Phase-dependent compensatory responses to perturbation applied during walking in humans

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

Task-Dependent Compensatory Responses to Perturbations Applied During Rhythmic Movements in Humans

Modulation of the responses to perturbation applied during different phases of three rhythmic movements in humans—running, cycling, and hopping—was studied. The perturbation was an electrical stimulus. The results showed gating and modulation of the responses in both ipsi-and contralateral limb muscles. The responses during running and cycling were only excitatory in nature, while during hopping an inhibitory response was observed. These responses were not correlated with the normal activity during the movement. The latency of the response in general was not altered for different stimulation phases. The alterations in the step cycle demonstrated overt behavioral changes due to the responses. There were differences between the responses observed during these movements and walking. In running, the major adaptation to perturbations appears to be in the contralateral side as seen in the changes in the step cycle. During cycling (except for one phase) and hopping, the same set of muscles was activated in response to perturbation. This represents a simplifying strategy in response organization. The dependency of the response on the task characteristics, postural stability requirement, and external constraints imposed on the subject is discussed. These studies provide insights into task-dependent strategies adopted by the nervous system to meet unexpected perturbation during rhythmic movement in humans.     

Task-dependent compensatory responses to perturbations applied during rhythmic movements in humans

Modulation of the responses to perturbation applied during different phases of three rhythmic movements in humans-running, cycling, and hopping-was studied. The perturbation was an electrical stimulus. The results showed gating and modulation of the responses in both ipsi- and contralateral limb muscles. The responses during running and cycling were only excitatory in nature, while during hopping an inhibitory response was observed. These responses were not correlated with the normal activity during the movement. The latency of the response in general was not altered for different stimulation phases. The alterations in the step cycle demonstrated overt behavioral changes due to the responses. There were differences between the responses observed during these movements and walking. In running, the major adaptation to perturbations appears to be in the contralateral side as seen in the changes in the step cycle. During cycling (except for one phase) and hopping, the same set of muscles was activated in response to perturbation. This represents a simplifying strategy in response organization. The dependency of the response on the task characteristics, postural stability requirement, and external constraints imposed on the subject is discussed. These studies provide insights into task-dependent strategies adopted by the nervous system to meet unexpected perturbation during rhythmic movements in humans.     

Effects of walking on various inclines on EMG patterns of lower limb muscles in humans

Myoelectric signals from several muscles of the lower limb were studied during treadmill locomotion over various inclines. A pattern recognition technique was used to analyse these activity patterns. The analyses revealed the following rules. These are common features among the various muscle activity patterns. The results suggest that the limb is controlled as a unit. Both phasic and average components of the muscle activity patterns are modulated to meet demands imposed by the various inclines. The distal muscles in general are more tightly controlled than the proximal muscles. The changes in average EMG values are muscle-specific, and are not similar for the stance and swing phases of the step cycle. On average, the proximal muscles show greater increases than the distal muscles. These results are compared with those found previously in the different speed and stride-length condition. Studies such as these shed light on the adaptability of the basic locomotor synergy.     

Influence of perturbation induced by an anticipated load on human motor regulatory systems.

To investigate how human motor regulatory systems are modified by prior knowledge of a predictable external perturbation, six normal human subjects, each when sitting on a chair, were required to maintain a stable elbow flexion angle (90 degrees) while different weight perturbations were applied (0.5 kg or 2-kg loads). Loads were applied either by the experimenter Without Anticipation or With Anticipation by the subject's own contralateral hand. Acceleration of the forearm movement (elbow extension and flexion) by loads and electromyograms (EMGs) of the biceps brachii (BB) and the triceps brachii (TB) muscle were recorded. Under With Anticipation conditions, preceding EMG activities of BB and TB muscles prior to the onset time of perturbation were clearly observed. Furthermore, the amount of these preceding EMG activities was larger in the heavy load perturbation than in the light load perturbation. Under Without Anticipation conditions, however, these preceding EMG activities were not observed. In the preceding EMG activities, EMG bursts (latency 20 msec.) of a presumed stretch reflex induced by the perturbation were clearly observed. Thus, the function of anticipatory adjustment of mainitaining the elbow angle definitely appears to optimize limb stability in the case of the mechanical self-applied perturbation. Furthermore, the extent of the anticipatory adjustment of the elbow angle was dependent on the predicted magnitude of load.     

