Afferent feedback in the control of human gait

Biomechanical and neurophysiological mechanisms related to postural control and efficiency of movement: a review

Understanding postural control requires considering various mechanisms underlying a person's ability to stand, to walk, and to interact with the environment safely and efficiently. The purpose of this paper is to summarize the functional relation between biomechanical and neurophysiological perspectives related to postural control in both standing and walking based on movement efficiency. Evidence related to the biomechanical and neurophysiological mechanisms is explored as well as the role of proprioceptive input on postural and movement control.

EEG during pedaling: evidence for cortical control of locomotor tasks

OBJECTIVE

This study characterized the brain electrical activity during pedaling, a locomotor-like task, in humans. We postulated that phasic brain activity would be associated with active pedaling, consistent with a cortical role in locomotor tasks.

METHODS

Sixty four channels of electroencephalogram (EEG) and 10 channels of electromyogram (EMG) data were recorded from 10 neurologically-intact volunteers while they performed active and passive (no effort) pedaling on a custom-designed stationary bicycle. Ensemble averaged waveforms, 2 dimensional topographic maps and amplitude of the β (13-35 Hz) frequency band were analyzed and compared between active and passive trials.

RESULTS

The peak-to-peak amplitude (peak positive-peak negative) of the EEG waveform recorded at the Cz electrode was higher in the passive than the active trials (p < 0.01). β-band oscillations in electrodes overlying the leg representation area of the cortex were significantly desynchronized during active compared to the passive pedaling (p < 0.01). A significant negative correlation was observed between the average EEG waveform for active trials and the composite EMG (summated EMG from both limbs for each muscle) of the rectus femoris (r = -0.77, p < 0.01) the medial hamstrings (r = -0.85, p < 0.01) and the tibialis anterior (r = -0.70, p < 0.01) muscles.

CONCLUSIONS

These results demonstrated that substantial sensorimotor processing occurs in the brain during pedaling in humans. Further, cortical activity seemed to be greatest during recruitment of the muscles critical for transitioning the legs from flexion to extension and vice versa.

SIGNIFICANCE

This is the first study demonstrating the feasibility of EEG recording during pedaling, and owing to similarities between pedaling and bipedal walking, may provide valuable insight into brain activity during locomotion in humans.

Precise temporal association between cortical potentials evoked by motor imagination and afference induces cortical plasticity

In monkeys, the repeated activation of somatosensory afferents projecting onto the motor cortex (M1) has a pivotal role in motor skill learning. Here we investigate if sensory feedback that is artificially generated at specific times during imagination of a dorsiflexion task leads to reorganization of the human M1. The common peroneal nerve was stimulated to generate an afferent volley timed to arrive during specific phases of the cortical potential generated when a movement was imagined (50 repetitions). The change in the output of M1 was quantified by applying single transcranial magnetic stimuli to the area of M1 controlling the tibialis anterior muscle. The results demonstrated that the concomitance between the cognitive process of movement (motor imagination) and the ascending volley due to the peripheral nerve stimulation can lead to significant increases in cortical excitability. These increases were critically dependent on the timing between the peripherally generated afferent volley and the cortical potential generated during the imagined movement. Only if the afferent volley arrived during the peak negative deflection of the potential, were significant alterations in motor cortical output attained. These results demonstrate that an artificially generated signal (the peripheral afferent volley) can interact with a physiologically generated signal in humans leading to plastic changes within the M1, the final output stage for movement generation within the human brain. The results presented may have implications in systems for artificially inducing cortical plasticity in patients with motor impairments (neuromodulation).

Effects of reduced plantar cutaneous afferent feedback on locomotor adjustments in dynamic stability during perturbed walking

This study examined the effects of reduced plantar cutaneous afferent feedback on predictive and feedback adaptive locomotor adjustments in dynamic stability during perturbed walking. Twenty-two matched participants divided between an experimental-group and a control-group performed a gait protocol, which included surface alterations to one covered exchangeable gangway-element (hard/soft). In the experimental-group, cutaneous sensation in both foot soles was reduced to the level of sensory peripheral neuropathy by means of intradermal injections of an anaesthetic solution, without affecting foot proprioception or muscles. The gait protocol consisted of baseline trials on a uniformly hard surface and an adaptation phase consisting of nineteen trials incorporating a soft gangway-element, interspersed with three trials using the hard surface-element (2nd, 8th and 19th). Dynamic stability was assessed by quantifying the margin of stability (MS), which was calculated as the difference between the base of support (BS) and the extrapolated centre of mass (CM). The horizontal velocity of the CM and its vertical projection in the anterior-posterior direction and the eigenfrequency of an inverted pendulum determine the extrapolated-CM. Both groups increased the BS at the recovery step in response to the first unexpected perturbation. These feedback corrections were used more extensively in the experimental-group, which led to a higher MS compared to the control-group, i.e. a more stable body-position. In the adaptation phase the MS returned to baseline similarly in both groups. In the trial on the hard surface directly after the first perturbation, both groups increased the MS at touchdown of the disturbed leg compared to baseline trials, indicating rapid predictive adjustments irrespective of plantar cutaneous input. Our findings demonstrate that the locomotor adaptational potential does not decrease due to the loss of plantar sensation.

Volitional muscle strength in the legs predicts changes in walking speed following locomotor training in people with chronic spinal cord injury

BACKGROUND

It is unclear which individuals with incomplete spinal cord injury best respond to body-weight-supported treadmill training.

OBJECTIVE

The purpose of this study was to determine the factors that predict whether a person with motor incomplete spinal cord injury will respond to body-weight-supported treadmill training.

DESIGN

This was a prognostic study with a one-group pretest-posttest design.

METHODS

Demographic, clinical, and electrophysiological measurements taken prior to training were examined to determine which measures best predicted improvements in walking speed in 19 individuals with chronic (>7 months postinjury), motor-incomplete spinal cord injuries (ASIA Impairment Scale categories C and D, levels C1-L1).

RESULTS

Two initial measures correlated significantly with improvements in walking speed: (1) the ability to volitionally contract a muscle, as measured by the lower-extremity manual muscle test (LE MMT) (r=.72), and (2) the peak locomotor electromyographic (EMG) amplitude in the legs (r=.56). None of the demographics (time since injury, age, body mass index) were significantly related to improvements in walking speed, nor was the clinical measure of balance (Berg Balance Scale). Further analysis of LE MMT scores showed 4 key muscle groups were significantly related to improvements in walking speed: knee extensors, knee flexors, ankle plantar flexors, and hip abductors (r=.82). Prediction using the summed MMT scores from those muscles and peak EMG amplitude in a multivariable regression indicated that peak locomotor EMG amplitude did not add significantly to the prediction provided by the LE MMT alone. Change in total LE MMT scores from the beginning to the end of training was not correlated with a change in walking speed over the same period.

LIMITATIONS

The sample size was limited, so the results should be considered exploratory.

CONCLUSIONS

The results suggest that preserved muscle strength in the legs after incomplete spinal cord injury, as measured by MMT, allows for improvements in walking speed induced by locomotor training.

