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

Influence of circumferential ankle pressure of shoe collar on the kinematics, dynamic stability, electromyography, and plantar pressure during normal walking

BACKGROUND

The shoe's collar plays a significant role in supporting the ankle during walking. Since the protective effect of the collar requires the circular embracing of the ankle and shank, a stiffer collar might be involved with increased circumferential ankle pressure (CAP). It is not clear how collar CAP affects walking performance. Therefore, this study was aimed at examining the influence of the collar CAP on the kinematics, dynamic stability, electromyography (EMG), and plantar pressure during normal walking.

METHOD

Sixteen healthy male participants walked on a treadmill while wearing a custom-designed high-collar shoe with 10 (low), 30 (medium), and 60 mmHg (high) CAP conditions, and the joint angles, dynamic stability index, EMG, and plantar pressure were measured.

RESULT

While the low CAP condition did not affect the ankle range of motion (ROM), The high CAP condition restricted both the ankle sagittal and frontal ROM, whereas the medium CAP condition limited only the ankle frontal ROM. The knee and hip ROM did not differ between conditions. The dynamic stability for the high and medium CAP cases was comparable but significantly higher than that of the low CAP condition. The ankle muscle activity and corresponding co-contraction increased with increasing CAP for gastrocnemius medialis (GM), soleus (SOL), peroneus longus (PL), tibialis anterior (TA) muscles in the weight acceptance and push-off phases but not in the single limb support. Knee muscle activity, including vastus lateralis (VL) and semitendinosus (SEMI) was similar between all conditions. A higher relative pressure was observed under the lateral aspect of the heel when walking in the high CAP condition.

CONCLUSION

The results suggest that a high-collar shoe with a high CAP may not be an appropriate choice for walking owing to the injury risk factors and limited walking efficiency. A medium CAP is associated with certain advantages and, thus, a superior choice for high-collar shoe design.

Control of Mammalian Locomotion by Somatosensory Feedback

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

Multiple strategies to correct errors in foot placement and control speed in human walking

Neural feedback plays a key role in maintaining locomotor stability in the face of perturbations. In this study, we systematically identified properties of neural feedback that contribute to stabilizing human walking by examining how the nervous system responds to small kinematic deviations away from the desired gait pattern. We collected data from 20 participants (9 men and 11 women). We simultaneously applied (1) small continuous mechanical perturbations, forces at the ankles that affected foot placement, and (2) small continuous sensory perturbations, movement of a virtual visual scene that produced the illusion of change in walking speed, to compare how neural feedback responds to actual and illusory kinematic deviations. We computed phase-dependent impulse response functions that describe kinematic and muscular responses to small brief perturbations to identify critical phases of the gait cycle when the nervous system modulates muscle activity. In addition to the known foot-placement strategies that counteract kinematic displacement, such as the modulation of the hamstring muscle group during swing, we identified phase-specific muscle modulations that compensated for the perturbations. In particular, our results suggested that an early-stance modulation of anterior leg muscles (i.e., dorsiflexors and quadriceps) is a general control mechanism that serves to control forward body propulsion and compensates for errors in foot placement. Another detected general compensatory strategy was the late-stance modulation of the rectus femoris and gastrocnemius muscles, which controls walking speed in the next cycle.

Systematic review of motor control and somatosensation assessment tests for the ankle

Background/Aim

Ankle sprains are frequent musculoskeletal injuries that can lead to sensorimotor deficits provoking long-term instability at the ankle joint. A broad variety of clinical tests currently exist to assess sensorimotor processing, and are commonly clinically referred to as proprioceptive tests. However, there is a discrepancy in the use of the term proprioception when looking at the main outcome of these tests. As identifying specific deficits is important for motor recovery, it is critical for clinicians to select the most appropriate tests.

Methods

A systematic review of four databases was performed to provide an up-to-date review of the psychometric properties of available tests referred to as proprioceptive tests. Seventy-nine articles on eight ankle proprioceptive tests were included and critically appraised. Data on validity, reliability and responsiveness were extracted from the included articles and synthesised. The tests reviewed were then divided into two categories based on their main outcome: motor control or somatosensation.

Results

Strong evidence showed that the Star Excursion Balance Test, a motor control test, is capable of differentiating between stable and unstable ankles. Moderate evidence suggests that somatosensation tests, such as Joint Position Sense, are also valid and reliable, but their responsiveness has yet to be evaluated.

Conclusions

Together, these findings indicate that the Star Excursion Balance Test can be used in the clinic to assess motor control based on its excellent psychometric properties. However, as ankle stability control involves complex sensorimotor interactions, care has to be taken regarding the use of this test as a specific tool for proprioception assessment.

Changes in human walking dynamics induced by uneven terrain are reduced with ongoing exposure, but a higher variability persists

During walking, uneven terrain alters the action of the ground reaction force from stride to stride. The extent to which such environmental inconsistencies are withstood may be revealed by the regulation of whole-body angular momentum (L) during walking. L quantifies the balance of momenta of the body segments (thigh, trunk, etc.) about their combined center of mass, and remains close to zero during level walking. A failure to constrain L has been linked to falls. The aim of this study was to explore the ability of young adults to orchestrate their movement on uneven terrain, illustrated by the range of L (L_R) and its variability (vL_R). In eleven male adults, we observed significant increases in sagittal plane L_R, and vL_R in all three planes of motion during walking on an uneven in comparison to a flat surface. No reductions in these measures were observed within a 12-minute familiarisation period, suggesting that unimpaired adults either are unable to, or do not need to eliminate the effects of uneven terrain. Transverse plane L_R, in contrast, was lower on immediate exposure, and then increased, pointing to the development of a less restrictive movement pattern, and would support the latter hypothesis.

