Early corrective reactions of the leg to perturbations at the torso during walking in humans

Low-force human-human hand interactions induce gait changes through sensorimotor engagement instead of direct mechanical effects

Physical human-robot interactions (pHRI) often provide mechanical force and power to aid walking without requiring voluntary effort from the human. Alternatively, principles of physical human-human interactions (pHHI) can inspire pHRI that aids walking by engaging human sensorimotor processes. We hypothesize that low-force pHHI can intuitively induce a person to alter their walking through haptic communication. In our experiment, an expert partner dancer influenced novice participants to alter step frequency solely through hand interactions. Without prior instruction, training, or knowledge of the expert's goal, novices decreased step frequency 29% and increased step frequency 18% based on low forces (< 20 N) at the hand. Power transfer at the hands was 3-700 × smaller than what is necessary to propel locomotion, suggesting that hand interactions did not mechanically constrain the novice's gait. Instead, the sign/direction of hand forces and power may communicate information about how to alter walking. Finally, the expert modulated her arm effective dynamics to match that of each novice, suggesting a bidirectional haptic communication strategy for pHRI that adapts to the human. Our results provide a framework for developing pHRI at the hand that may be applicable to assistive technology and physical rehabilitation, human-robot manufacturing, physical education, and recreation.

Coupling of single cutaneous afferents in the hand with ankle muscles, and their response to rapid light touch displacements

Light touch reduces sway during standing. Unexpected displacement of a light touch reference at the finger can produce rapid responses in ankle muscles when standing, suggesting cutaneous receptors in the hand are functionally coupled with ankle muscles. Using microneurography in the median nerve, we tested the hypotheses: 1) that cutaneous afferent activity of mechanoreceptors of the hand would modulate electromyographic (EMG) activity of ankle muscles, and 2) that displacement of a light touch contact across a receptor's sensory territory would be encoded in the afferent activity. Spike-triggered averaging of EMG activity of tibialis anterior (TA) and soleus (SOL) demonstrated thirty-four of forty-two (81%) cutaneous afferents recorded modulated activity of ankle muscles with latencies between 40 to 119 ms. Cutaneous afferents of all types (slow and fast adapting, types I and II) demonstrated responses in TA and SOL, in both the ipsilateral and contralateral leg. Activity from eleven cutaneous afferents were recorded while a light touch contact was displaced across their receptive fields. Afferent activity increased with stimulus onset and remained elevated for the stimulus duration for all afferents recorded. These results suggest cutaneous afferents from the hand consistently form connections with motor pools of the leg at latencies implicating spinal pathways. In addition, the same population of afferents is readily excited by the displacement of a light touch contact. Therefore, cutaneous receptors of the hand can be recruited and utilized to alter motoneuron pool excitability in muscles important to balance control, at latencies relevant for rapid balance responses.

Long-lasting changes in muscle activation and step cycle variables induced by repetitive sensory stimulation to discrete areas of the foot sole during walking

We examined whether repetitive electrical stimulation to discrete foot sole regions that are phase-locked to the step cycle modulates activity patterns of ankle muscles and induces neuronal adaptation during human walking. Nonnoxious repetitive foot sole stimulation (STIM; 67 pulses at 333 Hz) was given to the medial forefoot (f-M) or heel (HL) regions at 1) the stance-to-swing transition, 2) swing-to-stance transition, or 3) midstance, during every step cycle for 10 min. Stance, but not swing, durations were prolonged with f-M STIM delivered at stance-to-swing transition, and these changes remained for up to 20-30 min after the intervention. Electromyographic (EMG) burst durations and amplitudes in the ankle extensors were also prolonged and persisted for 20 min after the intervention. Interestingly, STIM to HL was ineffective at inducing modulation, suggesting stimulation location-specific adaptation. In contrast, STIM to HL (but not f-M), at the swing-to-stance phase transition, shortened the step cycle by premature termination of swing. Furthermore, the onset of EMG bursts in the ankle extensors appeared earlier than in the control condition. STIM delivered during the midstance phase was ineffective at modulating the step cycle, highlighting phase-dependent adaptation. These effects were absent when STIM was applied while mimicking static postures for each walking phase during standing. Our findings suggest that the combination of walking-related neuronal activity with repetitive sensory inputs from the foot can generate short-term adaptation that is phase-dependent and localized to the site of STIM.NEW & NOTEWORTHY Repetitive (∼10 min) long (200 ms) trains of sensory stimulation to discrete areas of the foot sole produce persistent changes in muscle activity and cycle timing during walking. Interactions between the delivery phase and stimulus location determine the expression of the adaptations. These observations bear striking similarities to those in decerebrate cat experiments and may be usefully translated to improving locomotor function after neurotrauma.