Interlimb coordination evoked by unilateral mechanical perturbation during body-weight supported gait

During locomotion, motor strategies can rapidly compensate for any obstruction or perturbation that could interfere with forward progression. Here we studied the contribution of interlimb pathways for evoking muscle activation patterns in the case where body weight is externally supported and vestibular feedback is limited. The experiments were conducted using a novel device intended for gait therapy: the MIT-Skywalker. The subject's body weight was supported by an underneath saddle-like seat, and a chest harness was used to provide stabilization of the torso. Eight neurologically healthy individuals were asked to walk on the MIT-Skywalker, while one side of its split belt treadmill was unexpectedly dropped either before heel-strike or during mid-stance. Leg kinematics will be reported. We found that unilateral perturbations evoked responses at the contralateral limb, which were observed in both kinematic and neuromuscular level. The latency of most responses exceeded 100 msec, which suggests a supraspinal (i.e. not local) pathway.

On the interlimb coordination and synchronization during gait

Human locomotion is based on the finely tuned coordination of the two legs. For this research, we studied the contribution of interlimb pathways for coordinating and synchronizing the legs' motion in the case where body weight is externally supported and vestibular feedback is limited. The experiments were conducted using a novel device intended for gait therapy: the MIT-Skywalker. The subject's body weight was supported by an underneath saddle-like seat, and a chest harness was used to provide stabilization of the torso. Two neurologically healthy individuals were asked to walk on the MIT-Skywalker, while one side of its split belt treadmill was unexpectedly dropped during the perturbed leg stance phase. Leg kinematics are reported as well as the effect of the timing of perturbation on the unperturbed leg. Presented here are the phase-response curves (PRCs) for both legs. We found that unilateral perturbations evoked responses at the contralateral limb, while the timing of the activation played a significant role in those responses.

Functioning of peripheral Ia pathways in infants with typical development: responses in antagonist muscle pairs

In muscle responses of proprioceptive origin, including the stretch/tendon reflex (T-reflex), the corresponding reciprocal excitation and irradiation to distant muscles have been described from newborn infants to older adults. However, the functioning of other responses mediated primarily by Ia-afferents has not been investigated in infants. Understanding the typical development of these multiple pathways is critical to determining potential problems in their development in populations affected by neurological disease, such as spina bifida or cerebral palsy. Hence, the goal of the present study was to quantify the excitability of Ia-mediated responses in lower limb muscles of infants with typical development. These responses were elicited by mechanical stimulation applied to the distal tendons of the gastrocnemius-soleus (GS), tibialis anterior (TA) and quadriceps (QAD) muscles of both legs in twelve 2- to 10-month-old infants and recorded simultaneously in antagonist muscle pairs by surface EMG. Tendon taps alone elicited responses in either, both or neither muscle. The homonymous response (T-reflex) was less frequent in the TA than the GS or QAD muscle. An 80 Hz vibration superimposed on tendon taps induced primarily an inhibition of monosynaptic responses; however, facilitation also occurred in either muscle of the recorded pair. These responses were not influenced significantly by age or gender. Vibration alone produced a tonic reflex response in the vibrated muscle (TVR) and/or the antagonist muscle (AVR). However, for the TA muscle the TVR was more frequently elicited in older than younger infants. High variability was common to all responses. Overall, the random distribution and inconsistency of muscle responses suggests that the gain of Ia-mediated feedback is unstable. We propose that during infancy the central nervous system needs to learn to set stable feedback gain, or destination of proprioceptive assistance, based on their use during functional movements. This will tailor the neuromuscular connectivity to support adaptive motor behaviors.

Ultrasonography as a tool to study afferent feedback from the muscle-tendon complex during human walking

In humans, one of the most common tasks in everyday life is walking, and sensory afferent feedback from peripheral receptors, particularly the muscle spindles and Golgi tendon organs (GTO), makes an important contribution to the motor control of this task. One factor that can complicate the ability of these receptors to act as length, velocity and force transducers is the complex pattern of interaction between muscle and tendinous tissues, as tendon length is often considerably greater than muscle fibre length in the human lower limb. In essence, changes in muscle-tendon mechanics can influence the firing behaviour of afferent receptors, which may in turn affect the motor control. In this review we first summarise research that has incorporated the use of ultrasound-based techniques to study muscle-tendon interaction, predominantly during walking. We then review recent research that has combined this method with an examination of muscle activation to give a broader insight to neuromuscular interaction during walking. Despite the advances in understanding that these techniques have brought, there is clearly still a need for more direct methods to study both neural and mechanical parameters during human walking in order to unravel the vast complexity of this seemingly simple task.

Short-term locomotor adaptation to a robotic ankle exoskeleton does not alter soleus Hoffmann reflex amplitude

BACKGROUND

To improve design of robotic lower limb exoskeletons for gait rehabilitation, it is critical to identify neural mechanisms that govern locomotor adaptation to robotic assistance. Previously, we demonstrated soleus muscle recruitment decreased by approximately 35% when walking with a pneumatically-powered ankle exoskeleton providing plantar flexor torque under soleus proportional myoelectric control. Since a substantial portion of soleus activation during walking results from the stretch reflex, increased reflex inhibition is one potential mechanism for reducing soleus recruitment when walking with exoskeleton assistance. This is clinically relevant because many neurologically impaired populations have hyperactive stretch reflexes and training to reduce the reflexes could lead to substantial improvements in their motor ability. The purpose of this study was to quantify soleus Hoffmann (H-) reflex responses during powered versus unpowered walking.

METHODS

We tested soleus H-reflex responses in neurologically intact subjects (n=8) that had trained walking with the soleus controlled robotic ankle exoskeleton. Soleus H-reflex was tested at the mid and late stance while subjects walked with the exoskeleton on the treadmill at 1.25 m/s, first without power (first unpowered), then with power (powered), and finally without power again (second unpowered). We also collected joint kinematics and electromyography.

RESULTS

When the robotic plantar flexor torque was provided, subjects walked with lower soleus electromyographic (EMG) activation (27-48%) and had concomitant reductions in H-reflex amplitude (12-24%) compared to the first unpowered condition. The H-reflex amplitude in proportion to the background soleus EMG during powered walking was not significantly different from the two unpowered conditions.

CONCLUSION

These findings suggest that the nervous system does not inhibit the soleus H-reflex in response to short-term adaption to exoskeleton assistance. Future studies should determine if the findings also apply to long-term adaption to the exoskeleton.