A spasticity model based on feedback from muscle force explains muscle activity during passive stretches and gait in children with cerebral palsy

Muscle spasticity is characterized by exaggerated stretch reflexes and affects about 85% of the children with cerebral palsy. However, the mechanisms underlying spasticity and its influence on gait are not well understood. Here, we first aimed to model the response of spastic hamstrings and gastrocnemii in children with cerebral palsy to fast passive stretches. Then, we evaluated how the model applied to gait. We developed three models based on exaggerated proprioceptive feedback. The first model relied on feedback from muscle fiber length and velocity (velocity-related model), the second model relied on feedback from muscle fiber length, velocity, and acceleration (acceleration-related model), and the third model relied on feedback from muscle force and its first time derivative (force-related model). The force-related model better reproduced measured hamstrings and gastrocnemii activity during fast passive stretches (coefficients of determination (R2): 0.73 ± 0.10 and 0.60 ± 0.13, respectively, and root mean square errors (RMSE): 0.034 ± 0.031 and 0.009 ± 0.007, respectively) than the velocity-related model (R2: 0.46 ± 0.15 and 0.07 ± 0.13, and RMSE: 0.053 ± 0.051 and 0.015 ± 0.009), and the acceleration-related model (R2: 0.47 ± 0.15 and 0.09 ± 0.14, and RMSE: 0.052 ± 0.050 and 0.015 ± 0.008). Additionally, the force-related model predicted hamstrings and gastrocnemii activity that better correlated with measured activity during gait (cross correlations: 0.82 ± 0.09 and 0.85 ± 0.06, respectively) than the activity predicted by the velocity-related model (cross correlations: 0.49 ± 0.17 and 0.71 ± 0.22) and the acceleration-related model (cross correlations: 0.51 ± 0.16 and 0.67 ± 0.20). Our results therefore suggest that force encoding in muscle spindles in combination with altered feedback gains and thresholds underlie activity of spastic muscles during passive stretches and gait. Our model of spasticity opens new perspectives for studying movement impairments due to spasticity through simulation.

Walking with perturbations: a guide for biped humans and robots

This paper provides an update on the neural control of bipedal walking in relation to bioinspired models and robots. It is argued that most current models or robots are based on the construct of a symmetrical central pattern generator (CPG). However, new evidence suggests that CPG functioning is basically asymmetrical with its flexor half linked more tightly to the rhythm generator. The stability of bipedal gait, which is an important problem for robots and biological systems, is also addressed. While it is not possible to determine how biological biped systems guarantee stability, robot solutions can be useful to propose new hypotheses for biology. In the second part of this review, the focus is on gait perturbations, which is an important topic in robotics in view of the frequent falls of robots when faced with perturbations. From the human physiology it is known that the initial reaction often consists of a brief interruption followed by an adequate response. For instance, the successful recovery from a trip is achieved using some basic reactions (termed elevating and lowering strategies), that depend on the phase of the step cycle of the trip occurrence. Reactions to stepping unexpectedly in a hole depend on comparing expected and real feedback. Implementation of these ideas in models and robotics starts to emerge, with the most advanced robots being able to learn how to fall safely and how to deal with complicated disturbances such as provided by walking on a split-belt.

Role of muscle spindle feedback in regulating muscle activity strength during walking at different speed in mice

Terrestrial animals increase their walking speed by increasing the activity of the extensor muscles. However, the mechanism underlying how this speed-dependent amplitude modulation is achieved remains obscure. Previous studies have shown that group Ib afferent feedback from Golgi tendon organs that signal force is one of the major regulators of the strength of muscle activity during walking in cats and humans. In contrast, the contribution of group Ia/II afferent feedback from muscle spindle stretch receptors that signal angular displacement of leg joints is unclear. Some studies indicate that group II afferent feedback may be important for amplitude regulation in humans, but the role of muscle spindle feedback in regulation of muscle activity strength in quadrupedal animals is very poorly understood. To examine the role of feedback from muscle spindles, we combined in vivo electrophysiology and motion analysis with mouse genetics and gene delivery with adeno-associated virus. We provide evidence that proprioceptive sensory feedback from muscle spindles is important for the regulation of the muscle activity strength and speed-dependent amplitude modulation. Furthermore, our data suggest that feedback from the muscle spindles of the ankle extensor muscles, the triceps surae, is the main source for this mechanism. In contrast, muscle spindle feedback from the knee extensor muscles, the quadriceps femoris, has no influence on speed-dependent amplitude modulation. We provide evidence that proprioceptive feedback from ankle extensor muscles is critical for regulating muscle activity strength as gait speed increases. NEW & NOTEWORTHY Animals upregulate the activity of extensor muscles to increase their walking speed, but the mechanism behind this is not known. We show that this speed-dependent amplitude modulation requires proprioceptive sensory feedback from muscle spindles of ankle extensor muscle. In the absence of muscle spindle feedback, animals cannot walk at higher speeds as they can when muscle spindle feedback is present.

Contribution of sensory feedback to plantar flexor muscle activation during push-off in adults with cerebral palsy

Exaggerated sensory activity has been assumed to contribute to functional impairment following lesion of the central motor pathway. However, recent studies have suggested that sensory contribution to muscle activity during gait is reduced in stroke patients and children with cerebral palsy (CP). We investigated whether this also occurs in CP adults and whether daily treadmill training is accompanied by alterations in sensory contribution to muscle activity. Seventeen adults with CP and 12 uninjured individuals participated. The participants walked on a treadmill while a robotized ankle-foot orthosis applied unload perturbations at the ankle, thereby removing sensory feedback naturally activated during push-off. Reduction of electromyographic (EMG) activity in the soleus muscle caused by unloads was compared and related to kinematics and ankle joint stiffness measurements. Similar measures were obtained after 6 wk of gait training. We found that sensory contribution to soleus EMG activation was reduced in CP adults compared with uninjured adults. The lowest contribution of sensory feedback was found in participants with lowest maximal gait speed. This was related to increased ankle plantar flexor stiffness. Six weeks of gait training did not alter the contribution of sensory feedback. We conclude that exaggerated sensory activity is unlikely to contribute to impaired gait in CP adults, because sensory contribution to muscle activity during gait was reduced compared with in uninjured individuals. Increased passive stiffness around the ankle joint is likely to diminish sensory feedback during gait so that a larger part of plantar flexor muscle activity must be generated by descending motor commands.NEW & NOTEWORTHY Findings suggest that adults with cerebral palsy have less contribution of sensory feedback to ongoing soleus muscle activation during push-off than uninjured individuals. Increased passive stiffness around the ankle joint is likely to diminish sensory feedback during gait, and/or sensory feedback is less integrated with central motor commands in the activation of spinal motor neurons. Consequently, muscle activation must to a larger extent rely on descending drive, which is already decreased because of the cerebral lesion.

Development and aging of human spinal cord circuitries

The neural motor circuitries in the spinal cord receive information from our senses and the rest of the nervous system and translate it into purposeful movements, which allow us to interact with the rest of the world. In this review, we discuss how these circuitries are established during early development and the extent to which they are shaped according to the demands of the body that they control and the environment with which the body has to interact. We also discuss how aging processes and physiological changes in our body are reflected in adaptations of activity in the spinal cord motor circuitries. The complex, multifaceted connectivity of the spinal cord motor circuitries allows them to generate vastly different movements and to adapt their activity to meet new challenges imposed by bodily changes or a changing environment. There are thus plenty of possibilities for adaptive changes in the spinal motor circuitries both early and late in life.