Biarticular muscles are most responsive to upper-body pitch perturbations in human standing

Balancing the upper body is pivotal for upright and efficient gait. While models have identified potentially useful characteristics of biarticular thigh muscles for postural control of the upper body, experimental evidence for their specific role is lacking. Based on theoretical findings, we hypothesised that biarticular muscle activity would increase strongly in response to upper-body perturbations. To test this hypothesis, we used a novel Angular Momentum Perturbator (AMP) that, in contrast to existing methods, perturbs the upper-body posture with only minimal effect on Centre of Mass (CoM) excursions. The impulse-like AMP torques applied to the trunk of subjects resulted in upper-body pitch deflections of up to 17° with only small CoM excursions below 2 cm. Biarticular thigh muscles (biceps femoris long head and rectus femoris) showed the strongest increase in muscular activity (mid- and long-latency reflexes, starting 100 ms after perturbation onset) of all eight measured leg muscles which highlights the importance of biarticular muscles for restoring upper-body balance. These insights could be used for improving technological aids like rehabilitation or assistive devices, and the effectiveness of physical training for fall prevention e.g. for elderly people.

Locomotor training improves reciprocal and nonreciprocal inhibitory control of soleus motoneurons in human spinal cord injury

Pathologic reorganization of spinal networks and activity-dependent plasticity are common neuronal adaptations after spinal cord injury (SCI) in humans. In this work, we examined changes of reciprocal Ia and nonreciprocal Ib inhibition after locomotor training in 16 people with chronic SCI. The soleus H-reflex depression following common peroneal nerve (CPN) and medial gastrocnemius (MG) nerve stimulation at short conditioning-test (C-T) intervals was assessed before and after training in the seated position and during stepping. The conditioned H reflexes were normalized to the unconditioned H reflex recorded during seated. During stepping, both H reflexes were normalized to the maximal M wave evoked at each bin of the step cycle. In the seated position, locomotor training replaced reciprocal facilitation with reciprocal inhibition in all subjects, and Ib facilitation was replaced by Ib inhibition in 13 out of 14 subjects. During stepping, reciprocal inhibition was decreased at early stance and increased at midswing in American Spinal Injury Association Impairment Scale C (AIS C) and was decreased at midstance and midswing phases in AIS D after training. Ib inhibition was decreased at early swing and increased at late swing in AIS C and was decreased at early stance phase in AIS D after training. The results of this study support that locomotor training alters postsynaptic actions of Ia and Ib inhibitory interneurons on soleus motoneurons at rest and during stepping and that such changes occur in cases with limited or absent supraspinal inputs.

Balance-corrective responses to unexpected perturbations at the arms during treadmill walking

The arms have been shown to be involved in the regulation of balance during walking. The use of a walking aid enhances balance by increasing the base of support and reducing the load on the legs by partly transferring it to the arms. However, when actively engaged during a balance task, perturbations to the arms can destabilize balance. Previous studies have investigated postural adjustments associated with focal arm movements during standing and walking. However, balance-corrective reactions to unexpected perturbations to the arms during walking have not been well studied. In the present study, subjects walked on a treadmill while grasping a pair of handles when sudden perturbations were delivered by displacing the handles in the forward or backward direction. Instructing subjects to oppose the displacement of the handles resulted in strong responses in the arms that were accompanied by activation of muscles in the legs, comparable to those observed in other balance disturbance studies. Conversely, when subjects were instructed to allow the handles to move when displaced, no responses were observed in the arms. However, similar responses were observed in the legs whether subjects opposed the displacement of the handles or not when perturbations were applied at heel strike. The results from this study show that balance reactions can be elicited in the legs in response to perturbations applied at the arms, and that the expression of these responses is affected by the task engaged in by the arms.

