Recumbent stepping has similar but simpler neural control compared to walking

Generalizability of motor modules across walking-based and in-place tasks - a distribution-based analysis on total knee replacement patients

Introduction: There are evidences that the nervous system produces motor tasks using a low-dimensional modular organization of muscle activations, known as motor modules. Previous studies have identified characteristic motor modules across similar tasks in healthy population. This study explored the generalizability of motor modules across two families of walking-based (level-walking, downhillwalking and stair-decent), in-place ascending (sit-to-stand, squat-to-stand), and in-place descending (stand-to-sit and stand-to-squat) motor tasks in a group of six individuals undergone total knee replacement (TKR) surgery. Methods: Motor modules were extracted from the EMG data of CAMS-Knee dataset using non-negative matrix factorization technique. A distribution-based approach, employing three levels of k-means clustering, was then applied to find the shared and task-specific modules, and assess their representability among the whole task-trial data. Results and Discussion: Results indicated a four- and a seven-subcluster arrangement of the shared and task-specific motor modules, depending upon the membership criteria. The first arrangement revealed motor modules which were shared across all tasks (min coverage index: 76%; modules' distinctness range: 7.08-8.91) and the latter among tasks of the same family mainly, although there remained some interfamily shared modules (min coverage index: 81%; modules' distinctness range: 7.17-9.89). It was concluded that there are shared motor modules across walking-based and in-place tasks in TKR individuals, with their generalizability and representability depending upon the analysis method. This finding highlights the importance of the analysis method in identifying the shared motor modules, as the main building blocks of motor control.

Alterations in the Neuromuscular Control Mechanism of the Legs During a Post-Fatigue Landing Make the Lower Limbs More Susceptible to Injury

Fatigue causes the lower limb to land in an injury-prone state, but the underlying neuromuscular control changes remain unclear. This study aims to investigate lower limb muscle synergies during landing in basketball players, both before and after fatigue, to examine alterations in neuromuscular control strategies induced by fatigue. Eighteen male recreational basketball players performed landing tasks pre- and post-fatigue induced by 10 × 10 countermovement jumps. Electromyographic (EMG) data from eight muscles, including the erector spinae (ES), rectus abdominus (RA), gluteus maximus (GM), rectus femoris (RF), biceps femoris (BF), lateral gastrocnemius (LG), soleus (SM), and tibialis anterior (TA) muscles, were analyzed using non-negative matrix factorization to extract muscle synergies. Post-fatigue results revealed significant changes: synergy primitive 1 decreased before landing (18-30% phase) and synergy primitive 2 decreased after landing (60-100% phase). Muscle weights of the LG and SM in synergy module 2 increased. Fatigue reduced synergistic muscle activation levels, compromising joint stability and increasing knee joint loading due to greater reliance on calf muscles. These changes heighten the risk of lower limb injuries. To mitigate fatigue-induced injury risks, athletes should improve thigh muscle endurance and enhance neuromuscular control, fostering better synergy between thigh and calf muscles during fatigued conditions.

Results of a feasibility and initial efficacy clinical trial of a high-intensity interval training program using adaptive equipment in persons with multiple sclerosis who have walking disability

BACKGROUND

High intensity interval training (HIIT) has been identified as potential stimulus for eliciting health-promoting physical activity in an efficient manner among persons with multiple sclerosis (MS). The current study aimed to examine the feasibility and initial efficacy of a 12-week HIIT program using a recumbent stepper (RSTEP) in persons with MS who have walking disability. Feasibility outcomes of interest included process (i.e., recruitment, adherence, and retention rates), resource (i.e., time and monetary costs), management (i.e., data management and safety reporting assessment), and science (i.e., safety, burden, and treatment effect assessment). We hypothesized that 12-weeks of HIIT will be feasible via meeting a priori benchmarks in process, resource, management, and scientific outcomes. The efficacy outcomes of interest included changes in aerobic fitness, physical activity, walking, upper arm function, cognition, fatigue, and depressive symptoms. We hypothesized that 12 weeks of HIIT would result in improvements in aerobic capacity, walking, upper arm function, cognition, fatigue, and depression.