Hysteresis in Center of Mass Velocity Control during the Stance Phase of Treadmill Walking

Achieving a soft landing during walking can be quantified by analyzing changes in the vertical velocity of the body center of mass (CoM) just prior to the landing of the swing limb. Previous research suggests that walking speed and step length may predictably influence the extent of this CoM control. Here we ask how stable this control is. We altered treadmill walking speed by systematically increasing or decreasing it at fixed intervals. We then reversed direction. We hypothesized that the control of the CoM vertical velocity during the late stance of the walking gait may serve as an order parameter which has an attribute of hysteresis. The presence of hysteresis implies that the CoM control is not based on simply knowing the current input conditions to predict the output response. Instead, there is also the influence of previous speed conditions on the ongoing responses. We found that the magnitudes of CoM control were different depending on whether the treadmill speed (as the control parameter) was ramped up or down. Changes in step length also influenced CoM control. A stronger effect was observed when the treadmill speed was speeded up compared to down. However, the effect of speed direction remained significant after controlling for step length. The hysteresis effect of CoM control as a function of speed history demonstrated in the current study suggests that the regulation of CoM vertical velocity during late stance is influenced by previous external conditions and constraints which combine to influence the desired behavioral outcome.

Human Spinal Motor Control

Human studies in the past three decades have provided us with an emerging understanding of how cortical and spinal networks collaborate to ensure the vast repertoire of human behaviors. Humans have direct cortical connections to spinal motoneurons, which bypass spinal interneurons and exert a direct (willful) muscle control with the aid of a context-dependent integration of somatosensory and visual information at cortical level. However, spinal networks also play an important role. Sensory feedback through spinal circuitries is integrated with central motor commands and contributes importantly to the muscle activity underlying voluntary movements. Regulation of spinal interneurons is used to switch between motor states such as locomotion (reciprocal innervation) and stance (coactivation pattern). Cortical regulation of presynaptic inhibition of sensory afferents may focus the central motor command by opening or closing sensory feedback pathways. In the future, human studies of spinal motor control, in close collaboration with animal studies on the molecular biology of the spinal cord, will continue to document the neural basis for human behavior.

The influence of orthosis options on walking parameters in spinal cord-injured patients: a literature review

OBJECTIVE

Orthoses for various joints sections are considered to greatly influence the gait function and energy expenditure in spinal cord-injured (SCI) patients. The aim of this review was to determine the influence of orthoses characteristics and options on the improvement of walking in patients with SCI.

METHODS

A search was performed using the Population Intervention Comparison Outcome (PICO) method, based on selected keywords; studies were identified electronically in the Science Direct, Google Scholar, Scopus, Web of Knowledge and PubMed databases. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) method was used to report the results. Assessment of the quality of all articles was performed based on the Physiotherapy Evidence Database (PEDro scale).

RESULTS

Twelve studies evaluated the effects of different hip joint options on walking parameters and energy expenditure. Five studies investigated the role of knee joint options on gait parameters and compensatory trunk motion. Only five studies analyzed modified ankle joints on gait parameters in SCI patients. Nine studies analyzed gait parameters in SCI patients as powered orthoses and exoskeleton. These studies had a low level of evidence according to the PEDro score (2/10).

CONCLUSION

The various joint types of orthoses appear to be critical in the improvement of walking in patients with SCI. In particular, 'user friendly' orthoses that support the related structure such as the hip joint with a reciprocating mechanism, activated knee joint and movable ankle joint with dorsiflexion assist enable SCI patients to optimize their walking pattern when wearing an orthoses system.

Efficient neuroplasticity induction in chronic stroke patients by an associative brain-computer interface

Brain-computer interfaces (BCIs) have the potential to improve functionality in chronic stoke patients when applied over a large number of sessions. Here we evaluated the effect and the underlying mechanisms of three BCI training sessions in a double-blind sham-controlled design. The applied BCI is based on Hebbian principles of associativity that hypothesize that neural assemblies activated in a correlated manner will strengthen synaptic connections. Twenty-two chronic stroke patients were divided into two training groups. Movement-related cortical potentials (MRCPs) were detected by electroencephalography during repetitions of foot dorsiflexion. Detection triggered a single electrical stimulation of the common peroneal nerve timed so that the resulting afferent volley arrived at the peak negative phase of the MRCP (BCIassociative group) or randomly (BCInonassociative group). Fugl-Meyer motor assessment (FM), 10-m walking speed, foot and hand tapping frequency, diffusion tensor imaging (DTI) data, and the excitability of the corticospinal tract to the target muscle [tibialis anterior (TA)] were quantified. The TA motor evoked potential (MEP) increased significantly after the BCIassociative intervention, but not for the BCInonassociative group. FM scores (0.8 ± 0.46 point difference, P = 0.01), foot (but not finger) tapping frequency, and 10-m walking speed improved significantly for the BCIassociative group, indicating clinically relevant improvements. Corticospinal tract integrity on DTI did not correlate with clinical or physiological changes. For the BCI as applied here, the precise coupling between the brain command and the afferent signal was imperative for the behavioral, clinical, and neurophysiological changes reported. This association may become the driving principle for the design of BCI rehabilitation in the future. Indeed, no available BCIs can match this degree of functional improvement with such a short intervention.

Can Treadmill Perturbations Evoke Stretch Reflexes in the Calf Muscles?

Disinhibition of reflexes is a problem amongst spastic patients, for it limits a smooth and efficient execution of motor functions during gait. Treadmill belt accelerations may potentially be used to measure reflexes during walking, i.e. by dorsal flexing the ankle and stretching the calf muscles, while decelerations show the modulation of reflexes during a reduction of sensory feedback. The aim of the current study was to examine if belt accelerations and decelerations of different intensities applied during the stance phase of treadmill walking can evoke reflexes in the gastrocnemius, soleus and tibialis anterior in healthy subjects. Muscle electromyography and joint kinematics were measured in 10 subjects. To determine whether stretch reflexes occurred, we assessed modelled musculo-tendon length and stretch velocity, the amount of muscle activity, as well as the incidence of bursts or depressions in muscle activity with their time delays, and co-contraction between agonist and antagonist muscle. Although the effect on the ankle angle was small with 2.8±1.0°, the perturbations caused clear changes in muscle length and stretch velocity relative to unperturbed walking. Stretched muscles showed an increasing incidence of bursts in muscle activity, which occurred after a reasonable electrophysiological time delay (163-191 ms). Their amplitude was related to the muscle stretch velocity and not related to co-contraction of the antagonist muscle. These effects increased with perturbation intensity. Shortened muscles showed opposite effects, with a depression in muscle activity of the calf muscles. The perturbations only slightly affected the spatio-temporal parameters, indicating that normal walking was retained. Thus, our findings showed that treadmill perturbations can evoke reflexes in the calf muscles and tibialis anterior. This comprehensive study could form the basis for clinical implementation of treadmill perturbations to functionally measure reflexes during treadmill-based clinical gait analysis.