Afferent Contribution to Locomotor Muscle Activity During Unconstrained Overground Human Walking: An Analysis of Triceps Surae Muscle Fascicles

Plantar flexor series elasticity can be used to dissociate muscle–fascicle and muscle–tendon behavior and thus afferent feedback during human walking. We used electromyography (EMG) and high-speed ultrasonography concomitantly to monitor muscle activity and muscle fascicle behavior in 19 healthy volunteers as they walked across a platform. On random trials, the platform was dropped (8 cm, 0.9 g acceleration) or held at a small inclination (up to ±3° in the parasagittal plane) with respect to level ground. Dropping the platform in the mid and late phases of stance produced a depression in the soleus muscle activity with an onset latency of about 50 ms. The reduction in ground reaction force also unloaded the plantar flexor muscles. The soleus muscle fascicles shortened with a minimum delay of 14 ms. Small variations in platform inclination produced significant changes in triceps surae muscle activity; EMG increased when stepping on an inclined surface and decreased when stepping on a declined surface. This sensory modulation of the locomotor output was concomitant with changes in triceps surae muscle fascicle and gastrocnemius tendon length. Assuming that afferent activity correlates to these mechanical changes, our results indicate that within-step sensory feedback from the plantar flexor muscles automatically adjusts muscle activity to compensate for small ground irregularities. The delayed onset of muscle fascicle movement after dropping the platform indicates that at least the initial part of the soleus depression is more likely mediated by a decrease in force feedback than length-sensitive feedback, indicating that force feedback contributes to the locomotor activity in human walking.

Joint kinetic response during unexpectedly reduced plantar flexor torque provided by a robotic ankle exoskeleton during walking

During human walking, plantar flexor activation in late stance helps to generate a stable and economical gait pattern. Because plantar flexor activation is highly mediated by proprioceptive feedback, the nervous system must modulate reflex pathways to meet the mechanical requirements of gait. The purpose of this study was to quantify ankle joint mechanical output of the plantar flexor stretch reflex response during a novel unexpected gait perturbation. We used a robotic ankle exoskeleton to mechanically amplify the ankle torque output resulting from soleus muscle activation. We recorded lower-body kinematics, ground reaction forces, and electromyography during steady-state walking and during randomly perturbed steps when the exoskeleton assistance was unexpectedly turned off. We also measured soleus Hoffmann- (H-) reflexes at late stance during the two conditions. Subjects reacted to the unexpectedly decreased exoskeleton assistance by greatly increasing soleus muscle activity about 60ms after ankle angle deviated from the control condition (p<0.001). There were large differences in ankle kinematic and electromyography patterns for the perturbed and control steps, but the total ankle moment was almost identical for the two conditions (p=0.13). The ratio of soleus H-reflex amplitude to background electromyography was not significantly different between the two conditions (p=0.4). This is the first study to show that the nervous system chooses reflex responses during human walking such that invariant ankle joint moment patterns are maintained during perturbations. Our findings are particularly useful for the development of neuromusculoskeletal computer simulations of human walking that need to adjust reflex gains appropriately for biomechanical analyses.

Similarity of joint kinematics and muscle demands between elliptical training and walking: implications for practice

BACKGROUND

People with physical disabilities often face barriers to regaining walking ability and fitness after discharge from rehabilitation. Physical therapists are uniquely positioned to teach clients the knowledge and skills needed to exercise on functionally relevant equipment available in the community, such as elliptical trainers. However, therapeutic use is hindered by a lack of empirical information.

OBJECTIVE

The purpose of this study was to examine joint kinematics and muscle activation recorded during walking and elliptical training to provide evidence-based data to guide clinical decision making.

DESIGN

This was a prospective, controlled laboratory study using a repeated-measures design.

METHODS

Twenty adults free from impairments that might hinder gait participated. After familiarization procedures, subjects walked and trained on 4 elliptical devices while kinematic, electromyographic (EMG), and stride characteristic data were recorded.

RESULTS

Movement similarities between elliptical training and walking were supported by the documentation of relatively high coefficients of multiple correlation for the hip (.85-.89), thigh (.92-.94), knee (.87-.89) and, to a lesser extent, the ankle (.57-.71). Significantly greater flexion was documented at the trunk, pelvis, hip, and knee during elliptical training than during walking. One of the elliptical trainers most closely simulated sagittal-plane walking kinematics, as determined from an assessment of key variables. During elliptical training, gluteus maximus and vastus lateralis muscle activation were increased; medial hamstring, gastrocnemius, soleus, and tibialis anterior muscle activation were decreased; and gluteus medius and lateral hamstring muscle activation were relatively unchanged compared with muscle activation of those muscles in walking. On the basis of EMG findings, no elliptical trainer clearly emerged as the best for simulating gait.

LIMITATIONS

To date, only 4 elliptical trainers have been studied, and the contributions of the upper extremities to movement have not been quantified.

CONCLUSIONS

Although one of the elliptical trainers best simulated sagittal-plane walking kinematics, EMG analysis failed to identify one clearly superior device. This research provides evidence-based data to help guide clinical decision making related to the use of elliptical trainers across the health care continuum and into the community.

Compression-caused peroneal neuropathy: commentary from a biopsychologist

Compression is the most common cause of damage to the fibular head, the site of most peroneal nerve injuries which cause foot drop. Compression injuries can be caused by prolonged immobility and habitual leg-crossing. A review of the literature does not reveal the existence of a nationwide study that investigates the prevalence of compression-caused foot drop, nor does the literature contain encouragement to arrange medical practices to prevent its occurrence (e.g., soft substrates for sitting, frequent reminders for the patient to uncross the legs). Treatments for foot drop do not appear to be strongly scientifically based and they do not incorporate the use of sensory integration, specifically use of the visual sense, during rehabilitation. Finally, compression-caused foot drop may be preventable, a conclusion that could ultimately have important implications in the context of Medicare and Medicaid reimbursement.

Reorganization and preservation of motor control of the brain in spinal cord injury: a systematic review

Reorganization of brain function in people with CNS damage has been identified as one of the fundamental mechanisms involved in the recovery of sensorimotor function. Spinal cord injury (SCI) brain mapping studies during motor tasks aim for assessing the reorganization and preservation of brain networks involved in motor control. Revealing the activation of cortical and subcortical brain areas in people with SCI can indicate principal patterns of brain reorganization when the neurotrauma is distal to the brain. This review assessed brain activation after SCI in terms of intensity, volume, and somatotopic localization, as well as preservation of activation during attempted and/or imagined movements. Twenty-five studies meeting the inclusion criteria could be identified in Medline (1980 to January 2008). Relevant characteristics of studies (level of lesion, time after injury, motor task) and mapping techniques varied widely. Changes in brain activation were found in both cortical and subcortical areas of individuals with SCI. In addition, several studies described a shift in the region of brain activation. These patterns appeared to be dynamic and influenced by the level, completeness, and time after injury, as well as extent of clinical recovery. In addition, several aspects of reorganization of brain function following SCI resembled those reported in stroke. This review demonstrates that brain networks involved in different demands of motor control remain responsive even in chronic paralysis. These findings imply that therapeutic strategies aimed at restoring spinal cord function, even in people with chronic SCI, can build on preserved competent brain control.