Axial muscle activation provides stabilization against perturbations while running

Incidence of traumatic brain injury is an important hazard in sports and recreation. Unexpected (blind-sided) impacts with other players, obstacles, and the ground can be particularly dangerous. We believe this is partially due to the lack of muscular activation which would have otherwise provided protective bracing. In this study participants were asked to run on the treadmill while undergoing perturbations applied at the waist which pulled participants in the fore-aft and lateral directions. To determine the effect of unexpected impacts, participants were given a directional audio-visual warning 0.5 s prior to the perturbation in half of the trials and were unwarned in the other half of the trials. Perturbations were given during the start of the stance phase and during the start of the flight phase to examine two distinct points within the locomotor cycle. Muscle activity was monitored in axial muscles before, during, and after the perturbations were given. We hypothesized that the presence of a warning would allow for voluntary axial muscle activity prior to and during perturbations that would provide bracing of the body, and decreased displacement and acceleration of the head compared to unwarned perturbations. Our results indicate that when a warning is given prior to perturbation, the body was displaced significantly less, and the linear acceleration of the head was also significantly lessened in response to some perturbations. The perturbations given in this study caused significant increases in axial muscle activity compared to activity present during control running. We found evidence that cervical and abdominal muscles increased activity in response to the warning and that typically the warned trials displayed a lower reflexive muscle activity response. Additionally, we found a stronger effect of the warnings on muscle activity within the perturbations given during flight phase than those given at stance phase. Results from this study support the hypothesis that knowledge regarding an impending perturbation is used by the neuromuscular system to activate relevant core musculature and provide bracing to the athlete.     

视觉预期和注意指向对姿势和动作肌肉预期和补偿姿势调节的影响

采用经典落球试验研究范式,同步观察视觉预期和注意指向对腰部姿势肌肉和上肢运动肌肉预期和补偿姿势调节的影响,探索视觉预期和注意指向影响姿势控制的早期心理生理机制。24名青年志愿者(10名男性,14名女性)参与完成本实验,分别在有、无视觉预期以及注意指向"托盘稳定"或者"重心稳定"的实验条件下观察外部姿势干扰对腰部姿势肌肉(L5~S1腰部多裂肌)和上肢动作肌肉(肱二头肌)预期姿势调节(anticipatory postural adjustments,APAs)和补偿姿势调节(compensatory postural adjustments,CPAs)相关时间和强度参数的影响。APAs和CPAs的时间和强度参数通过获取被检肌肉s EMG信号并参照相关检测规范进行。结果显示:(1)视觉预期对多裂肌的APAs启动时间,对肱二头肌的APAs启动时间、APAs强度和CPAs强度有显著影响;(2)注意指向对多裂肌的CPAs启动时间和肱二头肌APAs启动时间有显著影响;(3)视觉预期和注意指向对肱二头肌的APAs启动时间和APAs强度有交互作用。研究认为,突发外部姿势干扰条件下姿势肌肉和动作肌肉的姿势调节策略具有一定的差异,视觉预期和注意指向仅对姿势肌肉的时间参数有影响,对动作肌肉APAs和CPAs时间和强度参数都产生调节作用,表明在中枢神经系统的姿势控制中,人体姿势策略的调节是通过对姿势肌肉和动作肌肉的双重控制来完成的,视觉预期效应和心理指向效应反映在对不同功能肌肉前馈控制和反馈控制相应参数的调节。     

Some characteristics of EMG patterns during locomotion: implications for the locomotor control process

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

Some Characteristics of EMG Patterns During Locomotion

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

An analysis of air-stepping in normal infant vervet monkeys

Limb movements during air-stepping were analyzed in three neonatal vervet monkeys over a three-week period. The movements had similar temporal organization both across animals and across time. For example, the duration of both the hind and the forelimb cycle equaled about 500 ms, with hind limb return strokes lasting much longer than the hind limb power strokes. Furthermore, there were clear indications of both intra- and interlimb coordination. Specifically, all the joints of a limb tended to flex and extend simultaneously, and contralateral and ipsilateral limb pairs had an average phase relationship of approximately 50% of cycle duration. Despite a qualitative similarity between limb movements during air-stepping in the neonates and overground locomotion in older animals, there were notable differences both in temporal relationships and joint displacement patterns. Finally, there appeared to be important similarities between air-stepping in these monkeys and stepping in newborn humans. Most notably, both tended to disappear after a limited period. The implications of these similarities, as well as the overall results, are discussed in relation to the understanding of the development of locomotor behavior in human and nonhuman primates, using approaches based both upon the hard-wired and dynamic models.     