The effect of ankle joint mobility when using an isocentric reciprocating gait orthosis (IRGO) on energy consumption in people with spinal cord injury: preliminary results

OBJECTIVES

The aim of this study was to evaluate the effect of walking with isocentric reciprocating gait orthoses (IRGOs) utilizing two designs of ankle foot orthoses (AFOs) on specific outcome measures in people with spinal cord injury (SCI).

METHODS

Four volunteer SCI subjects participated in this study, and were fitted with an IRGO equipped with either solid or dorsiflexion-assisted AFOs in a randomized order. Subjects walked at their self-selected speed along a flat walkway to enable a comparison of walking speed, endurance and the resulting physiological cost index (PCI) to be performed.

RESULTS

Increased walking speed, increased distance walked and less PCI were demonstrated in walking with the IRGO incorporating dorsiflexion-assisted AFOs as compared to walking with an IRGO plus solid AFO as a control condition.

CONCLUSION

This study demonstrated that people with SCI could walk at relatively higher speeds and with greater endurance and a reduced PCI when utilizing an IRGO with dorsiflexion-assisted AFO components compared to solid ones. It is therefore concluded that the IRGO incorporating dorsiflexion-assisted AFOs may be an effective alternative in helping to reduce the energy consumption experienced by people with SCI.

Altering length and velocity feedback during a neuro-musculoskeletal simulation of normal gait contributes to hemiparetic gait characteristics

Background

Spasticity is an important complication after stroke, especially in the anti-gravity muscles, i.e. lower limb extensors. However the contribution of hyperexcitable muscle spindle reflex loops to gait impairments after stroke is often disputed. In this study a neuro-musculoskeletal model was developed to investigate the contribution of an increased length and velocity feedback and altered reflex modulation patterns to hemiparetic gait deficits.

Methods

A musculoskeletal model was extended with a muscle spindle model providing real-time length and velocity feedback of gastrocnemius, soleus, vasti and rectus femoris during a forward dynamic simulation (neural control model). By using a healthy subject’s base muscle excitations, in combination with increased feedback gains and altered reflex modulation patterns, the effect on kinematics was simulated. A foot-ground contact model was added to account for the interaction effect between the changed kinematics and the ground. The qualitative effect i.e. the directional effect and the specific gait phases where the effect is present, on the joint kinematics was then compared with hemiparetic gait deviations reported in the literature.

Results

Our results show that increased feedback in combination with altered reflex modulation patterns of soleus, vasti and rectus femoris muscle can contribute to excessive ankle plantarflexion/inadequate dorsiflexion, knee hyperextension/inadequate flexion and increased hip extension/inadequate flexion during dedicated gait cycle phases. Increased feedback of gastrocnemius can also contribute to excessive plantarflexion/inadequate dorsiflexion, however in combination with excessive knee and hip flexion. Increased length/velocity feedback can therefore contribute to two types of gait deviations, which are both in accordance with previously reported gait deviations in hemiparetic patients. Furthermore altered modulation patterns, in particular the reduced suppression of the muscle spindle feedback during swing, can contribute largely to an increased plantarflexion and knee extension during the swing phase and consequently to hampered toe clearance.

Conclusions

Our results support the idea that hyperexcitability of length and velocity feedback pathways, especially in combination with altered reflex modulation patterns, can contribute to deviations in hemiparetic gait. Surprisingly, our results showed only subtle temporal differences between length and velocity feedback. Therefore, we cannot attribute the effects seen in kinematics to one specific type of feedback.

Evidence for a time-invariant phase variable in human ankle control

Human locomotion is a rhythmic task in which patterns of muscle activity are modulated by state-dependent feedback to accommodate perturbations. Two popular theories have been proposed for the underlying embodiment of phase in the human pattern generator: a time-dependent internal representation or a time-invariant feedback representation (i.e., reflex mechanisms). In either case the neuromuscular system must update or represent the phase of locomotor patterns based on the system state, which can include measurements of hundreds of variables. However, a much simpler representation of phase has emerged in recent designs for legged robots, which control joint patterns as functions of a single monotonic mechanical variable, termed a phase variable. We propose that human joint patterns may similarly depend on a physical phase variable, specifically the heel-to-toe movement of the Center of Pressure under the foot. We found that when the ankle is unexpectedly rotated to a position it would have encountered later in the step, the Center of Pressure also shifts forward to the corresponding later position, and the remaining portion of the gait pattern ensues. This phase shift suggests that the progression of the stance ankle is controlled by a biomechanical phase variable, motivating future investigations of phase variables in human locomotor control.

Gait evaluation of the advanced reciprocating gait orthosis with solid versus dorsi flexion assist ankle foot orthoses in paraplegic patients

BACKGROUND

Mechanical orthoses are used for standing and walking after neurological injury. Most orthoses such as the advanced reciprocating gait orthosis typically use solid ankle-foot orthoses.

OBJECTIVES

The goal of this study was to test the effects of ankle dorsiflexion assistance in patients with spinal cord injury when ambulating with an advanced reciprocating gait orthosis compared to walking with fixed ankles.

STUDY DESIGN

Quasi-experimental.

METHODS

Four patients with spinal cord injury were fitted with an advanced reciprocating gait orthosis equipped with solid and dorsiflexion assist-type ankle-foot orthoses and walked at their self-selected speed. Joint angles and spatial-temporal parameters were measured and analyzed.

RESULTS

The mean walking speed and stride length were both significantly increased along with cadence by the volunteer subjects when ambulating using the advanced reciprocating gait orthosis fitted with dorsiflexion assist ankle-foot orthoses compared to the advanced reciprocating gait orthosis with solid ankle-foot orthoses. The mean ankle joint ranges of motion were significantly increased when walking with the advanced reciprocating gait orthosis with dorsiflexion assist ankle-foot orthoses compared to when using the advanced reciprocating gait orthosis with the solid ankle-foot orthoses. Knee joint ranges of motion were reduced, and hip joint ranges of motion were increased but not significantly.