METHODS

A pre-post clinical trial design was applied. Participants (N = 16) were recruited and enrolled in the 12-week RSTEP HIIT program who met the following inclusion criteria: age ≥18 years, self-reported diagnosis of MS, Patient Determined Disability Steps scale score 3.0-7.0, relapse free in past 30 days, willing to visit a University Laboratory for study protocol, asymptomatic status for maximal exercise testing, physician approval, and a self-reported ability to speak, read, and understand English. Measures of efficacy outcomes of interest included Six Minute Walk Test (6MW), Timed 25 Foot Walk Test (T25FW), the Brief International Cognitive Assessment in MS (BICAMS), 9-hole peg test (9-HPT), Expanded Disability Status Scale (EDSS), Fatigue Severity Survey (FSS), Hospital Anxiety and Depression Scale (HADS), Godin Leisure Time Exercise Questionnaire (GLTEQ), Multiple Sclerosis Walking Scale-12 (MSWS-12). Participants completed a graded maximal exercise test for measuring aerobic fitness (VO2peak) and prescription of exercise throughout the intervention. All outcomes were measured at baseline, mid-point (6-weeks), and post-intervention (12-weeks). The intervention involved 12 weeks of supervised, individualized HIIT sessions two times per week using RSTEP. The individual HIIT sessions included 10 cycles of 60 s intervals at the work rate associated with 90 % VO2peak followed by 60 s of active recovery intervals, totaling 20 minutes plus 5-minute warm-up and cool-down periods. Process, resources, management, and scientific feasibility outcomes were examined using descriptive statistics, percentage, and frequency analyses. The efficacy of the intervention was assessed using a 1-factor (Time), repeated measure analysis of variance to identify significant changes over time.

RESULTS

Fourteen of 16 participants were retained throughout the full study period and adherence with prescribed exercise sessions was 97 %. Twenty-three staff were comprehensively trained across two sites. There was only one adverse event reported that did not impact participation in the study and overall mean satisfaction rating with the program among participants was 4.7/5. There were statistically significant changes in cognitive processing speed (p = 0.002), GLTEQ (p = 0.005), and MSWS-12 (p = 0.04), but not the other outcomes of fitness, arm function, and walking. Of note, there were large effect sizes noted for peak power output (d = 1.10) and FSS (d = 1.05) despite the lack of statistically significant changes CONCLUSION: Feasibility of a 12-week individualized RSTEP HIIT program was established and participants significantly improved on measures of cognition, physical activity, and walking.

The Effect of Handlebar Height and Bicycle Frame Length on Muscular Activity during Cycling: A Pilot Study

The cycling literature is filled with reports of electromyography (EMG) analyses for a better understanding of muscle function during cycling. This research is not just limited to performance, as the cyclist’s goal may be rehabilitation, recreation, or competition, so a bicycle that meets the rider’s needs is essential for a more efficient muscular activity. Therefore, the purpose of this study was to understand the contribution of the activity of each of the following muscles: TD (trapezius descending), LD (latissimus dorsi), GM (gluteus maximus), and AD (anterior deltoid) in response to different bicycle-rider systems (handlebar height; bicycle frame length) and intensities in a bicycle equipped with a potentiometer. Surface EMG signals from muscles on the right side of the body were measured. A general linear model test was used to analyze the differences between muscle activation in the test conditions. Effect sizes were calculated using a partial Eta2 (η2). The level of significance was set at 0.05. Muscle activation of different muscles differs, depending on the cycling condition (Pillai’s trace = 2.487; F (36.69) = 9.300; p < 0.001. η2 = 0.958), mostly during low intensities. In high intensities, one specific pattern emerges, with a greater contribution of GM and TD and weaker participation of LD and AD, enhancing the cycling power output.

