contributed to the recovery from the stumble, but its movements were markedly more variable among the subjects than those of the thigh and knee. The observed kinematics to some extent resembled a crossed-extension reflex, which may have been triggered by muscle, joint, cutaneous or vestibular afferents. These data should provide a baseline by which to compare groups in which recovery from stumbling is known to be deficient (e.g., the elderly).

What functions do reflexes serve during human locomotion?

Studies on the reflex modulation of vertebrate locomotion have been conducted in many different laboratories and with many different preparations: for example, lamprey swimming, bird flight, quadrupedal walking in cats and bipedal walking in humans. Emerging concepts are that reflexes are task-, phase- and context-dependent. To function usefully in a behaviour such as locomotion wherein initial conditions change from step to step, reflexes would have to show modulation. Papers are reviewed in which the study of different reflexes have been conducted during different behaviours, with an emphasis on experiments in humans. A framework is developed in which the modulation and flexibility of reflexes are demonstrated. Alterations in cutaneous, and muscle (stretch and load receptor) reflexes between sitting, standing and walking are discussed. Studies in which both electrical, mechanical and 'natural' receptor activation have been conducted during walking are reviewed. Reflexes are shown to have important regulatory functions during human locomotion. A framework for discussion of reflex function throughout the step cycle is developed. The function of a given reflex pathway changes dynamically throughout the locomotor cycle. While all reflexes act in concert to a certain extent, generally cutaneous reflexes act to alter swing limb trajectory to avoid stumbling and falling. Stretch reflexes act to stabilize limb trajectory and assist force production during stance. Load receptor reflexes are shown to have an effect on both stance phase body weight support and step cycle timing. After neurotrauma or in disease, reflexes no longer function as during normal locomotion, but still have the potential to be clinically exploited in gait modification regimens.

Function of sural nerve reflexes during human walking

1. The functions of ipsilateral cutaneous reflexes were studied with short trains of stimuli presented pseudorandomly to the sural nerve during human walking. Electromyograms (EMG) of lower (tibialis anterior (TA), soleus, lateral (LG) and medial (MG) gastrocnemius) and upper leg (vastus lateralis and biceps femoris) muscles were recorded, together with ankle, knee and hip joint angles. Net reflex EMG responses were quantified in each of the sixteen parts of the step cycle. The kinematic measurements included ankle eversion- inversion, and ankle, knee and hip flexion-extension. 2. The function of the sural reflexes depended upon the part of the step cycle in which the nerve was stimulated and the intensity of stimulation. During stance, reflexes in MG and TA muscles in response to a medium intensity of stimulation (1.9 x radiating threshold, x RT) were closely associated with ankle eversion and dorsiflexion responses, respectively. These responses could assist in accommodation to uneven terrain that applies pressure to the lateral side of the foot (sural innervation area). Non-noxious, high intensity (2.3 x RT) stimulation resulted in strong suppression of LG and MG during stance which was correlated to a small reduction in ankle plantarflexion. At this higher intensity the response would function to prevent the foot from moving more forcefully onto a potentially harmful obstacle. 3. During swing, ankle dorsiflexion increased and was significantly correlated to the net TA EMG response after both medium and high intensity stimulation. Knee flexion was increased throughout swing at both intensities of stimulation. These responses may serve in an avoidance response in which the swing limb is brought past an obstacle without destabilizing contact. 4. The net EMG and kinematic responses suggest that cutaneous reflexes stabilize human gait against external perturbations produced by an uneven surface in stance or obstacles encountered during swing.