An Analysis of Air-Stepping in Normal Infant Vervet Monkeys

Limb movements during air-stepping were analyzed in three neonatal vervet monkeys over a three-week period. The movements had similar temporal organization both across animals and across time. For example, the duration of both the hind and the forelimb cycle equaled about 500 ms, with the hind limb return strokes lasting much longer than the hind limb power strokes. Furthermore, there were clear indications of both intra- and interlimb coordination. Specifically, all the joints of a limb tended to flex and extend simultaneously, and contralateral and ipsilateral limb pairs had an average phase relationship of approximately 50% of cycle duration. Despite a qualitative similarity between limb movements during air-stepping in the neonates and overground locomotion in older animals, there were notable differences both in temporal relationships and joint displacement patterns. Finally, there appeared to be important similarities between air-stepping in these monkeys and stepping in newborn humans. Most notably, both tended to disappear after a limited period. The implications of these similarities, as well as the overall results, are discussed in relation to the understanding of the development of locomotor behavior in human and nonhuman primates, using approaches based both upon the hard-wired and dynamic models.     

Human locomotor adjustments during perturbed walking

The locomotor adjustment induced by step perturbation of human subjects walking on a treadmill was described by a quantitative analysis of the EMG activities of selected trunk and leg muscles and by rotations of leg joints. The role of the proprioceptive input in the EMG reaction was also evaluated. The perturbation was obtained by a rapid and unexpected increase of belt speed. The motor response showed the stereotyped characteristics of a motor automatism and was accomplished without affecting the basic motor pattern of the gait. The EMG adjustment showed short-latency reflex responses (40–60 msec) of muscles acting at the joints more directly affected by the perturbing stimulus. This result supports the hypothesis of a spinal neuronal mechanism involved in the rapid adjustment of gait. The activity of primary spindle afferents seems to play an important role in the production of the faster EMG responses.     

Postural muscle responses following changing balance threats in young, stable older, and unstable older adults

The authors determined the postural muscle response to support surface perturbations, in relation to aging and level of stability of 16 young adults and 32 older adults who were classified into stable (SOA) and unstable (UOA) groups on the basis of their functional balance abilities. Forward and backward support surface translations of various amplitudes and velocities were used so that postural responses of the standing adults could be elicited. The thigh and leg postural muscle responses were recorded with surface electromyography (EMG). The older groups had significantly longer onset latency in the anterior postural muscles, smaller integrated EMG in the posterior muscles, and greater extent of integrated EMG attenuation over time. The UOA showed longer onset latency in the gastrocnemius following slow backward perturbation and used a greater percentage of the functional capacity of the gastrocnemius muscle than the SOA did. Those findings indicate that the SOA and UOA had limited ability to adapt to changing balance threats; the UOA were more limited than the SOA. When designing balance training programs, therefore, therapists should consider the adult's level of functional stability.     

Longitudinal Development of the Automatic Postural Response in Infants

In this longitudinal study, the development of automatic postural responses elicited during stance following perturbation of the support surface was investigated. Infants (N = 9) unable to stand without support were tested initially: follow-up tests were performed until the infants were able to stand and walk independently. Surface electromyographic recordings of leg and trunk muscle activity following a postural perturbation induced by a forward or backward translation of the support surface were made for each infant. Muscle onset latencies following the perturbation and the proportion of trials in which muscle activity was recorded were determined. First, infants activated appropriate muscles either in isolation or in pairs and then combined these muscles into functional synergies. Although activation of all three postural muscles was recorded in infants before they were able to stand and walk independently, the three-muscle response was not consistent in the youngest children. The proportion of trials eliciting muscle activity continued to increase (p <.O5) after infants began to walk independently, with postural muscle activity recorded in virtually every trial by late independent walking. Thus. the automatic postural response elicited during stance was shown to begin with activity in single or paired muscles, followed by activation of the postural muscles in functional synergies. These data illustrate the progressive development of an effective sensory-motor organization.     

Visual control of locomotion: strategies for changing direction and for going over obstacles.

Dynamics of gait adjustments required to go over obstacles and to alter direction of locomotion when cued visually were assessed through the measurement of ground reaction forces, muscle activity, and kinematics. The time of appearance of obstacles of varying heights, their position within the step cycle, and cue lights for direction change were varied. Direction change must be planned in the previous step to reduce the acceleration of the body center of mass toward the landing foot to 0. The inability of steering within the step cycle is due to the incapacity of muscles to rotate the body and translate it along the mediolateral axes. For obstacle avoidance, Ss systematically manipulated the gait patterns as a function of obstacle height and position and the time available within the ongoing step. Greater supraspinal involvement in control of locomotion is found.     

Discordance in Recovery Between Altered Locomotion and Muscle Atrophy Induced by Simulated Microgravity in Rats

Exposure to a microgravity environment leads to adverse effects in motion and musculoskeletal properties. However, few studies have investigated the recovery of altered locomotion and muscle atrophy simultaneously. The authors investigated altered locomotion in rats submitted to simulated microgravity by hindlimb unloading for 2 weeks. Motion deficits were characterized by hyperextension of the knees and ankle joints and forward-shifted limb motion. Furthermore, these locomotor deficits did not revert to their original form after a 2-week recovery period, although muscle atrophy in the hindlimbs had recovered, implying discordance in recovery between altered locomotion and muscle atrophy, and that other factors such as neural drives might control behavioral adaptations to microgravity.     