CONCLUSION

The advanced reciprocating gait orthosis fitted with the dorsiflexion assist ankle-foot orthoses had the effect of improving gait parameters when compared to the advanced reciprocating gait orthosis with solid ankle-foot orthoses. Clinical relevance The advanced reciprocating gait orthosis with dorsiflexion assist ankle-foot orthoses has the potential to improve hip and ankle joint kinematics and the temporal-spatial parameters of gait in spinal cord injury patients' walking.

Comparison of the effects of solid versus hinged ankle foot orthoses on select temporal gait parameters in patients with incomplete spinal cord injury during treadmill walking

BACKGROUND

Ankle foot orthoses (AFOs) are usually used for patients with incomplete spinal cord injury (ISCI) to provide support in walking.

OBJECTIVES

The aim of this study was to compare the effect of AFOs, with and without ankle hinges, on specific gait parameters during treadmill training by subjects with ISCI.

STUDY DESIGN

Quasi-experimental.

METHODS

Five patients with ISCI at the thoracic level participated in this study. Gait evaluation was performed when walking 1) barefoot 2) wearing a solid AFO and 3) wearing a hinged AFO.

RESULTS

The mean step length when walking barefoot was 26.3 ± 16.37 cm compared to 31.3 ± 17.27 cm with a solid AFO and 28.5 ± 15.86 cm with a hinged AFO. The mean cadence for walking barefoot was 61.59 ± 25.65 steps/min. compared to 50.94 ± 22.36 steps/min. with a solid AFO and 56.25 ± 24.44 steps/min with a hinged AFO. Significant differences in cadence and step length during walking were only demonstrated between the barefoot condition and when wearing a solid AFO. Significant difference was not observed between conditions in mean of ankle range of motion.

CONCLUSION

The solid AFO was the only condition which improved cadence and step length in patients during ISCI gait training. Clinical relevance A solid AFO could be used permanently to compensate for impaired ankle function or it could be used while retraining stepping.

Reciprocal inhibition versus unloading response during stretch reflex in humans

Rotation of an upper limb joint produces excitatory stretch reflex peaks M1 and M2 in the stretched muscles and simultaneous decrease in electromyographic (EMG) activity in the shortened muscles. The objective of this study was to examine whether the decreased activity in the antagonists (rINHIB) is purely from unloading of the spindles or receives active inhibition involving inhibitory interneurons. If rINHIB is due only to unloading, then the termination of rINHIB should vary with the duration of perturbation used to elicit stretch reflex, namely shorter stretches should result in shorter values of decreased periods of EMG. To examine this question, rectangular pulses, ranging in duration from 25 to 150 ms, were used to stretch wrist flexors or extensors with a torque motor. These rectangular pulses resulted in joint rotations which peaked at times (T(peak)) ranging from approximately 75-160 ms. As shown by previous authors, when the duration of rotation was shortened, the magnitude of M1 did not change, while the magnitude of M2 decreased. However, termination time of rINHIB in the shortened muscles did not change with change in T(peak), implying thereby that unloading of spindles of the antagonist muscles is not the only mechanism for the reduction in activity and that inhibitory reflex pathways most likely contribute. Possible sources of inhibition are discussed for the short- and long-latency inhibition.

Force control in the absence of visual and tactile feedback

Motor control tasks like stance or object handling require sensory feedback from proprioception, vision and touch. The distinction between tactile and proprioceptive sensors is not frequently made in dynamic motor control tasks, and if so, mostly based on signal latency. We previously found that force control tasks entail more compliant behavior than a passive, relaxed condition and by neuromuscular modeling we were able to attribute this to adaptations in proprioceptive force feedback from Golgi tendon organs. This required the assumption that both tactile and visual feedback are too slow to explain the measured adaptations in face of unpredictable force perturbations. Although this assumption was shown to hold using model simulations, so far no experimental data is available to validate it. Here we applied a systematic approach using continuous perturbations and engineering analyses to provide experimental evidence for the hypothesis that motor control adaptation in force control tasks can be achieved using proprioceptive feedback only. Varying task instruction resulted in substantial adaptations in neuromuscular behavior, which persisted after eliminating visual and/or tactile feedback by a nerve block of the nervus plantaris medialis. It is concluded that proprioception adapts dynamic human ankle motor control even in the absence of visual and tactile feedback.

Control of ankle extensor muscle activity in walking cats

Our objective was to gain insight into the relative importance of feedforward control and different proprioceptive feedback pathways to ongoing ankle extensor activity during walking in the conscious cat. We asked whether the modulation of stance phase muscle activity is due primarily to proprioceptive feedback and whether the same proprioceptive gains and feedforward commands can automatically generate the muscle activity required for changes in walking slope. To test these hypotheses, we analyzed previously collected muscle activity and mechanics data from cats with an isolated medial gastrocnemius muscle walking along a sloped pegway. Models of proprioceptor dynamics predicted afferent activity from the measured muscle mechanics. We modeled muscle activity as the weighted sum of the activity predicted from the different proprioceptive pathways and a simple model of central drive. We determined the unknown model parameters using optimization procedures that minimized the error between the predicted and measured muscle activity. We found that the modulation of muscle activity within the stance phase and across walking slopes is indeed well described by neural control that employs constant central drive and constant proprioceptive feedback gains. Furthermore, it is force feedback from Ib afferents that is primarily responsible for modulating muscle activity; group II afferent feedback makes a small contribution to tonic activity, and Ia afferent feedback makes no contribution. Force feedback combined with tonic central drive appears to provide a simple control mechanism for automatically compensating for changes in terrain without requiring different commands from the brain or even modification of central nervous system gains.

Triceps surae short latency stretch reflexes contribute to ankle stiffness regulation during human running

During human running, short latency stretch reflexes (SLRs) are elicited in the triceps surae muscles, but the function of these responses is still a matter of controversy. As the SLR is primarily mediated by Ia afferent nerve fibres, various methods have been used to examine SLR function by selectively blocking the Ia pathway in seated, standing and walking paradigms, but stretch reflex function has not been examined in detail during running. The purpose of this study was to examine triceps surae SLR function at different running speeds using Achilles tendon vibration to modify SLR size. Ten healthy participants ran on an instrumented treadmill at speeds between 7 and 15 km/h under 2 Achilles tendon vibration conditions: no vibration and 90 Hz vibration. Surface EMG from the triceps surae and tibialis anterior muscles, and 3D lower limb kinematics and ground reaction forces were simultaneously collected. In response to vibration, the SLR was depressed in the triceps surae muscles at all speeds. This coincided with short-lasting yielding at the ankle joint at speeds between 7 and 12 km/h, suggesting that the SLR contributes to muscle stiffness regulation by minimising ankle yielding during the early contact phase of running. Furthermore, at the fastest speed of 15 km/h, the SLR was still depressed by vibration in all muscles but yielding was no longer evident. This finding suggests that the SLR has greater functional importance at slow to intermediate running speeds than at faster speeds.