Neurophysiology of gait disorders: present and future applications

This article will review those electrophysiological investigations which have addressed the neuronal mechanisms underlying impaired gait. The aims of the review are to provide further insights to the underlying pathophysiology of impaired gait and also towards the selection of an appropriate treatment. From the patients' point of view the first indication of a central motor system lesion is an impairment of movement, most notably locomotion. These symptoms are characteristic in cases of spasticity, cerebellar lesion or Parkinson's disease. Clinical examination reveals typical changes in tendon tap reflexes and muscle tone which were believed to account for the movement disorder presented. However, we now know that there is only a weak relationship between the physical symptoms observed during clinical examination under passive motor conditions and the altered neuronal mechanisms underlying the impairment during active motion. By recording and analysing electrophysiological and biomechanical parameters during functional movements such as locomotion, the significance of impaired reflex behaviour or the pathophysiology of muscle tone and its contribution to the movement disorder can be reliably assessed. Consequently, the treatment should not be cosmetic, i.e. the correction of an isolated clinical parameter, but should be based on the pathophysiology and significance of those mechanisms underlying the impairment of the patients' movements. Data from electrophysiological and biomechanical investigations of locomotion of patients with spasticity, cerebellar disorder or Parkinson's disease are discussed in this review. The neuronal mechanisms, which are essentially central programs and afferent input, involved in disorders of gait are evaluated on the basis of their function in healthy subjects. The impact of this analysis in deciding an appropriate treatment are discussed with respect to the pathophysiology underlying the gait disorder (spasticity, cerebellar disorder or Parkinson's disease). At the present time we have only a basic understanding of the essential receptor systems, such as leg extensor load receptors, and their interaction with other systems involved in postural control. In the future, the knowledge gained from gait analysis may help in the selection of the appropriate pharmacological and physical treatment required even though the patient may only be at an early stage of motor impairment.

Cutaneous reflexes during human gait: electromyographic and kinematic responses to electrical stimulation

The functions of ipsilateral cutaneous reflexes were studied with short trains of stimuli presented pseudorandomly to the superficial peroneal (SP) and tibial nerves during human gait. Electromyograms (EMGs) of tibialis anterior (TA), soleus, lateral and medial gastrocnemius, vastus lateralis (VL), and biceps femoris (BF) muscle were recorded, together with ankle and knee joint angles. Net reflex EMG responses were quantified in each of the 16 parts of the step cycle according to a recently developed technique. After SP nerve stimulation, TA muscle showed a significant suppression during swing phase that was highly correlated to ankle plantarflexion. BF and VL muscles were both excited throughout swing and significantly correlated to knee flexion during early swing. Tibial nerve stimulation caused dorsiflexion during late stance, but plantarflexion during late swing. We argue that SP nerve reflexes are indicative of a stumbling corrective response to nonnoxious electrical stimulation in humans. The correlated kinematic responses after tibial nerve stimulation may allow smooth movement of the swing leg so as to prevent tripping during swing and to assist placing and weight acceptance at the beginning of stance.

Sensori-sensory afferent conditioning with leg movement: gain control in spinal reflex and ascending paths

Studies are reviewed, predominantly involving healthy humans, on gain changes in spinal reflexes and supraspinal ascending paths during passive and active leg movement. The passive movement research shows that the pathways of H reflexes of the leg and foot are down-regulated as a consequence of movement-elicited discharge from somatosensory receptors, likely muscle spindle primary endings, both ipsi- and contralaterally. Discharge from the conditioning receptors in extensor muscles of the knee and hip appears to lead to presynaptic inhibition evoked over a spinal path, and to long-lasting attenuation when movement stops. The ipsilateral modulation is similar in phase to that seen with active movement. The contralateral conditioning does not phase modulate with passive movement and modulates to the phase of active ipsilateral movement. There are also centrifugal effects onto these pathways during movement. The pathways of the cutaneous reflexes of the human leg also are gain-modulated during active movement. The review summarizes the effects across muscles, across nociceptive and non-nociceptive stimuli and over time elapsed after the stimulus. Some of the gain changes in such reflexes have been associated with central pattern generators. However, the centripetal effect of movement-induced proprioceptive drive awaits exploration in these pathways. Scalp-recorded evoked potentials from rapidly conducting pathways that ascend to the human somatosensory cortex from stimulation sites in the leg also are gain-attenuated in relation to passive movement-elicited discharge of the extensor muscle spindle primary endings. Centrifugal influences due to a requirement for accurate active movement can partially lift the attenuation on the ascending path, both during and before movement. We suggest that a significant role for muscle spindle discharge is to control the gain in Ia pathways from the legs, consequent or prior to their movement. This control can reduce the strength of synaptic input onto target neurons from these kinesthetic receptors, which are powerfully activated by the movement, perhaps to retain the opportunity for target neuron modulation from other control sources.