A neuromechanical strategy for mediolateral foot placement in walking humans

Stability is an important concern during human walking and can limit mobility in clinical populations. Mediolateral stability can be efficiently controlled through appropriate foot placement, although the underlying neuromechanical strategy is unclear. We hypothesized that humans control mediolateral foot placement through swing leg muscle activity, basing this control on the mechanical state of the contralateral stance leg. Participants walked under Unperturbed and Perturbed conditions, in which foot placement was intermittently perturbed by moving the right leg medially or laterally during the swing phase (by ∼50-100 mm). We quantified mediolateral foot placement, electromyographic activity of frontal-plane hip muscles, and stance leg mechanical state. During Unperturbed walking, greater swing-phase gluteus medius (GM) activity was associated with more lateral foot placement. Increases in GM activity were most strongly predicted by increased mediolateral displacement between the center of mass (CoM) and the contralateral stance foot. The Perturbed walking results indicated a causal relationship between stance leg mechanics and swing-phase GM activity. Perturbations that reduced the mediolateral CoM displacement from the stance foot caused reductions in swing-phase GM activity and more medial foot placement. Conversely, increases in mediolateral CoM displacement caused increased swing-phase GM activity and more lateral foot placement. Under both Unperturbed and Perturbed conditions, humans controlled their mediolateral foot placement by modulating swing-phase muscle activity in response to the mechanical state of the contralateral leg. This strategy may be disrupted in clinical populations with a reduced ability to modulate muscle activity or sense their body's mechanical state.

A neuromechanical strategy for mediolateral foot placement in walking humans

Stability is an important concern during human walking and can limit mobility in clinical populations. Mediolateral stability can be efficiently controlled through appropriate foot placement, although the underlying neuromechanical strategy is unclear. We hypothesized that humans control mediolateral foot placement through swing leg muscle activity, basing this control on the mechanical state of the contralateral stance leg. Participants walked under Unperturbed and Perturbed conditions, in which foot placement was intermittently perturbed by moving the right leg medially or laterally during the swing phase (by ∼50-100 mm). We quantified mediolateral foot placement, electromyographic activity of frontal-plane hip muscles, and stance leg mechanical state. During Unperturbed walking, greater swing-phase gluteus medius (GM) activity was associated with more lateral foot placement. Increases in GM activity were most strongly predicted by increased mediolateral displacement between the center of mass (CoM) and the contralateral stance foot. The Perturbed walking results indicated a causal relationship between stance leg mechanics and swing-phase GM activity. Perturbations that reduced the mediolateral CoM displacement from the stance foot caused reductions in swing-phase GM activity and more medial foot placement. Conversely, increases in mediolateral CoM displacement caused increased swing-phase GM activity and more lateral foot placement. Under both Unperturbed and Perturbed conditions, humans controlled their mediolateral foot placement by modulating swing-phase muscle activity in response to the mechanical state of the contralateral leg. This strategy may be disrupted in clinical populations with a reduced ability to modulate muscle activity or sense their body's mechanical state.