A new approach for detecting and analyzing cutaneous reflexes during locomotion

During human walking, due to their small amplitude, individual cutaneous reflex responses are difficult to detect in surface EMG recordings. In this study, we present a new algorithm to automatically detect individual cutaneous reflex responses and to extract their corresponding onset latency, amplitude, duration, and sign. To discriminate reflex responses from the intrinsic variability of the background EMG, each stimulated cycle is compared with 10 adjacent nonstimulated cycles, looking for consistent differences. In the first 200 ms after stimulation, reflex responses are detected when ≥9/10 of these differences are either positive or negative. This approach does not require amplitude thresholds or fixed time windows for reflex detection. To reduce false detections, a postprocessing step selects 50 nonstimulated cycles randomly, processes them through the algorithm as stimulated cycles, and establishes a minimal reflex duration criterion that it then used to validate the detected responses. Validated responses from an entire test session are then reported on a colormap (reflex activity map) from which specific responses can be identified and quantified. The new method was validated in ten participants, three cutaneous nerves, and two protocols (phase modulation and recruitment curves). Compared with the classical method, the new algorithm showed better performance in terms of detection accuracy, specificity, and reliability. Although tested here to evaluate cutaneous reflexes during human walking, the simplicity of this method is such that it could easily be used with other reflexes, signals, and preparations.

Involvement of the corticospinal tract in the control of human gait

Given the inherent mechanical complexity of human bipedal locomotion, and that complete spinal cord lesions in human leads to paralysis with no recovery of gait, it is often suggested that the corticospinal tract (CST) has a more predominant role in the control of walking in humans than in other animals. However, what do we actually know about the contribution of the CST to the control of gait? This chapter will provide an overview of this topic based on the premise that a better understanding of the role of the CST in gait will be essential for the design of evidence-based approaches to rehabilitation therapy, which will enhance gait ability and recovery in patients with lesions to the central nervous system (CNS). We review evidence for the involvement of the primary motor cortex and the CST during normal and perturbed walking and during gait adaptation. We will also discuss knowledge on the CST that has been gained from studies involving CNS lesions, with a particular focus on recent data acquired in people with spinal cord injury.

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.

Feedback and feedforward locomotor adaptations to ankle-foot load in people with incomplete spinal cord injury

Humans with spinal cord injury (SCI) modulate locomotor output in response to limb load. Understanding the neural control mechanisms responsible for locomotor adaptation could provide a framework for selecting effective interventions. We quantified feedback and feedforward locomotor adaptations to limb load modulations in people with incomplete SCI. While subjects airstepped (stepping performed with kinematic assistance and 100% bodyweight support), a powered-orthosis created a dorisflexor torque during the "stance phase" of select steps producing highly controlled ankle-load perturbations. When given repetitive, stance phase ankle-load, the increase in hip extension work, 0.27 J/kg above baseline (no ankle-load airstepping), was greater than the response to ankle-load applied during a single step, 0.14 J/kg (P = 0.029). This finding suggests that, at the hip, subjects produced both feedforward and feedback locomotor modulations. We estimate that, at the hip, the locomotor response to repetitive ankle-load was modulated almost equally by ongoing feedback and feedforward adaptations. The majority of subjects also showed after-effects in hip kinetic patterns that lasted 3 min in response to repetitive loading, providing additional evidence of feedforward locomotor adaptations. The magnitude of the after-effect was proportional to the response to repetitive ankle-foot load (R(2) = 0.92). In contrast, increases in soleus EMG amplitude were not different during repetitive and single-step ankle-load exposure, suggesting that ankle locomotor modulations were predominately feedback-based. Although subjects made both feedback and feedforward locomotor adaptations to changes in ankle-load, between-subject variations suggest that walking function may be related to the ability to make feedforward adaptations.

Achilles tendon length changes during walking in long-term diabetes patients

BACKGROUND

Diabetes leads to numerous side effects, including an increased density of collagen fibrils and thickening of the Achilles tendon. This may increase tissue stiffness and could affect stretch distribution between muscle and tendinous tissues during walking. The primary aim of this study was to examine stretch distribution between muscle and tendinous tissues in the medial gastrocnemius muscle-tendon unit in long-term diabetes patients and control subjects during walking.

METHODS

Achilles tendon length changes were investigated in 13 non-neuropathic diabetes patients and 12 controls, whilst walking at a self selected speed across a 10 m force platform. Electromyographic activity was recorded in the medial gastrocnemius, soleus and tibialis anterior muscles, goniometers were used to detect joint angle changes, and ultrasound was used to estimate tendon length changes.

FINDINGS

Achilles tendon length changes were attenuated in diabetes patients compared to controls, and were inversely correlated with diabetes duration (r=-0.628; P<0.05), as was ankle range of motion (r=-0.693; P<0.01). Tendon length changes were also independent of walking speed (r=-0.299; P=0.224) and age (r=0.115; P=0.721) in the diabetic group.

INTERPRETATION

Stretch distribution between muscle and tendon during walking is altered in diabetic patients, which could decrease walking efficiency, a factor that may be exacerbated with increasing diabetes duration. Diabetes-induced changes in mechanical tendon properties may be at least partly responsible for attenuated tendon length changes during walking in this patient group.

Load rather than length sensitive feedback contributes to soleus muscle activity during human treadmill walking

Walking requires a constant adaptation of locomotor output from sensory afferent feedback mechanisms to ensure efficient and stable gait. We investigated the nature of the sensory afferent feedback contribution to the soleus motoneuronal drive and to the corrective stretch reflex by manipulating body load and ankle joint angle. The volunteers walked on a treadmill ( approximately 3.6 km/h) connected to a body weight support (BWS) system. To manipulate the load sensitive afferents the level of BWS was switched between 5 and 30% of body weight. The effect of transient changes in BWS on the soleus stretch reflex was measured by presenting dorsiflexion perturbations ( approximately 5 degrees, 360-400 degrees/s) in mid and late stances. Short (SLRs) and medium latency reflexes (MLRs) were quantified in a 15 ms analysis window. The MLR decreased with decreased loading (P = 0.045), but no significant difference was observed for the SLR (P = 0.13). Similarly, the effect of the BWS was measured on the unload response, i.e., the depression in soleus activity following a plantar-flexion perturbation ( approximately 5.6 degrees, 203-247 degrees/s), quantified over a 50 ms analysis window. The unload response decreased with decreased load (P > 0.001), but was not significantly affected (P = 0.45) by tizanidine induced depression of the MLR (P = 0.039, n = 6). Since tizanidine is believed to depress the group II afferent pathway, these results are consistent with the idea that force-related afferent feedback contributes both to the background locomotor activity and to the medium latency stretch reflex. In contrast, length-related afferent feedback may contribute to only the medium latency stretch reflex.