Fast responses to stepping on an unexpected surface height depend on intact large-diameter nerve fibers: a study on Charcot-Marie-Tooth type 1A disease

The contribution of reflexes from the large myelinated afferents in the control of normal and perturbed gait in humans is a highly debated issue. One way to investigate this topic is by studying normal and perturbed gait in patients lacking large myelinated fibers in the distal limb (Charcot-Marie-Tooth [CMT] type 1A disease). Such patients should have delayed and decreased reflexes if the latter depend on these large myelinated fibers. To elicit the reflexes, both patients and controls had to step on a platform that was either at the same level or lowered by 5 cm. In control subjects, landing on a level surface induced short-latency responses in the biceps femoris and tibialis anterior muscles, whereas such responses were largely absent in the patients. Similarly, stepping down unexpectedly induced a very fast muscle synergy, leading to a brake of the forward propulsion in the controls, which was significantly reduced and delayed (on average 32 ms) in the patients. The observed changes correlated with both sensory and motor deficits. Nevertheless, it is concluded that the results are primarily related to the sensory deficits, since the delayed or absent responses appeared in both upper and lower leg muscles, whereas only the latter showed motor deficits. The data are taken as evidence that large-diameter afferents from the distal leg are essential for fast reflex activations induced by stepping on a level or lowered surface unexpectedly.

Mechanical and neural stretch responses of the human soleus muscle at different walking speeds

During human walking, a sudden trip may elicit a Ia afferent fibre mediated short latency stretch reflex. The aim of this study was to investigate soleus (SOL) muscle mechanical behaviour in response to dorsiflexion perturbations, and to relate this behaviour to short latency stretch reflex responses. Twelve healthy subjects walked on a treadmill with the left leg attached to an actuator capable of rapidly dorsiflexing the ankle joint. Ultrasound was used to measure fascicle lengths in SOL during walking, and surface electromyography (EMG) was used to record muscle activation. Dorsiflexion perturbations of 6 deg were applied during mid-stance at walking speeds of 3, 4 and 5 km h(-1). At each walking speed, perturbations were delivered at three different velocities (slow: approximately 170 deg s(-1), mid: approximately 230 deg s(-1), fast: approximately 280 deg s(-1)). At 5 km h(-1), fascicle stretch amplitude was 34-40% smaller and fascicle stretch velocity 22-28% slower than at 3 km h(-1) in response to a constant amplitude perturbation, whilst stretch reflex amplitudes were unchanged. Changes in fascicle stretch parameters can be attributed to an increase in muscle stiffness at faster walking speeds. As stretch velocity is a potent stimulus to muscle spindles, a decrease in the velocity of fascicle stretch at faster walking speeds would be expected to decrease spindle afferent feedback and thus stretch reflex amplitudes, which did not occur. It is therefore postulated that other mechanisms, such as altered fusimotor drive, reduced pre-synaptic inhibition and/or increased descending excitatory input, acted to maintain motoneurone output as walking speed increased, preventing a decrease in short latency reflex amplitudes.

Timing-specific transfer of adapted muscle activity after walking in an elastic force field

Human locomotion results from interactions between feedforward (central commands from voluntary and automatic drive) and feedback (peripheral commands from sensory inputs) mechanisms. Recent studies have shown that locomotion can be adapted when an external force is applied to the lower limb. To better understand the neural control of this adaptation, the present study investigated gait modifications resulting from exposure to a position-dependent force field. Ten subjects walked on a treadmill before, during, and after exposure to a force field generated by elastic tubing that pulled the foot forward and up during swing. Lower limb kinematics and electromyographic (EMG) activity were recorded during each walking period. During force field exposure, peak foot velocity was initially increased by 38%. As subjects adapted, peak foot velocity gradually returned to baseline in <or=125 strides. In the adapted state, hamstring EMG activity started earlier (16% before toe off) and remained elevated throughout swing. After force field exposure, foot velocity was initially reduced by 22% and returned to baseline in 9-51 strides. Aftereffects in hamstring EMGs consisted of increased activity around toe off. Contrary to the adapted state, this increase was not maintained during the rest of swing. Together, these results suggest that while the neural control of human locomotion can adapt to force field exposure, the mechanisms underlying this adaptation may vary according to the timing in the gait cycle. Adapted hamstring EMG activity may rely more on feedforward mechanisms around toe off and more on feedback mechanisms during the rest of swing.

Sudden Drop in Ground Support Produces Force-Related Unload Response in Human Overground Walking

Humans maneuver easily over uneven terrain. To maintain smooth and efficient gait the motor system needs to adapt the locomotor output to the walking environment. In the present study we investigate the role of sensory feedback in adjusting the soleus muscle activity during overground walking in 19 healthy volunteers. Subjects walked unrestrained over a hydraulically actuated platform. On random trials the platform was accelerated downward at 0.8 g, unloading the plantar flexor muscles in midstance or late stance. The drop of the platform resulted in a significant depression of the soleus muscle activity of −17.9% (SD 2) and −21.4% (SD 2), with an onset latency of 49 ms (SD 1) and 45 ms (SD 1) in midstance and late stance, respectively. Input to the vestibular apparatus (i.e., the head acceleration) occurred at a latency 10.0 ms (SD 2.4) following the drop and ankle dorsiflexion velocity was decreased starting 22 ms (SD 15) after the drop. To investigate the role of length- and velocity-sensitive afferents on the depression in soleus muscle activity, the ankle rotation was arrested by using an ankle foot orthotic as the platform was dropped. Preventing the ankle movement did not significantly change the soleus depression in late stance [−18.2% (SD 15)], whereas the depression in midstance was removed [+4.9% (SD 13)]. It is concluded that force feedback from ankle extensors increases the locomotor output through positive feedback in late stance. In midstance the effect of force feedback was not observed, suggesting that spindle afferents may have a more significant effect on the output during this phase of the step cycle.

Asymmetric operation of the locomotor central pattern generator in the neonatal mouse spinal cord

The rhythmic voltage oscillations in motor neurons (MNs) during locomotor movements reflect the operation of the pre-MN central pattern generator (CPG) network. Recordings from MNs can thus be used as a method to deduct the organization of CPGs. Here, we use continuous conductance measurements and decomposition methods to quantitatively assess the weighting and phase tuning of synaptic inputs to different flexor and extensor MNs during locomotor-like activity in the isolated neonatal mice lumbar spinal cord preparation. Whole cell recordings were obtained from 22 flexor and 18 extensor MNs in rostral and caudal lumbar segments. In all flexor and the large majority of extensor MNs the extracted excitatory and inhibitory synaptic conductances alternate but with a predominance of inhibitory conductances, most pronounced in extensors. These conductance changes are consistent with a "push-pull" operation of locomotor CPG. The extracted excitatory and inhibitory synaptic conductances varied between 2 and 56% of the mean total conductance. Analysis of the phase tuning of the extracted synaptic conductances in flexor and extensor MNs in the rostral lumbar cord showed that the flexor-phase-related synaptic conductance changes have sharper locomotor-phase tuning than the extensor-phase-related conductances, suggesting a modular organization of premotor CPG networks consisting of reciprocally coupled, but differently composed, flexor and extensor CPG networks. There was a clear difference between phase tuning in rostral and caudal MNs, suggesting a distinct operation of CPG networks in different lumbar segments. The highly asymmetric features were preserved throughout all ranges of locomotor frequencies investigated and with different combinations of locomotor-inducing drugs. The asymmetric nature of CPG operation and phase tuning of the conductance profiles provide important clues to the organization of the rodent locomotor CPG and are compatible with a multilayered and distributed structure of the network.