Spinal plasticity in stroke patients after botulinum neurotoxin A injection in ankle plantar flexors

The effect of botulinum neurotoxin A (BoNT-A) in stroke patients' upper limbs has been attributed to its peripheral action only. However, BoNT-A depressed recurrent inhibition of lumbar motoneurons, likely due to its retrograde transportation along motor axons affecting synapses to Renshaw cells. Because Renshaw cells control group Ia interneurons mediating reciprocal inhibition between antagonists, we tested whether this inhibition, particularly affected after stroke, could recover after BoNT-A. The effect of posterior tibial nerve (PTN) stimulation on tibialis anterior (TA) electromyogram (EMG) was investigated in 13 stroke patients during treadmill walking before and 1 month after BoNT-A injection in ankle plantar flexors. Before BoNT-A, PTN stimuli enhanced TA EMG all during the swing phase. After BoNT-A, the PTN-induced reciprocal facilitation in TA motoneurons was depressed at the beginning of swing and reversed into inhibition in midswing, but at the end of swing, the reciprocal facilitation was enhanced. This suggests that BoNT-A induced spinal plasticity leading to the recovery of reciprocal inhibition likely due to the withdrawal of inhibitory control from Renshaw cells directly blocked by the toxin. At the end of swing, the enhanced reciprocal facilitation might be due to BoNT-induced modification of peripheral afferent inputs. Therefore, both central and peripheral actions of BoNT-A can modify muscle synergies during walking: (1) limiting ankle muscle co-contraction in the transition phase from stance to swing, to assist dorsiflexion, and (2) favoring it from swing to stance, which blocks the ankle joint and thus assists the balance during the single support phase on the paretic limb.

The contribution of light touch sensory cues to corrective reactions during treadmill locomotion

The arms play an important role in balance regulation during walking. In general, perturbations delivered during walking trigger whole-body corrective responses. For instance, holding to stable handles can largely attenuate and even suppress responses in the leg muscles to perturbations during walking. Particular attention has been given to the influence of light touch on postural control. During standing, lightly touching a stable contact greatly reduces body sway and enhances corrective responses to postural perturbations, whereas light touch during walking allows subjects to continue to walk on a treadmill with the eyes closed. We hypothesized that in the absence of mechanical support from the arms, sensory cues from the hands would modulate responses in the legs to balance disturbing perturbations delivered at the torso during walking. To test this, subjects walked on a treadmill while periodically being pulled backwards at the waist while walking. The amplitude of the responses evoked in tibialis anterior to these perturbations was compared across 4 test conditions, in a 2 × 2 design. Subjects either (a) lightly touched or (b) did not touch a stable contact, while the eyes were (c) open or (d) closed. Allowing the subjects to touch a stable contact resulted in a reduction in the amount of fore-aft oscillation of the body on the treadmill, which was accompanied by a reduction in the ongoing electromyographic activity in both tibialis anterior and soleus during undisturbed walking. In contrast, the provision of touch resulted in an increase in the amplitude of the evoked responses in tibialis anterior to the backward perturbations that was more evident when subjects walked with the eyes closed. These results indicate that light touch provides a sensory cue that can be used to assist in stabilizing the body while walking. In addition, the sensory information provided by light touch contributes to the regulation of corrective reactions initiated by balance disturbances encountered during walking.

Robust control of CPG-based 3D neuromusculoskeletal walking model

This paper proposes a method for enhancing the robustness of the central pattern generator (CPG)-based three-dimensional (3D) neuromusculoskeletal walking controller. The CPG has been successfully applied to walking controllers and controllers for walking robots. However, the robustness of walking motion with the CPG-based controller is not sufficient, especially when subjected to external forces or environmental variations. To achieve a realistic and stable walking motion of the controller, we propose the use of an attracting controller in parallel with the CPG-based controller. The robustness of the proposed controller is confirmed through simulation results.

A novel approach to mechanical foot stimulation during human locomotion under body weight support

Input from the foot plays an essential part in perceiving support surfaces and determining kinematic events in human walking. To simulate adequate tactile pressure inputs under body weight support (BWS) conditions that represent an effective form of locomotion training, we here developed a new method of phasic mechanical foot stimulation using light-weight pneumatic insoles placed inside the shoes (under the heel and metatarsus). To test the system, we asked healthy participants to walk on a treadmill with different levels of BWS. The pressure under the stimulated areas of the feet and subjective sensations were higher at high levels of BWS and when applied to the ball and toes rather than heels. Foot stimulation did not disturb significantly the normal motor pattern, and in all participants we evoked a reliable step-synchronized triggering of stimuli for each leg separately. This approach has been performed in a general framework looking for "afferent templates" of human locomotion that could be used for functional sensory stimulation. The proposed technique can be used to imitate or partially restore surrogate contact forces under body weight support conditions.