Using a simple rope-pulley system that mechanically couples the arms, legs, and treadmill reduces the metabolic cost of walking

BACKGROUND

Emphasizing the active use of the arms and coordinating them with the stepping motion of the legs may promote walking recovery in patients with impaired lower limb function. Yet, most approaches use seated devices to allow coupled arm and leg movements. To provide an option during treadmill walking, we designed a rope-pulley system that physically links the arms and legs. This arm-leg pulley system was grounded to the floor and made of commercially available slotted square tubing, solid strut channels, and low-friction pulleys that allowed us to use a rope to connect the subject's wrist to the ipsilateral foot. This set-up was based on our idea that during walking the arm could generate an assistive force during arm swing retraction and, therefore, aid in leg swing.

METHODS

To test this idea, we compared the mechanical, muscular, and metabolic effects between normal walking and walking with the arm-leg pulley system. We measured rope and ground reaction forces, electromyographic signals of key arm and leg muscles, and rates of metabolic energy consumption while healthy, young subjects walked at 1.25 m/s on a dual-belt instrumented treadmill (n = 8).

RESULTS

With our arm-leg pulley system, we found that an assistive force could be generated, reaching peak values of 7% body weight on average. Contrary to our expectation, the force mainly coincided with the propulsive phase of walking and not leg swing. Our findings suggest that subjects actively used their arms to harness the energy from the moving treadmill belt, which helped to propel the whole body via the arm-leg rope linkage. This effectively decreased the muscular and mechanical demands placed on the legs, reducing the propulsive impulse by 43% (p < 0.001), which led to a 17% net reduction in the metabolic power required for walking (p = 0.001).

CONCLUSIONS

These findings provide the biomechanical and energetic basis for how we might reimagine the use of the arms in gait rehabilitation, opening the opportunity to explore if such a method could help patients regain their walking ability.

TRIAL REGISTRATION

 Study registered on 09/29/2018 in ClinicalTrials.gov (ID-NCT03689647).

Effects of Visual Feedback During Recumbent Stepping in Individuals With Chronic Stroke

Objective

To investigate the effects of intermittent visual feedback (using the Balanced Power program on the NuStep Transitt) during recumbent stepping on strength, balance, and functional mobility in individuals with chronic stroke.

Design

Quasi-experimental 1-group pretest-posttest study.

Setting

Human performance research laboratory.

Participants

Adults (N=11; 7 female; mean age, 58.7±13.6y), >6 months post stroke.

Interventions

Eight 45-minute training sessions on the NuStep Transitt (visits 2-9) twice a week (5-minute warm-up and cooldown with 35 minutes of training [5min with and then without visual feedback regarding left/right lower extremity percentage effort]). Visits 1 and 10: pre- and post assessment.

Main Outcome Measures

Self-selected and fast gait speeds; maximum voluntary contractions (MVCs) of knee extension and flexion and ankle dorsiflexion and plantarflexion; and 5 times sit-to-stand (5TSTS).

Results

Significant improvements in 5TSTS (14.2s, P=.007) and fast gait (hemi: 4.9 cm [P=.024], nonhemi: 4.5cm (P=.019) stride length; nonhemi step length 2.3 cm (P=.024]). MVC and self-selected gait parameters showed no significant changes.

Conclusions

The NuStep Transitt is a valuable tool that provides real-time feedback about percentage of use of the hemiparetic leg. This intervention study has demonstrated that the addition of visual feedback about left/right percentage effort while exercising on the Transitt has significant and clinically relevant effects on the functional mobility of individuals with chronic stroke.