Differential effects of plantar desensitization on locomotion dynamics

Reduced plantar sensation secondary to chronic diffuse polyneuropathy (PN) is believed to reduce locomotor stability, especially when walking at non-preferred speeds. However, the contribution of plantar sensation to the maintenance of locomotor stability is not entirely clear. The purpose of this study was to examine the effects of acute loss of plantar sensation on the stability-related kinematic properties of walking at different speeds. Lower-extremity joint kinematics were acquired as healthy young adults walked on a treadmill at their preferred walking speed (PWS) and three predetermined speeds (0.8, 1.0, and 1.2m/s) under both normal and desensitized conditions. Desensitization of the foot soles was induced by ice-exposure, and plantar pressure sensation was assessed by a 5.07 monofilament. The average magnitude of stride duration variability (SDvar) and lower-extremity joint angle variability (JTvar), as well as short- and long-term "finite-time" Lyapunov exponents (lambda(ST)(*), lambda(LT)(*)) associated with lower-extremity joint angles were computed. Ice-induced plantar desensitization led to increased lambda(ST)(*) ( approximately 40%) and lambda(LT)(*) (approximately 8%) values but did not affect SDvar or JTvar. Higher treadmill speed led to greater lambda(ST)(*) and lambda(LT)(*) values, but the speed effects were not influenced by plantar desensitization.While acute loss of plantar sensation does not appear to influence the magnitude of spatial or temporal variability, it did attenuate the state-space trajectory divergence caused by stride-to-stride variability (i.e., lambda(ST)( *) and lambda(LT)(*)). However, as opposed to walking at PWS, otherwise healthy locomotor systems do not appear to place increased reliance on plantar sensation when walking at non-preferred treadmill speeds.

Immobilization induces changes in presynaptic control of group Ia afferents in healthy humans

Neural plasticity occurs throughout adult life in response to maturation, use and disuse. Recent studies have documented that H-reflex amplitudes increase following a period of immobilization. To elucidate the mechanisms contributing to the increase in H-reflex size following immobilization we immobilized the left foot and ankle joint for 2 weeks in 12 able-bodied subjects. Disynaptic reciprocal inhibition of soleus (SOL) motoneurons and presynaptic control of SOL group Ia afferents was measured before and after the immobilization as well as following 2 weeks of recovery. Following immobilization, maximal voluntary plantar- and dorsiflexion torque (MVC) was significantly reduced and the maximal SOL H-reflex amplitude increased with no changes in the maximal compound motor response (M(max)). Decreased presynaptic inhibition of the Ia afferents probably contributed to the increase of the H-reflex size, since we observed a significant decrease in the long-latency depression of the SOL H-reflex evoked by peroneal nerve stimulation (D2 inhibition) and an increase in the size of the monosynaptic Ia facilitation of the SOL H-reflex evoked by femoral nerve stimulation. These two measures provide independent evidence of changes in presynaptic inhibition of SOL Ia afferents and taken together suggest that GABAergic presynaptic inhibition of the SOL Ia afferents is decreased following 2 weeks of immobilization. The depression of the SOL H-reflex when evoked at intervals shorter than 10 s (homosynaptic post-activation depression) also decreased following immobilization, suggesting that the activity-dependent regulation of transmitter release from the afferents was also affected by immobilization. We observed no significant changes in disynaptic reciprocal Ia inhibition. Two weeks after cast removal measurements returned to pre-immobilization levels. Together, these observations suggest that disuse causes plastic changes in spinal interneuronal circuitries responsible for presynaptic control of sensory input to the spinal cord. This may be of significance for the motor disabilities seen following immobilization as well as the development of spasticity following central motor lesions.

Modelling gait processes as a combination of sensory-motor and cognitive controls in an attempt to describe accidents on the level in occupational situations

In occupational situations, accidents referred to as accidents on the level (AoLs) occur most of the time when locomotion control fails. This control is determined by the interactions between the operator and the environment, the task and the used tools. Hence, AoLs prevention requires developing ways to optimise these interactions. More fundamentally, AoLs prevention requires understanding locomotion control in situations where this control is at sake, that is in situations involving one or more AoLs factors. The purpose of this article is to propose a comprehensive model for the control of locomotion in occupational environments. This model featuring the operator, the task and the working space should be an appropriate tool to understand AoLs in the scope of their prevention. Firstly, we describe what occupational AoLs are. In a second part, we present a review of the theoretical and experimental knowledge related to the locomotion system through the various means developed by the Central Nervous System to cope with perturbations of the environment and/or particular constraints from the task. Finally, we propose a simplified systemic model presenting the various levels of control (sensory-motor to cognitive levels) describing locomotion in occupational situations, and we suggest experiments likely to produce the appropriate data to construct the final comprehensive model.

Strategy adoption and locomotor adjustment in obstacle clearance of newly walking toddlers with Down syndrome after different treadmill interventions

This study investigated how newly walking toddlers with Down syndrome (DS), after different treadmill interventions, adopted clearance strategies and modified anticipatory locomotor adjustment patterns to negotiate an obstacle in their travel path. Thirty infants with DS (about 10 months of age) were recruited and randomly assigned to either a lower-intensity, generalized (LG) treadmill training group, or a higher-intensity, individualized (HI) treadmill training group. Thirteen in each group completed a one-year-gait follow-up after the treadmill intervention. Initially, both groups chose to either crawl or walk over an obstacle. However, walking over the obstacle became their preferred clearance strategy over the course of the gait follow-up even though the height of the obstacle increased from visit to visit. The HI group used the strategy of walking over the obstacle at a considerably higher percentage than the LG group within 6 months after the training. When approaching the obstacle, both groups started to show consistent anticipatory locomotor adjustments about 6 months after the training. Both groups decreased velocity, cadence and step length, and increased step width at the last three pre-obstacle steps. It was concluded that the retention of the HI training effects led the HI group to predominantly walk over an obstacle earlier than the LG group within 6 months after treadmill intervention, and the two groups produced similar anticipatory locomotor adjustments in the last three steps before negotiating the obstacle.

Relative contribution of walking velocity and stepping frequency to the neural control of locomotion

Stepping frequency is tightly coupled to walking velocity during natural locomotion. In a recent model, we demonstrated that walking velocity determines stride frequency, governs the active feedback control of the swing and determines the swing phase dynamics that governs foot movement. Here, we questioned whether the swing phase dynamics reflect independent effects of stride frequency and walking velocity. Foot movements were measured with a motion detection system (Optotrak) while subjects walked at 0.6-2.1 m/s on a treadmill. Stepping frequencies of 1.3-2.8 Hz were generated with pacing cues at each walking velocity. In the 'iso-velocity' condition, peak forward toe velocity during the swing phases was related to walking velocity and did not vary with alterations in stride frequency. In the 'iso-frequency' condition, in contrast, stepping frequency altered the relationship between toe acceleration and toe position in the fore-aft direction. The cycle frequency, main sequence (peak velocity vs. amplitude) relationships, and the shape of the phase-plane trajectories of the swing phases also reflected this relationship. The data were modeled by decoupling stepping frequency from walking velocity, while maintaining active feedback control dependent on frequency. The latter predicted both the dominant shape of the phase plane trajectories and the main sequence relationships. Thus, according to the model, walking velocity and stride frequency are independent central variables that control the dynamics of the swing phases and stepping. The ability to decouple stride frequency from walking velocity may help in navigating over uneven terrain or when executing curved trajectories while maintaining a constant velocity.