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.

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.

Effects of prolonged walking on neural and mechanical components of stretch responses in the human soleus muscle

After repeated passive stretching, tendinous tissue compliance increases in the human soleus (SOL) muscle-tendon unit. During movement, such changes would have important consequences for neural and mechanical stretch responses. This study examined the existence of such effects in response to a 75 min walking intervention. Eleven healthy subjects walked on a treadmill at 4 km h(1) with a robotic stretch device attached to the left leg. Ultrasonography was used to measure SOL fascicle lengths, and surface EMG activity was recorded in the SOL and tibialis anterior (TA) muscles. Perturbations of 6 deg were imposed at three different measurement intervals: Pre (immediately before the walking intervention), Mid (after approximately 30 min of walking) and Post (immediately after the intervention). Between the Pre-Mid and Mid-Post intervals, subjects walked for 30 min at a gradient of 3%. After the intervention, the amplitude and velocity of fascicle stretch both decreased (by 46 and 59%, respectively; P < 0.001) in response to a constant external perturbation, as did short (33%; P < 0.01) and medium (25%; P < 0.01) latency stretch reflex amplitudes. A faster perturbation elicited at the end of the protocol resulted in a recovery of fascicle stretch velocities and short latency reflex amplitudes to the pre-exercise values. These findings suggest that repeated stretching and shortening of a muscle-tendon unit can induce short-term structural changes in the tendinous tissues during human walking. The data also highlight the effect of these changes on neural feedback from muscle sensory afferents.

Using an electrohydraulic ankle foot orthosis to study modifications in feedforward control during locomotor adaptation to force fields applied in stance

Background

Adapting to external forces during walking has been proposed as a tool to improve locomotion after central nervous system injury. However, sensorimotor integration during walking varies according to the timing in the gait cycle, suggesting that adaptation may also depend on gait phases. In this study, an ElectroHydraulic AFO (EHO) was used to apply forces specifically during mid-stance and push-off to evaluate if feedforward movement control can be adapted in these 2 gait phases.

Methods

Eleven healthy subjects walked on a treadmill before (3 min), during (5 min) and after (5 min) exposure to 2 force fields applied by the EHO (mid-stance/push-off; ~10 Nm, towards dorsiflexion). To evaluate modifications in feedforward control, strides with no force field ('catch strides') were unexpectedly inserted during the force field walking period.

Results

When initially exposed to a mid-stance force field (FF_20%), subjects showed a significant increase in ankle dorsiflexion velocity. Catches applied early into the FF_20% were similar to baseline (P > 0.99). Subjects gradually adapted by returning ankle velocity to baseline over ~50 strides. Catches applied thereafter showed decreased ankle velocity where the force field was normally applied, indicating the presence of feedforward adaptation. When initially exposed to a push-off force field (FF_50%), plantarflexion velocity was reduced in the zone of force field application. No adaptation occurred over the 5 min exposure. Catch strides kinematics remained similar to control at all times, suggesting no feedforward adaptation. As a control, force fields assisting plantarflexion (-3.5 to -9.5 Nm) were applied and increased ankle plantarflexion during push-off, confirming that the lack of kinematic changes during FF_50% catch strides were not simply due to a large ankle impedance.

Conclusion

Together these results show that ankle exoskeletons such as the EHO can be used to study phase-specific adaptive control of the ankle during locomotion. Our data suggest that, for short duration exposure, a feedforward modification in torque output occurs during mid-stance but not during push-off. These findings are important for the design of novel rehabilitation methods, as they suggest that the ability to use resistive force fields for training may depend on targeted gait phases.

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.

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.

Force Regulation of Ankle Extensor Muscle Activity in Freely Walking Cats

To gain insight into the relative importance of force feedback to ongoing ankle extensor activity during walking in the conscious cat, we isolated the medial gastrocnemius muscle (MG) by denervating the other ankle extensors and measured the magnitude of its activity at different muscle lengths, velocities, and forces accomplished by having the animals walk up and down a sloped pegway. Mathematical models of proprioceptor dynamics predicted afferent activity and revealed that the changes in muscle activity under our experimental conditions were strongly correlated with Ib activity and not consistently associated with changes in Ia or group II activity. This allowed us to determine the gains within the force feedback pathway using a simple model of the neuromuscular system and the measured relationship between MG activity and force. Loop gain increased with muscle length due to the intrinsic force–length property of muscle. The gain of the pathway that converts muscle force to motoneuron depolarization was independent of length. To better test for a causal relationship between modulation of force feedback and changes in muscle activity, a second set of experiments was performed in which the MG muscle was perturbed during ground contact of the hind foot by dropping or lifting the peg underfoot. Collectively, these investigations support a causal role for force feedback and indicate that about 30% of the total muscle activity is due to force feedback during level walking. Force feedback's role increases during upslope walking and decreases during downslope walking, providing a simple mechanism for compensating for changes in terrain.

Within-step modulation of leg muscle activity by afferent feedback in human walking

To maintain smooth and efficient gait the motor system must adjust for changes in the ground on a step-to-step basis. In the present study we investigated the role of sensory feedback as 19 able-bodied human subjects walked over a platform that mimicked an uneven supporting surface. Triceps surae muscle activation was assessed during stance as the platform was set to different inclinations (+/-3 deg, +/-2 deg and 0 deg rotation in a parasagittal plane about the ankle). Normalized triceps surae muscle activity was significantly increased when the platform was inclined (2 deg: 0.153 +/- 0.051; 3 deg: 0.156 +/- 0.053) and significantly decreased when the platform was declined (-3 deg: 0.133 +/- 0.048; -2 deg: 0.132 +/- 0.049) compared with level walking (0.141 +/- 0.048) for the able-bodied subjects. A similar experiment was performed with a subject who lacked proprioception and touch sensation from the neck down. In contrast with healthy subjects, no muscle activation changes were observed in the deafferented subject. Our results demonstrate that the ability to compensate for small irregularities in the ground surface relies on automatic within-step sensory feedback regulation rather than conscious predictive control.