Differential Theta-Band Signatures of the Anterior Cingulate and Motor Cortices During Seated Locomotor Perturbations

Quantifying motor and cortical responses to perturbations during seated locomotor tasks such as recumbent stepping and cycling will expand and improve the understanding of locomotor adaptation processes beyond just perturbed gait. Using a perturbed recumbent stepping protocol, we hypothesized motor errors and anterior cingulate activity would decrease with time, and perturbation timing would influence electrocortical elicitation. Young adults (n = 17) completed four 10-minute arms and legs stepping tasks, with perturbations applied at every left or right leg extension-onset or mid-extension. A random no-perturbation "catch" stride occurred in every five perturbed strides. We instructed subjects to follow a pacing cue and to step smoothly, and we quantified temporal and spatial motor errors. We used high-density electroencephalography to estimate sources of electrocortical fluctuations shared among >70% of subjects. Temporal and spatial errors did not decrease from early to late for either perturbed or catch strides. Interestingly, spatial errors post-perturbation did not return to pre-perturbation levels, suggesting use-dependent learning occurred. Theta (3-8 Hz) synchronization in the anterior cingulate cortex and left and right supplementary motor areas (SMA) emerged near the perturbation event, and extension-onset perturbations elicited greater theta-band power than mid-extension perturbations. Even though motor errors did not adapt, anterior cingulate theta synchronization decreased from early to late perturbed strides, but only during the right-side tasks. Additionally, SMA mainly demonstrated specialized, not contralateral, lateralization. Overall, seated locomotor perturbations produced differential theta-band responses in the anterior cingulate and SMAs, suggesting that tuning perturbation parameters, e.g., timing, can potentially modify electrocortical responses.

Feasibility and initial efficacy of a high-intensity interval training program using adaptive equipment in persons with multiple sclerosis who have walking disability: study protocol for a single-group, feasibility trial

Background

There is considerable evidence for the efficacy of moderate-intensity continuous exercise benefitting clinically relevant outcomes in persons with multiple sclerosis (MS). However, persons with MS who have walking disability (pwMS-wd) are severely deconditioned and may achieve superior benefits by engaging in high-intensity interval training (HIIT), especially while utilizing adaptive equipment, such as recumbent arm/leg stepping (RSTEP). The proposed study will assess the feasibility of a 12-week, RSTEP HIIT program in pwMS-wd. The secondary aim will examine changes in aerobic fitness, physical activity, ambulation, upper arm function, cognition, fatigue, and depression as clinically relevant efficacy outcomes following the 12-week, RSTEP HIIT intervention.

Methods

The study will recruit 15 pwMS-wd. Feasibility will be measured via process, resource, management, and scientific outcomes throughout the entirety of the research study. The secondary, clinically relevant outcomes will consist of a neurological exam, aerobic capacity, physical activity, ambulation, cognition, upper arm function, fatigue, and depression. Outcomes will be assessed at baseline (T1), midpoint (T2, following 6 weeks), and post-intervention (T3, following 12 weeks). The intervention will involve 12 weeks of supervised, individualized HIIT sessions two to three times per week. The individual HIIT sessions will each involve 10 cycles of 60-s intervals at the wattage associated with 90% VO_2peak followed by 60 s of active recovery intervals at 15 W, totaling 20 min in length plus 5-min warm-up and cool-down periods.

Discussion

The feasibility design of the proposed study will provide experience and preliminary data for advancing towards a proof-of-concept study comparing HIIT to moderate-intensity continuous RSTEP for improving clinically relevant outcomes in a randomized control trial design. The results will be disseminated via manuscripts for publication and a report for distribution among the National Multiple Sclerosis Society.