Training locomotor networks

For a complete adult spinal rat to regain some weight-bearing stepping capability, it appears that a sequence of specific proprioceptive inputs that are similar, but not identical, from step to step must be generated over repetitive step cycles. Furthermore, these cycles must include the activation of specific neural circuits that are intrinsic to the lumbosacral spinal cord segments. For these sensorimotor pathways to be effective in generating stepping, the spinal circuitry must be modulated to an appropriate excitability level. This level of modulation is sustained from supraspinal input in intact, but not spinal, rats. In a series of experiments with complete spinal rats, we have shown that an appropriate level of excitability of the spinal circuitry can be achieved using widely different means. For example, this modulation level can be acquired pharmacologically, via epidural electrical stimulation over specific lumbosacral spinal cord segments, and/or by use-dependent mechanisms such as step or stand training. Evidence as to how each of these treatments can "tune" the spinal circuitry to a "physiological state" that enables it to respond appropriately to proprioceptive input will be presented. We have found that each of these interventions can enable the proprioceptive input to actually control extensive details that define the dynamics of stepping over a range of speeds, loads, and directions. A series of experiments will be described that illustrate sensory control of stepping and standing after a spinal cord injury and the necessity for the "physiological state" of the spinal circuitry to be modulated within a critical window of excitability for this control to be manifested. The present findings have important consequences not only for our understanding of how the motor pattern for stepping is formed, but also for the design of rehabilitation intervention to restore lumbosacral circuit function in humans following a spinal cord injury.

Decreased contribution from afferent feedback to the soleus muscle during walking in patients with spastic stroke

We investigated the contribution of afferent feedback to the soleus (SOL) muscle activity during the stance phase of walking in patients with spastic stroke. A total of 24 patients with hemiparetic spastic stroke and age-matched healthy volunteers participated in the study. A robotic actuator attached to the foot and leg was used to apply 3 types of ankle perturbations during treadmill walking. First, fast dorsiflexion perturbations were applied to elicit stretch reflexes in the SOL muscle. The SOL short-latency stretch reflex was facilitated in the patients (1.4 +/- 0.3) compared with the healthy volunteers (1.0 +/- 0.3, P = .05). Second, fast plantar flexion perturbations were applied during the stance phase to unload the plantar flexor muscles, thus, removing the afferent input from these muscles to the SOL motoneurons. These perturbations produced a distinct decrease in SOL activity that was significantly smaller in the patients (-30 +/- 3%) compared with the control subjects (-43 +/- 4%, P = .03). Third, slow-velocity, small-amplitude ankle trajectory modifications mimicking small deviations in the walking surface were applied to evaluate the afferent-mediated amplitude modulation of the locomotor SOL electromyogram (EMG). In the healthy volunteers these perturbations generated gradual increments and decrements on the SOL EMG; however, in the patients the SOL EMG modulation was significantly depressed (P = .04). Moreover, this depression was related to the spasticity level measured by the Ashworth score. These results indicate that although the stretch reflex response is facilitated during spastic gait, the contribution of afferent feedback to the ongoing locomotor SOL activity is depressed in patients with spastic stroke.

Modulation of multisegmental monosynaptic responses in a variety of leg muscles during walking and running in humans

Motor responses evoked by stimulating the spinal cord percutaneously between the T11 and T12 spinous processes were studied in eight human subjects during walking and running. Stimulation elicited responses bilaterally in the biceps femoris, vastus lateralis, rectus femoris, medial gastrocnemius, soleus, tibialis anterior, extensor digitorum brevis and flexor digitorum brevis. The evoked responses were consistent with activation of Ia afferent fibres through monosynaptic neural circuits since they were inhibited when a prior stimulus was given and during tendon vibration. Furthermore, the soleus motor responses were inhibited during the swing phase of walking as observed for the soleus H-reflex elicited by tibial nerve stimulation. Due to the anatomical site and the fibre composition of the peripheral nerves it is difficult to elicit H-reflex in leg muscles other than the soleus, especially during movement. In turn, the multisegmental monosynaptic responses (MMR) technique provides the opportunity to study modulation of monosynaptic reflexes for multiple muscles simultaneously. Phase-dependent modulation of the MMR amplitude throughout the duration of the gait cycle period was observed in all muscles studied. The MMR amplitude was large when the muscle was activated whereas it was generally reduced, or even suppressed, when the muscle was quiescent. However, during running, there was a systematic anticipatory increase in the amplitude of the MMR at the end of swing in all proximal and distal extensor muscles. The present findings therefore suggest that there is a general control scheme by which the transmission in the monosynaptic neural circuits is modulated in all leg muscles during stepping so as to meet the requirement of the motor task.

Physiological evaluation of gait disturbances post stroke

A large proportion of stroke survivors have to deal with problems in mobility. Proper evaluations must be undertaken to understand the sensorimotor impairments underlying locomotor disorders post stroke, so that evidence-based interventions can be developed. The current electrophysiological, biomechanical, and imagery evaluations that provide insight into locomotor dysfunction post stroke, as well as their advantages and limitations, are reviewed in this paper. In particular, electrophysiological evaluations focus on the contrast of electromyographic patterns and integrity of spinal reflex pathways during perturbed and unperturbed locomotion between persons with stroke and healthy individuals. At a behavioral level, biomechanical evaluations that include temporal distance factors, kinematic and kinetic analyses, as well as the mechanical energy and metabolic cost, are useful when combined with electrophysiological measures for the interpretation of gait disturbances that are related to the control of the central nervous system or secondary to biomechanical constraints. Finally, current methods in imaging and transcranial magnetic stimulation can provide further insight into cortical control of locomotion and the integrity of the corticospinal pathways.

Positive force feedback in human walking

The objective of this study was to determine if load receptors contribute to the afferent-mediated enhancement of ankle extensor muscle activity during the late stance phase of the step cycle. Plantar flexion perturbations were presented in late stance while able-bodied human subjects walked on a treadmill that was declined by 4%, inclined by 4% or held level. The plantar flexion perturbation produced a transient, but marked, presumably spinally mediated decrease in soleus EMG that varied directly with the treadmill inclination. Similarly, the magnitude of the control step soleus EMG and Achilles' tendon force also varied directly with the treadmill inclination. In contrast, the ankle angular displacement and velocity were inversely related to the treadmill inclination. These results suggest that Golgi tendon organ feedback, via the group Ib pathway, is reduced when the muscle-tendon complex is unloaded by a rapid plantar flexion perturbation in late stance phase. The changes in the unload response with treadmill inclination suggest that the late stance phase soleus activity may be enhanced by force feedback.