Whole-Body Responses: Neural Control and Implications for Rehabilitation and Fall Prevention

Humans are one of the unique species that utilize bipedal gait to ambulate in our environment. Despite this fact, coordination of the arms with the legs and the rest of body is essential for many daily activities. As such, whole-body responses have emerged as the preferred strategy following perturbations to balance during both standing and walking. Complex neural circuitry may allow for this coordination through the use of propriospinal pathways linking lumbar and cervical pattern generators in the spinal cord, with supraspinal centers altering this control depending on the context of the situation. Based on these findings, we argue that whole-body reactions may be exploited for rehabilitation purposes. Preliminary results have indicated training programs designed to elicit whole-body responses are effective in reducing falls and improving functional mobility in older adults with and without neurological impairment.

Phase-specific modulation of the soleus H-reflex as a function of threat to stability during walking

The purpose of the present study was to determine whether the soleus H-reflex is modulated with changes in the level of postural threat during walking. H-reflexes were tested at four points in the step cycle when subjects walked in 5 conditions representing different levels of postural threat. H-reflexes were significantly increased in amplitude at heelstrike in conditions of increased postural threat compared to normal treadmill walking with only minimal changes in H-reflex amplitude at other step cycle points. Conversely when subjects walked while holding stable handles, to decrease postural threat, the amplitude of the H-reflex was significantly smaller at heelstrike and midstance compared to normal walking. The changes in the amplitude of the H-reflex between walking conditions were not accompanied by changes in ongoing electromyographic activity or movements. Our findings suggest that the amplitude of the reflex is adjusted in a phase-specific manner, related to the postural uncertainty of the task. These adaptations in reflex amplitude may be related to changes in the amplitude of corrective responses following perturbations during walking. The adaptations in the amplitude of the H-reflex specific to heelstrike may be important in the control of foot placement at ground contact.

Human standing and walking: comparison of the effects of stimulation of the vestibular system

The adoption of bipedalism by hominids including man has complicated the tasks of balance control and the minimisation of body sway. We have investigated the role of the vestibular organs in controlling sway in the roll direction using galvanic vestibular stimulation (GVS). Two stance conditions were studied: during forward lean posterior compartment muscles are activated and during backward lean anterior compartment muscles are activated. GVS-evoked vestibular signals in stance control leg muscles as a group: all the active muscles in the leg on the GVS cathode side are excited together and those in the contralateral leg (anode side) relax. The subject sways towards the anode side. During treadmill walking, vestibular actions are subtly different: the actions are largely restricted to muscles acting at the ankle joint, occur at longer latencies, are not reciprocal in the opposite limb, are modulated throughout the step cycle (largest early in stance) and are reversed in sign in the peroneus longus muscle. The subject deviates towards the anode side. Hand contact with a firm object reduces GVS-evoked responses in leg muscles during treadmill walking. Responses to GVS are observed during over-ground walking but not significantly during bicycling on an ergometer. The observations suggest that these vestibular actions are part of a roll stabilisation mechanism. They may be mediated through different spinal premotor mechanisms during standing and walking and turned off during bicycling, when leg muscles have no balance control function.

Postural uncertainty leads to dynamic control of cutaneous reflexes from the foot during human walking