Mathematical Models of Central Pattern Generators in Locomotion

Possible neural connective patterns and functions with respect to interlimb coordination are studied theoretically with a mathematical model of the central pattern generating system for cat locomotion. Activities in populations of neurons controlling limb joint flexors and extensors in all four limbs are represented by a system of nonlinear differential equations. Solutions of the system for various parameter values simulate various gaits of the cat. The model is shown to be capable of generating all gaits of the cat and accounting for corresponding phase changes in interlimb coordination. The model also exhibits smooth changes of gait, and smooth initiation and termination of stepping. Further, within each limb, muscle sequencing, step cycle phases, and flexor-extensor interactions can be studied. The model suggests that one of the simplest mechanisms for a central command system to change the gait is via inhibition of specific interlimb propriospinal pathways. In a final section, properties of both proposed single limb and interlimb models are reviewed with specific reference to planning future experimental and theoretical studies.     

Perturbations of human posture: influence of impulse modality on EMG responses and cerebral evoked potentials

Leg muscle EMG responses and cerebral evoked potentials (CP), elicited by perturbations of stance while on a treadmill with split belts, were analyzed in order to study the relationship between compensatory leg muscle responses and afferent input to supraspinal centers. Various conditions of perturbation were used to establish the extent to which compensatory EMG responses and CPs show congruent behavior. Four different treadmill acceleration rates were applied in three different conditions (unilateral perturbation, directed forward or backward; bilateral perturbation, directed forward or backward; and opposing bilateral perturbation). EMG responses and CPs showed parallel increases in amplitude with increasing displacement velocity. The EMG responses showed distinct differences, predominantly in the response amplitude, between the different perturbation conditions, whereas the CPs were affected only to a minor degree. Tibialis anterior EMG responses were more closely related to the CP following forward perturbation than the corresponding gastrocnemius responses were to the CP following backward perturbation. We conclude that the EMG responses are more closely related than the CPs to displacement parameters and suggest that this is due to the further spinal processing of the afferent input needed to generate an appropriate EMG response. The closer relationship between the tibialis anterior response and CP may reflect a predominant central representation and control of tibialis anterior activation in the regulation of posture. The functional implications of these findings are discussed.     

Mathematical models of central pattern generators in locomotion: III. Interlimb model for the cat

Possible neural connective patterns and functions with respect to interlimb coordination are studied theoretically with a mathematical model of the central pattern generating system for cat locomotion. Activities in populations of neurons controlling limb joint flexors and extensors in all four limbs are represented by a system of nonlinear differential equations. Solutions of the system for various parameter values simulate various gaits of the cat. The model is shown to be capable of generating all gaits of the cat and accounting for corresponding phase changes in interlimb coordination. The model also exhibits smooth changes of gait, and smooth initiation and termination of stepping. Further, within each limb, muscle sequencing, step cycle phases, and flexor-extensor interactions can be studied. The model suggests that one of the simplest mechanisms for a central command system to change the gait is via inhibition of specific interlimb propriospinal pathways. In a final section, properties of both proposed single limb and interlimb models are reviewed with specific reference to planning future experimental and theoretical studies.     

Perturbations of Human Posture

Leg muscle EMG responses and cerebral evoked potentials (CP), elicited by perturbations of stance while on a treadmill with split belts, were analyzed in order to study the relationship between compensatory leg muscle responses and afferent input to supraspinal centers. Various conditions of perturbation were used to establish the extent to which compensatory EMG responses and CPs show congruent behavior. Four different treadmill acceleration rates were applied in three different conditions (unilateral perturbation, directed forward or backward; bilateral perturbation, directed forward or backward; and opposing bilateral perturbation). EMG responses and CPs showed parallel increases in amplitude with increasing displacement velocity. The EMG responses showed distinct differences, predominantly in the response amplitude, between the different perturbation conditions, whereas the CPs were affected only to a minor degree. Tibialis anterior EMG responses were more closely related to the CP following forward perturbation than the corresponding gastrocnemius responses were to the CP following backward perturbation.

We conclude that the EMG responses are more closely related than the CPs to displacement parameters and suggest that this is due to the further spinal processing of the afferent input needed to generate an appropriate EMG response. The closer relationship between the tibialis anterior response and CP may reflect a predominant central representation and control of tibialis anterior activation in the regulation of posture. The functional implications of these findings are discussed.