Afferent-mediated modulation of the soleus muscle activity during the stance phase of human walking

The aim of this study was to investigate the contribution of proprioceptive feedback to the amplitude modulation of the soleus muscle activity during human walking. We have previously shown that slow-velocity, small-amplitude ankle dorsiflexion enhancements and reductions applied during the stance phase of the step cycle generate, respectively, increments and decrements on the ongoing soleus activity. We have also shown that the increments in soleus activity are at least partially mediated by feedback from group Ia fibres. In the present study, we further investigated the afferent-mediated contribution from muscle group II afferents, cutaneous and proprioceptive afferents from the foot, and load-sensitive afferents to the soleus EMG. Slow-velocity, small-amplitude ankle trajectory modifications were combined with the pharmaceutical depression of group II polysynaptic pathways with tizanidine hydrochloride, anaesthetic blocking of sensory information from the foot with injections of lidocaine hydrochloride, and modulation of load feedback by increasing and decreasing the body load. The depression of the group II afferents significantly reduced the soleus response to the ankle trajectory modifications. Blocking sensory feedback from the foot did not have an effect on the soleus muscle activity. Changes in body load affected the ongoing soleus activity level; however, it did not affect the amplitude of the soleus EMG responses to the ankle trajectory modifications. These results suggest that the feedback from group II afferents, and possibly from load-sensitive afferents, contribute to the amplitude modulation of the soleus muscle activity during the stance phase of the step cycle. However, feedback from cutaneous afferents and instrinsic proprioceptive afferents from the foot does not seem to contribute to this muscle activation.

Contribution of feedback and feedforward strategies to locomotor adaptations

The aim of this study was to examine the strategies used by human subjects to adapt their walking pattern to a velocity-dependent resistance applied against hip and knee movements. Subjects first walked on a treadmill with their lower limbs strapped to an exoskeletal robotic gait orthosis with no resistance against leg motions (null condition). Afterward, a velocity-dependent resistance was applied against left hip and knee movements (force condition). Catch trials were interspersed throughout the experiment to track the development of adaptive changes in the walking pattern. After 188 steps in the force condition, subjects continued to step in the null condition for another 100 steps (washout period). Leg muscle activity and joint kinematics were recorded and analyzed. The adaptive modifications in the locomotor pattern suggest the involvement of both feedback and feedforward control strategies. Feedback-driven adaptations were reflected in increases in rectus femoris and tibialis anterior activity during swing, which occurred immediately, only in the presence of resistance, and not during the catch trials. Locomotor adaptations involving feedforward strategies were reflected in enhanced pre-swing activity in the biceps femoris and medial hamstrings muscles, which required experience and persisted in the catch trials. During washout subjects showed a gradual deadaptation of locomotor activity to control levels. In summary, adaptive changes in the walking pattern were driven by both feedback and feedforward adjustments in the walking pattern appropriate for overcoming the effects of resistance.

Spinal reflexes, mechanisms and concepts: From Eccles to Lundberg and beyond

This review focuses on investigations by Sir John Eccles and co-workers in Canberra, AUS in the 1950s, in which they used intracellular recordings to unravel the organization of neuronal networks in the cat spinal cord. Five classical spinal reflexes are emphasized: recurrent inhibition of motoneurons via motor axon collaterals and Renshaw cells, pathways from muscle spindles and Golgi tendon organs, presynaptic inhibition, and the flexor reflex. To set the scene for these major achievements I first provide a brief account of the understanding of the spinal cord in "reflex" and "voluntary" motor activities from the beginning of the 20th century. Next, subsequent work is reviewed on the convergence on spinal interneurons from segmental sensory afferents and descending motor pathways, much of which was performed and inspired by Anders Lundberg's group in Gothenburg, SWE. This work was the keystone for new hypotheses on the role of spinal circuits in normal motor control. Such hypotheses were later tested under more natural conditions; either by recording directly from interneurons in reduced animal preparations or by use of indirect non-invasive techniques in humans performing normal movements. Some of this latter work is also reviewed. These developments would not have been possible without the preceding work on spinal reflexes by Eccles and Lundberg. Finally, there is discussion of how Eccles' work on spinal reflexes remains central (1) as new techniques are introduced on direct recording from interneurons in behaving animals; (2) in experiments on plastic neuronal changes in relation to motor learning and neurorehabilitation; (3) in experiments on transgenic animals uncovering aspects of human pathophysiology; and (4) in evaluating the function of genetically identified classes of neurons in studies on the development of the spinal cord.

Inter-segmental coordination: Motor pattern in humans stepping over an obstacle with mechanical ankle joint friction

This study examined the influence of a mechanical perturbation of the ankle joint on obstacle avoidance pattern. A decoupled control between the distal joint and the combined (hip-knee) proximal joints was observed according to the task requirement. In this context, a greater mechanical friction at the ankle should be compensated at this joint (local compensation) or alternatively, by regulating more combined proximal joints (knee and/or hip). The leading limb inter-segmental coordination was evaluated in both no constraint and constraint conditions in calculating ranges of motion (ROM), moments of force and powers (from heel-off to obstacle) at the ankle, knee and hip joints. Electromyographic activities were also analyzed. With the constraint, the dorsiflexor moment and the tibialis anterior activity remained unchanged while both ROM and power bursts (absorbed and generated) decreased. The hip and knee ROM remain invariant. At heel-off the absorption by hip extensors decreased and the forthcoming generation by knee flexors increased in the constraint condition. To quantify the inter-joint coordination, principal component analysis was used and indicated a high level of inter-joint coupling (synergy) that decreased with the constraint (i.e. less inter-joint coupling). At the ankle joint, the results suggest that the central command was the same in both conditions thus, not be adapted. At both the hip and knee joints, a combined joints modulation occurred to overcome additional friction.

Lack of On-Going Adaptations in the Soleus Muscle Activity During Walking in Patients Affected by Large-Fiber Neuropathy

The aim of this study was to investigate the contribution of feedback from large-diameter sensory fibers to the adaptation of soleus muscle activity after small ankle trajectory modifications during human walking. Small-amplitude and slow-velocity ankle dorsiflexion enhancements and reductions were applied during the stance phase of the gait cycle to mimic the normal variability of the ankle trajectory during walking. Patients with demyelination of large sensory fibers (Charcot-Marie-Tooth type 1A and antibodies to myelin-associated glycoprotein neuropathy) and age-matched controls participated in this study. The patients had absent light-touch sense in the toes and feet and absent quadriceps and Achilles tendon reflexes, indicating functional loss of large sensory fibers. Moreover, their soleus stretch reflex response consisted of a single electromyographic (EMG) burst with delayed onset and longer duration ( P < 0.01) than the short- and medium-latency reflex responses observed in healthy subjects. In healthy subjects, the soleus EMG gradually increased or decreased when the ankle dorsiflexion was, respectively, enhanced or reduced. In the patients, the soleus EMG increased during the dorsiflexion enhancements; however, the velocity sensitivity of this response was decreased compared with the healthy volunteers. When the dorsiflexion was reduced, the soleus EMG was unchanged. These results indicate that the enhancement of the soleus EMG is mainly sensitive to feedback from primary and secondary muscle spindle afferents and that the reduction may be mediated by feedback from the group Ib pathways. This study provides evidence for the role of sensory feedback in the continuous adaptation of the soleus activity during the stance phase of human walking.