Cutaneous reflexes evoked by stimulation of nerves innervating the foot are modulated in a phase-dependent manner during locomotion. The pattern of modulation of these reflexes has been suggested to indicate a functional role of cutaneous reflexes in assisting to maintain stability during walking. We hypothesized that if cutaneous reflexes assist in maintaining stability during gait, then these reflexes should be modulated in a context-dependent manner when subjects are asked to walk in an environment in which stability is challenged. To do this, we asked subjects to walk on a treadmill under five conditions: (1) normally, (2) with the arms crossed, (3) while receiving unpredictable anterior-posterior (AP) perturbations, (4) with the arms crossed while receiving unpredictable AP perturbations, and (5) with the hands holding onto fixed handles. Cutaneous reflexes arising from electrical stimulation of the superficial peroneal (SP; relevant to stumbling) or distal tibial (TIB; relevant to ground contact sensation) nerves were recorded bilaterally, at four points in the step cycle. Reflexes evoked with SP nerve stimulation showed marked facilitation during the most unstable walking condition in 4 of the 7 muscles tested. SP nerve-evoked reflexes in the muscles of the contralateral leg also showed suppression during the most stable walking condition. Reflexes evoked with TIB nerve stimulation were less affected by changes in the walking task. We argue that the specific adaptation of cutaneous reflexes observed with SP nerve stimulation supports the hypothesis that cutaneous reflexes from the foot contribute to the maintenance of stability during walking.

Restricting arm use enhances compensatory reactions of leg muscles during walking

A number of recent studies have indicated that whole-body coordinated reactions are employed to regain balance following disturbances during walking. However, it is not always the case that all body segments are available to contribute to balance corrective strategies. We hypothesize that balance corrective strategies will adapt to task and environment constraints such that greater responses are generated in the available body segments when other body segments are unable to participate. In this study, we tested the hypothesis that voluntarily restricting the arms during walking would result in an increase in the amplitude of the electromyographic responses evoked in leg muscles when subjects are perturbed at the torso during walking. To do so, subjects were asked to walk on a motorized treadmill while either crossing their arms across their front or back, or with their arms swinging normally. Periodic perturbations, forwards and backwards, were applied at the pelvis randomly throughout the step cycle. This resulted in short latency responses in leg muscles. The amplitude of these responses was increased when subjects walked with their arms crossed, as compared with normal, unrestricted walking. Facilitation of these evoked responses was restricted to the early part of the stance phase, particularly at heel-strike. The pattern of muscle activation and the latency of the responses were not affected by restricting the arms. We suggest that this finding indicates that whole-body balance corrective strategies employed during walking are selected based upon the demands of the general features of the task, but that components of the strategy are scaled according to the specific context-dependent needs of the task.

Control of dynamic stability during gait termination on a slippery surface

There are three common ways by which to successfully terminate gait: decreased acceleration of whole-body center of mass (COM) through a flexor synergy in the trail leg, increased deceleration of whole-body COM through an extensor synergy in the front limb, and an energy/momentum transfer to dissipate any remaining momentum if the first two strategies are unsuccessful. Healthy individuals were asked to stop on a slippery surface while we examined their unexpected response to the slippery surface. Kinetic data from the forceplates revealed lower braking forces in the slip trials compared with normal gait-termination trials. Subjects were unable to control their center of pressure (COP) to manipulate the COM as revealed by increased deviations and maximum absolute ranges of COP movement. Subject COM deviated farther in both horizontal planes and lowered further during the slip compared with normal gait-termination trials. Arm movements were effective in dissipating forward COM movement. In addition, there likely was a transfer of forward to lateral momentum to stop forward progression. All recorded muscle activity in the lower limbs and back increased during the slip to provide support to the lower limbs and correct upright balance. The trailing limb shortened its final step to provide support to the lowering COM. The balance-correction response seen here resembles previous reactions to perturbations during locomotion suggesting there is a generalized strategy employed by the nervous system to correct for disturbances and maintain balance.

Is the use of vestibular information weighted differently across the initiation of walking?