Extensor muscle responses to tibial nerve stimulation are enhanced during dynamic tilts in standing humans

We studied the interaction between muscle responses evoked in standing by electrical stimulation (ES) of the tibial nerve and dynamic tilts of the head-and-body in 13 young healthy subjects. Subjects were attached to an L-shaped tilting apparatus and underwent sudden forward tilting of their head-and-body as a whole, without ankle rotation. During such tilts, the area of response evoked in the ipsilateral vastus lateralis (iVL) muscle by the ES was significantly increased by 74% as compared to quiet supported stance (P = 0.01). The response latency of the contralateral VL and soleus muscles i.e. the crossed extension reflex, was significantly shorter during tilt (54 +/- 22 ms) than during quiet supported stance (115 +/- 13 ms, P < 0.01). The increased excitability of extensor muscles activated by ES during tilt seems appropriate to maintain stance during a forward perturbation of the body.

Modification of reflexes in normal and abnormal movements

The trajectories observed for the limb during human locomotion are determined by a mixture of influences, some arising from neural circuits entirely within the central nervous system and others arising from a variety of sensory receptors. Muscle reflexes are highly modulated during locomotion in an adaptive manner within each phase of the step cycle. Furthermore, the modulation can be modified quickly for different tasks such as standing, walking and running, probably by changes in presynaptic inhibition. This modulation is often lost or severely reduced in patients with spasticity after spinal cord or head injury. In normal subjects cutaneous reflexes can be completely reversed from exciting to inhibiting a muscle during each step cycle, particularly in muscles that normally show two bursts of activity per cycle (e.g., tibialis anterior). In some patients stimulation of a mixed nerve (e.g., common peroneal) can directly produce muscle contraction, generate a reflex response (flexor reflex) and transiently reduce spasticity in antagonist (extensor) muscles. Thus, simple systems employing stimulation can enhance gait to a certain extent in patients with incomplete injuries.

Gating and reversal of reflexes in ankle muscles during human walking

Phase-dependent reflex modulation was studied by recording the electromyographic (EMG) responses in ankle flexors (Tibialis Anterior, TA) and extensors (Gastrocnemius Medialis, GM and Soleus, SOL) to a 20 ms train of electrical pulses, applied to the tibial or sural nerve at the ankle, in human volunteers walking on a treadmill at 4 km/h. For low intensity stimuli (i.e. 1.6 times perception threshold), given during the swing phase, the most common response was a suppression of the TA activity with a latency of 67 to 118 ms. With high intensity of stimulation (i.e. 2.8 x T), a facilitatory response appeared in TA with a latency of 74 ms. This latter response was largest during the middle of the swing phase, when it was correlated with exaggerated ankle dorsiflexion. The TA reflex amplitude was not a simple function of the level of spontaneous ongoing activity. During stance, TA responses were small or absent and accompanied by a suppression of the GM activity with a latency ranging from 62 to 101 ms. A few subjects showed an early facilitatory, instead of a suppressive, GM response (88 to 136 ms latency). They showed a phase-dependent reflex reversal from a dominant TA response during swing to a facilitatory GM response with an equivalent latency during stance. The GM facilitation occurred exclusively during the early stance phase and habituated more than the TA responses. It is concluded that phase-dependent gating of reflexes occurs in ankle muscles of man, but only when vigorous extensor reflexes are present. More commonly, a phase-dependent modulation is seen, both of facilitatory and suppressive responses.

Reflex actions of knee joint afferents during contraction of the human quadriceps

(1) The spinal reflex actions of afferents stimulated by knee joint distension have been investigated in man. (2) Cannulation of the knee and infusion of saline raised intra-articular pressure, especially during quadriceps contraction. High pressures did not induce any sensation of pain. Pressure was taken as an index of joint proprioceptor activation. (3) Increased pressure progressively depressed the quadriceps H-reflex, both at rest and during quadriceps contraction. There was no indication of a threshold pressure for this inhibitory action. (4) It is concluded that joint distension inhibits quadriceps motoneurons through spinal pathways that still operate during voluntary contraction. These pathways could thus contribute to pathological weakness after joint injury. (5) Joint distension produced spatial facilitation of non-reciprocal inhibition of quadriceps H-reflexes from afferents in the tibial nerve.