Regulation of arm and leg movement during human locomotion

Walking can be a very automated process, and it is likely that central pattern generators (CPGs) play a role in the coordination of the limbs. Recent evidence suggests that both the arms and legs are regulated by CPGs and that sensory feedback also regulates the CPG activity and assists in mediating interlimb coordination. Although the strength of coupling between the legs is stronger than that between the arms, arm and leg movements are similarly regulated by CPG activity and sensory feedback (e.g., reflex control) during locomotion.

Contribution of afferent feedback to the soleus muscle activity during human locomotion

During the stance phase of the human step cycle, the ankle undergoes a natural dorsiflexion that stretches the soleus muscle. The afferent feedback resulting from this stretch enhances the locomotor drive. In this study a robotic actuator was used to slightly enhance or reduce the natural ankle dorsiflexion, in essence, mimicking the small variations in the ankle dorsiflexion movement that take place during the stance phase of the step cycle. The soleus (SOL) and tibialis anterior EMG were analyzed in response to the ankle trajectory modifications. The dorsiflexion enhancements and reductions generated gradual increments and decrements, respectively, in the ongoing SOL EMG. We exercised care to ensure that the imposed ankle movements were too slow to elicit distinct burst-like stretch reflex responses that have been investigated previously. The increased SOL EMG after the dorsiflexion enhancements was reduced when the group Ia afferents were blocked with peripheral ischemia at the thigh, and during high-frequency Achilles tendon vibration. However, neither ischemia nor tendon vibration affected the decrements in the SOL EMG during the dorsiflexion reductions. These findings give evidence of the contribution of afferent feedback to the SOL activity in an ongoing basis during the stance phase. The results suggest that mainly feedback from the group Ia pathways is responsible for the increments in the SOL EMG during the dorsiflexion enhancements. However, the decrements in the SOL activity might be mediated by different afferent mechanisms.

Ankle extensor proprioceptors contribute to the enhancement of the soleus EMG during the stance phase of human walking

A rapid plantar flexion perturbation applied to the ankle during the stance phase of the step cycle during human walking unloads the ankle extensors and produces a marked decline in the soleus EMG. This demonstrates that sensory activity contributes importantly to the enhancement of the ankle extensor muscle activation during human walking. On average, the EMG begins to decline approximately 52 ms after the perturbation. In contrast, a rapid dorsi flex ion perturbation produces a group Ia mediated short-latency stretch reflex burst with an onset latency of approximately 36 ms. The transmission of sensory traffic from the foot and ankle was suppressed in 10 subjects by an anaesthetic nerve block produced with local injections of lidocaine hydrochloride. The anaesthetic block had no effect on the stance phase soleus EMG, the latencies of the EMG responses, or the magnitude of the EMG decline following the plantar flexion perturbation. Therefore, it is more likely that proprioceptive afferents, rather than cutaneous afferents, contribute to the background soleus EMG during the late stance phase of the step cycle. The large difference in onset latencies between the short-latency reflex and unload responses suggests that the largest of the active group Ia afferents might not contribute strongly to the background soleus EMG, although it remains to be determined which of the proprioceptive pathways provide the more important contributions.Key words: afferent feedback, gait, locomotion, stretch reflex.

Variation of magnitude and timing of wrist flexor stretch reflex across the full range of voluntary activation

This paper reports an investigation of the magnitude and timing of the stretch reflex over the full range of activation of flexor carpi radialis. While it is well established that the magnitude of the reflex increases with the level of muscle activation, there have been few studies of reflex magnitude above 50% of maximum voluntary contraction (MVC) and virtually no study of the timing of the response in relation to activation level. Continuous small amplitude (approximately 2 degrees) perturbations were applied to the wrist of 12 normal subjects while they maintained contraction levels between 2.5-95% MVC, monitored via surface electromyography (EMG). Both narrow band (4-5 Hz) and broad band (0-10 Hz) stretch perturbations were employed. The gain (EMG output/stretch input) and phase advance of the reflex varied with the level of muscle activation in a similar manner for both types of stretch, but there were significant differences in the patterns of change due to stretch bandwidth. Consistent with previous studies, the group average reflex gain initially increased with muscle activation level and then saturated. Inspection of individual data, however, revealed that the gain reached a peak at about 60% MVC and then decreased at higher contraction levels, the pattern across the full range of activation being well described by quadratic functions (mean r2=0.82). This quadratic pattern has not been reported previously for the neural reflex response in any muscle but is consistent with the pattern that has been reliably observed in studies of the mechanical reflex response in lower limb muscles. In contrast to the pattern for reflex gain, the phase advance of the reflex (at a stretch frequency of 4.5 Hz) decreased linearly from approximately 130 degrees at the lowest contraction levels to approximately 50 degrees as maximum voluntary contraction was reached (mean r2=0.69). This decrease corresponds to a delay of 49 ms introduced centrally in reflex pathways. All subjects showed clearly defined quadratic functions relating reflex gain and linear functions relating reflex phase to activation level, but there were considerable individual differences in the slopes of these functions which point to systematic differences in synaptic behaviour of the motoneuron pool. Thus, there was wide inter-subject variation in both the contraction level at which the reflex gain reached a peak (31-69% MVC) and the highest target contraction level that could be sustained during reflex measurement (47-95% MVC). A high correlation between these variables (r2=0.78) suggests a linear relation between afferent support of contraction and muscle fatigability. The decline in reflex gain at high levels of muscle activation signals a failure of muscle afferent input and subjects in whom the gain reached a peak and declined early were unable to sustain higher target contraction levels. The results of the study show that both the timing and magnitude of the stretch reflex vary markedly over the full range of voluntary muscle activation. The pattern of variation may account for why the stretch reflex contributes most effectively to muscle mechanics over the lower half of the range of activation, while progressive reductions in both gain and phase advance at higher levels render the reflex mechanically less effective and make tremor more likely.

Human walking along a curved path. II. Gait features and EMG patterns

We recorded basic gait features and associated patterns of leg muscle activity, occurring during continuous body progression when humans walked along a curved trajectory, in order to gain insight into the nervous mechanisms underlying the control of the asymmetric movements of the two legs. The same rhythm was propagated to both legs, in spite of inner and outer strides diverging in length (P < 0.001). There was a phase lag in limb displacement between the inner and outer leg of 7% of the total cycle duration (P = 0.0001). Swing velocity was greater for outer than inner foot (P < 0.001). The duration of the stance phase diminished and increased in the outer and inner leg (P < 0.01), respectively, and was associated with trunk leaning toward the inside of the path. Muscle activity was not dramatically altered during curved walking. The amplitude of soleus burst during stance increased in the outer (P < 0.05) and decreased in the inner leg (P < 0.05), without changes in timing. Tibialis anterior activity increased in both legs during the swing phase (P < 0.05); it was advanced on the outer and delayed on the inner side (P < 0.01; 2% of the cycle). The peroneus longus burst decreased in both legs, but more in the inner than the outer leg, and lasted longer in the inner leg at the onset of swing. Closing the eyes did not affect the gait pattern and muscle activity during turning. The command to walk along a curved path may exploit the basic mechanisms of the spinal locomotor generator, thereby limiting the computational cost of turning.