Vertical perturbations of human gait: organisation and adaptation of leg muscle responses

During the last several years, evidence has arisen that the neuronal control of human locomotion depends on feedback from load receptors. The aim of the present study was to determine the effects and the course of sudden and unexpected changes in body load (vertical perturbations) on leg muscle activity patterns during walking on a treadmill. Twenty-two healthy subjects walking with 25% body weight support (BWS) were repetitively and randomly loaded to 5% or unloaded to 45% BWS during left mid-stance. At the new level of BWS, the subjects performed 3-11 steps before returning to 25% BWS (base level). EMG activity of upper and lower leg muscles was recorded from both sides. The bilateral leg muscle activity pattern changed following perturbations in the lower leg muscles and the net effect of the vertical perturbations showed onset latencies with a range of 90-105 ms. Body loading enhanced while unloading diminished the magnitude of ipsilateral extensor EMG amplitude, compared to walking at base level. Contralateral leg flexor burst activity was shortened following loading and prolonged following unloading perturbation while flexor EMG amplitude was unchanged. A general decrease in EMG amplitudes occurred during the course of the experiment. This is assumed to be due to adaptation. Only the muscles directly activated by the perturbations did not significantly change EMG amplitude. This is assumed to be due to the required compensation of the perturbations by polysynaptic spinal reflexes released following the perturbations. The findings underline the importance of load receptor input for the control of locomotion.

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.

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.

Muscle reflexes and synergies triggered by an unexpected support surface height during walking

An important phase in the step cycle is foot contact. When the moment of foot contact differs from the one expected, a fast response is needed. Such a mismatch can be caused by hitting a support surface earlier or later than expected. To study this, experiments were performed with healthy young adults who walked on a platform that was unexpectedly at a lowered (5 cm) or at a level height. Glasses blocked the lower visual field. In the unexpectedly lowered trials, the absence of expected heel contact triggered responses in the ipsilateral anti-gravity muscles [ipsilateral medial gastrocnemius (MGi), ipsilateral rectus femoris (RFi)] and contralateral flexor muscles [contralateral tibialis anterior (TAc), contralaterial biceps femoris (BFc)] with latencies of 47-69 ms. After the delayed heel contact, enhanced activity was found in the MGi, RFi, and TAc muscles. This specific muscle synergy was presumably activated to arrest the forward propulsion of the body. In contrast, when the surface was unexpectedly at level height, the subjects expected to step down, and the leg briefly yielded. A muscle synergy was activated at 46-81 ms that flexed the ipsilateral knee (TAi, BFi, RFi) and extended the contralateral one (MGc, BFc) to unload the perturbed leg and delay the contralateral swing phase. Both conditions triggered a fast functionally relevant muscle synergy because of a mismatch between the expected and actual sensory feedback at the moment of foot contact. The results are consistent with an internal model that compares the expected with the actual sensory feedback. The short latency of the response suggests a subcortical, possibly cerebellar pathway.

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.

Spinal control of locomotion--from cat to man

For a large number of vertebrate species it is now indisputable that spinal networks have the capability of generating the basic locomotor rhythm. The aim of this review is to summarize the evidence for spinal pattern generators in cats and primates, including man and its interaction with sensory signals from the limbs. For all species the sensory feed-back from the moving limb is very important to achieve effective locomotor behaviour by adapting to the environment and compensating for unexpected postural disturbances. Sensory regulation of stepping can occur via reflex pathways to motoneurones (by-passing the locomotor rhythm generators) or by acting on the spinal locomotor networks themselves. The sensory feed-back serves to control the timing of the different phases in the step cycle, to shape the pattern of muscle activity, to contribute to the excitatory drive of the motoneurones and to the long-term adaptation of the locomotor activity. In this review we discuss the spinal locomotor circuits and the sensory feed-back in animals (mainly the cat) and human subjects. Special emphasis is given to work that has been of importance for the development of new rehabilitation paradigms following spinal cord injury.

Assessing sensory function in locomotor systems using neuro-mechanical simulations

Computer simulations are being used increasingly to gain an understanding of the complex interactions between the neuronal, sensory, muscular and mechanical components of locomotor systems. Recent neuro-mechanical simulations of walking in humans, cats and insects, and of swimming in lampreys, have provided new information on the functional role of specific groups of sensory receptors in regulating locomotion. As we discuss in this review, these studies also make it clear that a full understanding of the neural and mechanical mechanisms that underlie locomotion can be achieved only by using simulations in parallel with physiological investigations. The widespread implementation of this approach would be enhanced by the development of freely available and easy-to-use software tools.

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.

The effects of powered ankle-foot orthoses on joint kinematics and muscle activation during walking in individuals with incomplete spinal cord injury

Background

Powered lower limb orthoses could reduce therapist labor during gait rehabilitation after neurological injury. However, it is not clear how patients respond to powered assistance during stepping. Patients might allow the orthoses to drive the movement pattern and reduce their muscle activation. The goal of this study was to test the effects of robotic assistance in subjects with incomplete spinal cord injury using pneumatically powered ankle-foot orthoses.

Methods

Five individuals with chronic incomplete spinal cord injury (ASIA C-D) participated in the study. Each subject was fitted with bilateral ankle-foot orthoses equipped with artificial pneumatic muscles to power ankle plantar flexion. Subjects walked on a treadmill with partial bodyweight support at four speeds (0.36, 0.54, 0.72 and 0.89 m/s) under three conditions: without wearing orthoses, wearing orthoses unpowered (passively), and wearing orthoses activated under pushbutton control by a physical therapist. Subjects also attempted a fourth condition wearing orthoses activated under pushbutton control by them. We measured joint angles, electromyography, and orthoses torque assistance.

Results

A therapist quickly learned to activate the artificial pneumatic muscles using the pushbuttons with the appropriate amplitude and timing. The powered orthoses provided ~50% of peak ankle torque. Ankle angle at stance push-off increased when subjects walked with powered orthoses versus when they walked with passive-orthoses (ANOVA, p < 0.05). Ankle muscle activation amplitudes were similar for powered and passive-orthoses conditions except for the soleus (~13% lower for powered condition; p < 0.05).

Two of the five subjects were able to control the orthoses themselves using the pushbuttons. The other three subjects found it too difficult to coordinate pushbutton timing. Orthoses assistance and maximum ankle angle at push-off were smaller when the subject controlled the orthoses compared to when the therapist-controlled the orthoses (p < 0.05). Muscle activation amplitudes were similar between the two powered conditions except for tibialis anterior (~31% lower for therapist-controlled; p < 0.05).

Conclusion

Mechanical assistance from powered ankle-foot orthoses improved ankle push-off kinematics without substantially reducing muscle activation during walking in subjects with incomplete spinal cord injury. These results suggest that robotic plantar flexion assistance could be used during gait rehabilitation without promoting patient passivity.

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.