The purpose of this experiment was to examine vestibular contributions at specific times during the initiation of walking in human subjects. Subjects began walking forward at the sound of an auditory tone, with vision present or occluded. Galvanic vestibular stimulation (GVS) was delivered with the anode electrode on the right or left side at either: (1) onset of the anticipatory postural adjustment (APA), (2) toe-off of the first swing limb (TO) or (3) heel contact of the first swing limb (HC). Ground reaction forces and kinematic data were collected. Upper body (roll angles from head, trunk and pelvis) and lower body (foot placement) data were analysed to determine whether the timing and magnitude of the response to GVS, and therefore the level of vestibular contribution, was modulated at different points during the initiation of gait. With vision present and occluded, the magnitude of the lower body response varied depending on the event in the gait cycle at which the stimulation was delivered. These novel results demonstrate evidence that vestibular weighting during gait initiation is dependent upon the specific gait initiation events. Upper body roll also exhibited magnitude differences between events. However, these changes are proposed to occur due to the transition from a stationary position into a dynamic state, prompting the increased weighting of vestibular information. With vision present no significant changes were seen in the segment roll response. The observations suggest a distinction in vestibular regulation of upper body roll versus foot placement for successful completion of the gait initiation task. Changes in upper body roll are influenced by the dynamic nature of the task, whereas foot placement changes are modulated based on the event during gait initiation at which GVS is delivered.

Early activation of arm and leg muscles following pulls to the waist during walking

Many studies have investigated the compensatory reactions in humans elicited during walking when the support surface is perturbed. This has led to the description of characteristic responses generated in the muscles of the legs and torso, and recently the arms. The present study aimed to investigate the compensatory reactions elicited when balance was challenged by a perturbation applied to the waist, to determine to what extent balance corrective responses are generalized across perturbation modalities. A second aim was to characterize the arm responses elicited by the perturbations applied to the waist. We measured muscle activity of the left arm and leg following application of backward pulls of the waist while the subjects walked on a motorized treadmill. This resulted in robust activation of tibialis anterior and vastus lateralis, with co-activation of soleus and biceps femoris also evident when perturbations were applied at heel strike. These early responses occurred with a distal to proximal temporal organization. The responses in the leg muscles displayed a phase-dependent modulation in amplitude, decreasing in amplitude later in the stance phase. Leg muscle responses were not evident during the swing phase, except for the end of swing, just prior to heel strike. Arm muscle responses were observed in all subjects; however, the pattern of the arm responses varied considerably between subjects. Generally, shoulder muscles were more likely to respond than elbow muscles, at latencies consistent with the leg responses. Two important conclusions are drawn from the present study. First, the responses evoked in the legs with a pull to the waist are very similar to what has been reported for perturbations of the support surface, despite the very different locus of the perturbation. This suggests that balance control during walking may be achieved by preprogrammed reactions or synergies, which are triggered by multiple sensory cues. Second, rapid arm actions are integrated with these leg responses. However, the arm responses are more flexible, likely reflecting the fewer constraints imposed upon the actions of the arms, compared to the legs, during normal locomotion.

Progressive adaptation of the soleus H-reflex with daily training at walking backward

When untrained subjects walk backward on a treadmill the amplitude of the soleus H-reflex in midswing is equal to or exceeds the value in stance. This is a surprising result because during the swing phase of backward walking the soleus is inactive and its antagonist, the tibialis anterior, is active. We suggested that the high amplitude of the soleus H-reflex in late swing reflects task uncertainties, such as estimating the moment of foot contact with the ground and losing balance. In support of this idea we show that when untrained subjects held on to handrails the unexpected high-amplitude H-reflex during midswing was no longer present. We therefore asked whether daily training at this task without grasping the handrails would adaptively modify the H-reflex modulation pattern. In this event, within 10 days of training for 15 min daily, the anticipatory reflex activity at the beginning of training was gradually abated as the subjects reported gaining confidence at the task. However, when adapted subjects were made to walk backward with their eyes shut, the anticipatory reflex activity in midswing returned immediately. The reflex changes as a result of training were not due to changes in the motor activity or kinematics; they are likely part of the motor program controlling backward walking. This adaptive phenomenon may prove to be a useful model for studying the neural mechanisms of motor learning and adaptive plasticity in humans and may be relevant to rehabilitation programs for neurological patients.