Phase-Dependent Compensatory Responses to Perturbation Applied During Walking in Humans

The interaction between the peripheral and the central regulation of locomotion was studied by examining the dependency of the response to unexpected perturbation on the phase of the step cycle. The changes in the latency and magnitude of various muscle responses to electrical stimulation of the toe and applied unexpectedly at different phases of the locomotor cycle in humans are described. The results show that response to perturbation is gated and modulated in both ipsi- and contralateral limb muscles. These muscle responses, when present, were always excitatory in nature. They were not correlated with the normal locomotor activity, thus suggesting a more complex organization of the response. Except for one muscle in the contralateral limb, the latency of the other muscle responses did not vary across the step cycle. in response to the perturbation, the appropriate phase of the step cycle was shortened. The results from this study suggest that the perturbation applied elicits a phase-independent, normal ipsilateral flexor response in the tibialis anterior muscle, while the gating and modulation of other ipsi- and contralateral muscles provide appropriate phase-dependent adaptive response to maintain postural stability and continue with the ongoing task of locomotion.

Excitability of the soleus H-reflex arc during walking and stepping in man

In eight normal subjects, the excitability of the soleus (Sol) H-reflex was tested in parallel with Sol length changes, EMGs of leg and thigh muscles and ground contact phases, during three different pacing movements: bipedal treadmill walking, single limb treadmill walking, and single-limb stepping on one spot. A computerized procedure was used which compensated for changes in stimulus effectiveness that occurred during free motion. In the three paradigms examined, significant excitability modulations were observed with respect to a control level determined in standing weight-bearing position. During bipedal treadmill walking, excitability was decreased in the early stance, maximally enhanced in the second half of the stance, and again decreased during the end-stance and the whole swing phase, with a minimum value around the toe off period. The main modulation pattern was retained during single-limb treadmill walking. During single-limb stepping on one spot, the stance-phase increase in excitability and the swing phase depression were still present. However, in the second half of the swing phase, reflex responsiveness returned to reference level, which was maintained during the subsequent contact period. Moreover, a decrease in reflex excitability was detected around the mid-stance. The time course of the described modulations was only partly correlated with the EMG and length changes of the Sol muscle. Furthermore, in the three movements tested, during the early stance phase, the excitability of the H-reflex arc did not correspond to the one expected on the basis of the available H-reflex studies performed under static conditions. It is suggested that, at least in certain stride phases (e.g. around the early contact period), an active regulation affects the transmission in the Sol myotatic arc during the pacing movements investigated.

Adaptation of postural response to voluntary arm raises during locomotion in humans

Adaptation of the response to voluntary rapid arm flexion during 3 phases of the step cycle in humans are examined. The onset of arm movement was similar to the standing condition and was unaffected by the phase of the step cycle. The locomotor cycle in contrast was altered. These changes improved the stability of the subjects by shortening the appropriate stance or swing phase of the step cycle. Only the onset of the ipsilateral biceps femoris response which represents the postural adaptation was dependent on the phase of the step cycle. When the subjects were asked to flex their arm in early swing, the biceps femoris was enhanced after the onset of the arm movement. This suggests that the postural responses during locomotion may not necessarily be anticipatory in nature, but more functional.

Some Characteristics of EMG Patterns During Locomotion

Myoelectric signals from several muscles of the lower limb were studied under various speed and stride length conditions. The main purpose was to determine invariant and variant features among these myoelectric patterns. A pattern recognition algorithm was used to analyze these activity patterns. Within-condition analysis revealed some common features among the EMG patterns. This suggests that the nervous system does not have to generate all the muscle activity patterns, only the common features that can, in appropriate combination, produce the necessary activity patterns. From the across condition analysis, the following rules emerged. First, both phasic component and magnitude (d.c. level) of the muscle activity patterns have to be modulated to meet the demands imposed by the various conditions. Second, the variability in the proximal muscle activity patterns across conditions are higher than the distal muscle activity patterns. Within each group, the extensor muscles and double-jointed muscles show greater variability than the flexor muscles and single-jointed muscles. And finally, the changes in the average value (d.c. level) of the muscle activity patterns across conditions are not uniform but show muscle and task specificity. For example, within the speed condition, the increase in d.c. level of the extensors with speed of locomotion show a proximal to distal trend. Based on these results, a conceptual model for the human locomotor control process is proposed.