Trial registration

ClinicalTrials.gov NCT04416243. Retrospectively registered on June 4, 2020

Kinematics and neuromuscular recruitment during vertical treadmill exercise

The vertical treadmill (VertiRun) is an unresearched, partial weight-bearing exercise mode for lower limb rehabilitation. The user undertakes a “running-like” action whilst body weight is supported by a bench and the limb is drawn downwards against overhanging resistance cables on a vertically hung nonmotorised treadmill. This study sought to describe the kinematics and neuromuscular recruitment during VertiRun exercise in the supine, 40°, and 70° postures. Twenty-one healthy male participants (age, 25±7 years; stature, 1.79±0.07 m; body mass, 77.7±8.8 kg) volunteered for sagittal plane kinematic analysis of the ankle, knee and hip and electromyography of lower limb musculature in all three postures. Results indicated similar kinematic and neuromuscular profiles in the 40° and 70° postures which differed from the supine. Regardless of posture, a basic movement pattern was observed where the hamstrings and gastrocnemius muscles were active to extend the hip, flex the knee, plantarflex the ankle and draw the leg down the treadmill belt in the contact phase. The rectus femoris and tibialis anterior were active to flex the hip and knee, and dorsiflex the ankle to draw the leg upwards during the swing phase. The vasti muscles were not active during VertiRun exercise. The VertiRun demonstrated similar kinematic and neuro-muscular patterns to overground gait, allows workload progression based on effort and posture changes, and is a low-impact exercise mode that could maintain physical fitness without loading injured tissues. This study suggests that the VertiRun could supplement rehabilitation programmes for lower-limb injuries.

Neuromechanical interactions between the limbs during human locomotion: an evolutionary perspective with translation to rehabilitation

During bipedal locomotor activities, humans use elements of quadrupedal neuronal limb control. Evolutionary constraints can help inform the historical ancestry for preservation of these core control elements support transfer of the huge body of quadrupedal non-human animal literature to human rehabilitation. In particular, this has translational applications for neurological rehabilitation after neurotrauma where interlimb coordination is lost or compromised. The present state of the field supports including arm activity in addition to leg activity as a component of gait retraining after neurotrauma.

Effects of aerobic exercise training on fitness and walking-related outcomes in ambulatory individuals with chronic incomplete spinal cord injury

Study Design

Single group, pretest-posttest study.

Objectives

To determine the effects of a non-task-specific, voluntary, progressive aerobic exercise training (AET) intervention on fitness and walking-related outcomes in ambulatory adults with chronic motor-incomplete SCI.

Setting

Rehabilitation research center.

Methods

Ten ambulatory individuals (50% female; 57.94 ± 9.33 years old; 11.11 ± 9.66 years post injury) completed voluntary, progressive moderate-to-vigorous intensity AET on a recumbent stepper three days per week for six weeks. The primary outcome measures were aerobic capacity (VO_2peak) and self-selected overground walking speed (OGWS). Secondary outcome measures included: walking economy, six-minute walk test (6MWT), daily step counts, Walking Index for Spinal Cord Injury (WISCI-II), Dynamic Gait Index (DGI), and Berg Balance Scale (BBS).

Results

Nine participants completed all testing and training. Significant improvements in aerobic capacity (P=0.011), OGWS (P=0.023), the percentage of VO_2peak utilized while walking at self-selected speed (P=0.03), and daily step counts (P=0.025) resulted following training.

Conclusions

The results indicate that total-body, voluntary, progressive AET is safe, feasible, and effective for improving aerobic capacity, walking speed, and select walking-related outcomes in an exclusively ambulatory SCI sample. This study suggests the potential for non-task-specific aerobic exercise to improve walking following incomplete SCI and builds a foundation for further investigation aimed at the development of exercise based rehabilitation strategies to target functionally limiting impairments in ambulatory individuals with chronic SCI.

Muscle Synergies During a Single-Leg Drop-Landing in Boys and Girls

The purpose of this study was to investigate muscle activation patterns during a landing task in boys and girls through the use of muscle synergies. Electromyographical data from six lower extremity muscles were collected from 11 boys and 16 girls while they performed single-leg drop-landings. Electromyographical data from six leg muscles were rectified, smoothed, and normalized to maximum dynamic muscle activity during landing. Data from 100 ms before to 100 ms after touchdown were submitted to factor analyses to extract muscle synergies along with the associated activation and weighing coefficients. Boys and girls both used three muscle synergies. The activation coefficients of these synergies captured muscle activity during the prelanding, touchdown, and postlanding phases of the single-leg drop-landing. Analysis of the weighing coefficients indicated that within the extracted muscle synergies the girls emphasized activation of the medial hamstring muscle during the prelanding and touchdown synergy whereas boys emphasized activation of the vastus medialis during the postlanding synergy. Although boys and girls use similar muscle synergies during single-leg drop-landings, they differed in which muscles were emphasized within these synergies. The observed differences in aspects related to the muscle synergies during landing may have implications with respect to knee injury risk.