The special nature of human walking and its neural control

Walking the way we do is inherently unstable. Sophisticated neurological control systems are required to ensure that we progress and maintain our balance at the same time. Most of what is known about the functional organization of these neurological control systems is inferred from studies on animals. Here, I compare selected studies on the neural control of human walking with similar studies in reduced animal preparations. The simple monosynaptic reflex appears to be controlled by comparable mechanisms in walking cats and humans. However, peripheral feedback mechanisms suggested to contribute to the switch from stance to swing on the basis of experiments in reduced cat preparations have little influence during human walking. A cat whose spinal cord has been completely transected can be made to walk on a treadmill by drug injections, but such an immediate effect of pharmacological intervention is not seen in humans. However, there have been reports that pharmacological intervention can improve the walking of patients with incomplete spinal cord injury, especially when pharmacological treatment is combined with training.

Could enhanced reflex function contribute to improving locomotion after spinal cord repair?

Although numerous treatments have been found to improve locomotion in spinal cord injured mammals, the underlying mechanisms are very poorly understood. Some of the main possibilities are: (1) regeneration of axons across the injury site and the re-establishment of descending pathways needed to voluntarily initiate and maintain stepping in the hind legs, (2) enhanced effectiveness of undamaged neurons in preparations with incomplete transections of the cord, (3) non-specific facilitation of reflexes and intrinsic spinal networks by transmitters released from regenerated axons and/or by substances introduced by the treatment, and (4) enhanced trunk movements close to the injury site strengthening the mechanical coupling of the trunk to the hind legs via spinal reflexes. In addition, any procedure that even slightly improves stepping may be further enhanced by use-dependent modification of reflex pathways and interneuronal networks in the lumbar cord. The emphasis of this review is on the contribution of spinal reflexes to the patterning of motor activity for walking, and how enhancing reflex function may contribute to the improvement of locomotion by treatments aimed at restoring locomotion after complete transection of the spinal cord.

Interlimb co-ordination in human infant stepping

1. We held infants (aged 4-12 months) over a treadmill to study how they co-ordinated the two limbs during stepping. We disturbed one limb during the stance or swing phase and recorded the responses (muscle activity and movement) from both lower limbs. Manual disturbances were applied during the stance phase by sliding the foot backward, forcing the limb into the swing phase. Disturbances were also applied in the swing phase by manually extending the hip, interfering with the forward motion of the limb. Additional disturbances were applied to see if both limbs could perform the stance and swing phase synchronously. 2. When the limb was forced to initiate the swing phase on one side, the contralateral limb either prolonged its contact with the ground or quickly established ground contact. When the forward motion of the limb was interrupted in the swing phase, the swing phase was prolonged on the disturbed side and the stance phase prolonged on the contralateral side. In most cases, one leg maintained ground contact. Moreover, it was easy to elicit bilateral, simultaneous stance phase, whereas it was difficult to elicit simultaneous swing phase. In cases where swing phase in the two limbs was initiated close in time, rhythmic alternate stepping was immediately restored in the following step. 3. We conclude that human infants can generate co-ordinated motor responses bilaterally in response to unilateral perturbations, well before the onset of independent walking.

The initiation of the swing phase in human infant stepping: importance of hip position and leg loading

Hip extension and low load in the extensor muscles are important sensory signals that allow a decerebrate or spinal cat to advance from the stance phase to the swing phase during walking. We tested whether the same sensory information controlled the phases of stepping in human infants. Twenty-two infants between the ages of 5 and 12 months were studied during supported stepping on a treadmill. Forces exerted by the lower limbs, surface electromyography (EMG) from muscles, and the right hip angle were recorded. The whole experimental session was videotaped. The hip position and the amount of load experienced by the right limb were manipulated during stepping by changing the position of the foot during the stance phase or by applying manual pressure on the pelvic crest. Disturbances with different combinations of hip position and load were used. The stance phase was prolonged and the swing phase delayed when the hip was flexed and the load on the limb was high. In contrast, stance phase was shortened and swing advanced when the hip was extended and the load was low. The results were remarkably similar to those in reduced preparations of the cat. They thus suggest that the behaviour of the brainstem and spinal circuitry for walking may be similar between human infants and cats. There was an inverse relationship between hip position and load at the time of swing initiation, indicating the two factors combine to regulate the transition.