Modular control of varied locomotor tasks in children with incomplete spinal cord injuries

A module is a functional unit of the nervous system that specifies functionally relevant patterns of muscle activation. In adults, four to five modules account for muscle activation during walking. Neurological injury alters modular control and is associated with walking impairments. The effect of neurological injury on modular control in children is unknown and may differ from adults due to their immature and developing nervous systems. We examined modular control of locomotor tasks in children with incomplete spinal cord injuries (ISCIs) and control children. Five controls (8.6 ± 2.7 yr of age) and five children with ISCIs (8.6 ± 3.7 yr of age performed treadmill walking, overground walking, pedaling, supine lower extremity flexion/extension, stair climbing, and crawling. Electromyograms (EMGs) were recorded in bilateral leg muscles. Nonnegative matrix factorization was applied, and the minimum number of modules required to achieve 90% of the "variance accounted for" (VAF) was calculated. On average, 3.5 modules explained muscle activation in the controls, whereas 2.4 modules were required in the children with ISCIs. To determine if control is similar across tasks, the module weightings identified from treadmill walking were used to reconstruct the EMGs from each of the other tasks. This resulted in VAF values exceeding 86% for each child and each locomotor task. Our results suggest that 1) modularity is constrained in children with ISCIs and 2) for each child, similar neural control mechanisms are used across locomotor tasks. These findings suggest that interventions that activate the neuromuscular system to enhance walking also may influence the control of other locomotor tasks.

Effect of axon misdirection on recovery of electromyographic activity and kinematics after peripheral nerve injury

In this study, patterns of activity in the soleus (Sol) and tibialis anterior (TA) muscles and hindlimb kinematics were evaluated during slope walking in rats after transection and surgical repair either of the entire sciatic nerve (Sci group) or of its two branches separately, the tibial and common fibular nerves (T/CF group). With the latter method, axons from the tibial and common fibular nerves could not reinnervate targets of the other nerve branch after injury, reducing the opportunity for misdirection. Activity in the TA shifted from the swing phase in intact rats to nearly the entire step cycle in both injured groups. Since these changes occur without misdirection of regenerating axons, they are interpreted as centrally generated. Sol activity was changed from reciprocal to that of TA in intact rats to coactivate with TA, but only in the Sci group rats. In the T/CF group rats, Sol activity was not altered from that observed in intact rats. Despite effects of injury that limited foot movements, hindlimb kinematics were conserved during downslope walking in both injury groups and during level walking in the T/CF group. During level walking in the Sci group and during upslope walking in both groups of injured rats, the ability to compensate for the effects of the nerve injury was less effective and resulted in longer limb lengths held at more acute angles throughout the step cycle. Changes in limb movements occur irrespective of axon misdirection and reflect compensatory changes in the outputs of the neural circuits that drive locomotion.

Upper and lower limb muscle activation is bidirectionally and ipsilaterally coupled

PURPOSE

There are neural connections between the upper and lower limbs of humans that enable muscle activation in one limb pair (upper or lower) to modulate muscle activation in the other limb pair (lower or upper, respectively). The aims of this study were to extend previous findings regarding submaximal exercise to maximal effort exercise and determine whether there is an ipsilateral or contralateral bias to the neural coupling during a rhythmic locomotor-like task.

METHODS

We measured upper and lower limb muscle activity, joint kinematics, and limb forces in neurologically intact subjects (n = 16) as they performed recumbent stepping using different combinations of upper and lower limb efforts.

RESULTS

We found increased muscle activation in passive lower limbs during active upper limb effort compared with passive upper limb effort. Likewise, increased muscle activation in passive upper limbs occurred during active lower limb effort compared with passive lower limb effort, suggesting a bidirectional effect. Maximal muscle activation in the active lower limbs was not different between conditions with active upper limb effort and conditions with passive upper limb movement. Similarly, maximal muscle activation in the active upper limbs was not different between conditions with active lower limb effort and conditions with passive lower limb movement. Further comparisons revealed that neural coupling was primarily from active upper limb muscles to passive ipsilateral lower limb muscles.

CONCLUSIONS

These findings indicate that interlimb neural coupling affects muscle recruitment during maximal effort upper and lower limb rhythmic exercise and provides insight into the architecture of the neural coupling.

Neuromechanical considerations for incorporating rhythmic arm movement in the rehabilitation of walking

We have extensively used arm cycling to study the neural control of rhythmic movements such as arm swing during walking. Recently rhythmic movement of the arms has also been shown to enhance and shape muscle activity in the legs. However, restricted information is available concerning the conditions necessary to maximally alter lumbar spinal cord excitability. Knowledge on the neuromechanics of a task can assist in the determination of the type, level, and timing of neural signals, yet arm swing during walking and arm cycling have not received a detailed neuromechanical comparison. The purpose of this research was to provide a combined neural and mechanical measurement approach that could be used to assist in the determination of the necessary and sufficient conditions for arm movement to assist in lower limb rehabilitation after stroke and spinal cord injury. Subjects performed three rhythmic arm movement tasks: (1) cycling (cycle); (2) swinging while standing (swing); and (3) swinging while treadmill walking (walk). We hypothesized that any difference in neural control between tasks (i.e., pattern of muscle activity) would reflect changes in the mechanical constraints unique to each task. Three-dimensional kinematics were collected simultaneously with force measurement at the hand and electromyography from the arms and trunk. All data were appropriately segmented to allow a comparison between and across conditions and were normalized and averaged to 100% movement cycle based on shoulder excursion. Separate mathematical principal components analysis of kinematic and neural variables was performed to determine common task features and muscle synergies. The results highlight important neural and mechanical features that distinguish differences between tasks. For example, there are considerable differences in the anatomical positions of the arms during each task, which relate to the moments experienced about the elbow and shoulder. Also, there are differences between tasks in elbow flexion/extension kinematics alongside differential muscle activation profiles. As well, mechanical assistance and constraints during all tasks could affect muscle recruitment and the functional role of muscles. Overall, despite neural and mechanical differences, the results are consistent with conserved common central motor control mechanisms operational for cycle, walk, and swing but appropriately sculpted to demands unique to each task. However, changing the mechanical parameters could affect the role of afferent feedback altering neural control and the coupling to the lower limbs.

Neural regulation of rhythmic arm and leg movement is conserved across human locomotor tasks

It has been proposed that different forms of rhythmic human limb movement have a common central neural control ('common core hypothesis'), just as in other animals. We compared the modulation patterns of background EMG and cutaneous reflexes during walking, arm and leg cycling, and arm-assisted recumbent stepping. We hypothesized that patterns of EMG and reflex modulation during cycling and stepping (deduced from mathematical principal components analysis) would be comparable to those during walking because they rely on similar neural substrates. Differences between the tasks were assessed by evoking cutaneous reflexes via stimulation of nerves in the foot and hand in separate trials. The EMG was recorded from flexor and extensor muscles of the arms and legs. Angular positions of the hip, knee and elbow joints were also recorded. Factor analysis revealed that across the three tasks, four principal components explained more than 93% of the variance in the background EMG and middle-latency reflex amplitude. Phase modulation of reflex amplitude was observed in most muscles across all tasks, suggesting activity in similar control networks. Significant correlations between EMG level and reflex amplitude were frequently observed only during static voluntary muscle activation and not during rhythmic movement. Results from a control experiment showed that strong correlation between EMG and reflex amplitudes was observed during discrete, voluntary leg extension but not during walking. There were task-dependent differences in reflex modulation between the three tasks which probably arise owing to specific constraints during each task. Overall, the results show strong correlation across tasks and support common neural patterning as the regulator of arm and leg movement during various rhythmic human movements.