Motion parallax is used to control postural sway during walking

Three experiments tested the hypothesis that postural sway during locomotion is visually regulated by motion parallax as well as optical expansion. Oscillating displays of three-dimensional scenes were presented to participants walking on a treadmill, while postural sway was recorded. Displays simulated: (a) a cloud, in which parallax and expansion are congruent, (b) a hallway, (c) the side walls of the hallway, (d) a ground surface, (e) a wall, (f) the wall with a central hole, (g) a hall farther from the observer, and (h) a wall farther from the observer. In contrast to previous results with a hallway, responses with the cloud were isotropic and directionally specific. The other displays demonstrated that motion parallax was more effective than simple horizontal flow in eliciting lateral sway. These results are consistent with the hypothesis that adaptive control of sway during walking is based on congruent expansion and parallax in natural environments.     

Adaptation reveals independent control networks for human walking

Human walking is remarkably adaptable on short and long timescales. We can immediately transition between directions and gait patterns, and we can adaptively learn accurate calibrations for different walking contexts. Here we studied the degree to which different motor patterns can adapt independently. We used a split-belt treadmill to adapt the right and left legs to different speeds and in different directions (forward versus backward). To our surprise, adults could easily walk with their legs moving in opposite directions. Analysis of aftereffects showed that walking adaptations are stored independently for each leg and do not transfer across directions. Thus, there are separate functional networks controlling forward and backward walking in humans, and the circuits controlling the right and left legs can be trained individually. Such training could provide a new therapeutic approach for correcting various walking asymmetries.     

Hypothetical neural control of human bipedal walking with voluntary modulation

A hypothetical neuromusculoskeletal model is developed to simulate human normal walking and its modulated behaviors. A small set of neural periodic patterns drive spinal muscle synergies which in turn lead to specific pattern of muscle activation and supraspinal feedback systems maintain postural balance during walking. Then, the model demonstrates modulated behaviors by superimposing voluntary perturbations on the underlying walking pattern. Motions of kicking a ball and obstacle avoidance during walking are simulated as examples. The superposition of the new pulse command to a set of invariant pulses representing spino-locomotor is sufficient to achieve the coordinated behaviors. Also, forward bent walking motion is demonstrated by applying similar superposition. The composition of activations avoids a complicated computation of motor program for a specific task and presents a simple control scheme for different walking patterns.     

Functional coupling of motor units is modulated during walking in human subjects

Time- and frequency-domain analysis of the coupling between pairs of electromyograms (EMG) recorded from leg muscles was investigated during walking in healthy human subjects. For two independent surface EMG signals from the tibialis anterior (TA) muscle, coupling estimated from coherence measurements was observed at frequencies 相似文献   

Visual control of walking velocity

Even if optical correlates of self-motion velocity have already been identified, their contribution to the control of displacement velocity remains to be established. In this study, we used a virtual reality set-up coupled to a treadmill to test the role of both Global Optic Flow Rate (GOFR) and Edge Rate (ER) in the regulation of walking velocity. Participants were required to walk at a constant velocity, corresponding to their preferred walking velocity, while eye height and texture density were manipulated. This manipulation perturbed the natural relationship between the actual walking velocity and its optical specification by GOFR and ER, respectively. Results revealed that both these sources of information are indeed used by participants to control walking speed, as demonstrated by a slowing down of actual walking velocity when the optical specification of velocity by either GOFR or ER gives rise to an overestimation of actual velocity, and vice versa. Gait analyses showed that these walking velocity adjustments result from simultaneous adaptations in both step length and step duration. The role of visual information in the control of self-motion velocity is discussed in relation with other factors.     

Changes in corticospinal excitability following adaptive modification to human walking

Locomotor adaptations to a novel environment can be measured through changes in muscle activity patterns and lower limb kinematics. The location and mechanisms underlying these adaptive changes are unknown. The purposes of the current study were (1) to determine whether corticospinal tract (CST) excitability is altered by resisted walking and (2) to ascertain whether changes in cortical excitability are muscle specific. Forty healthy participants walked with a robotic gait device (Lokomat) that applied a velocity-dependent resistance against hip and knee movements during walking. CST excitability was assessed by quantifying motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation immediately before and after baseline and resisted walking. MEPs were measured in either the biceps femoris (BF) or the rectus femoris (RF). Recruitment curves were collected by stimulating in 5 % increments from 105 to 145 % of active motor threshold. Results demonstrated a significant increase in MEP amplitude in the BF following baseline walking in the Lokomat. The RF did not demonstrate these changes. There was no further change in MEP size following resisted walking in either muscle group. These results suggest that locomotion increases CST excitability in a muscle-specific fashion. As such, it may be important for determining how to enhance the central nervous system’s ability to integrate adaptive strategies during walking.     

Recumbent stepping has similar but simpler neural control compared to walking

The purpose of this study was to compare muscle activation patterns and kinematics during recumbent stepping and walking to determine if recumbent stepping has a similar motor pattern as walking. We measured joint kinematics and electromyography in ten neurologically intact humans walking on a treadmill at 0 and 50% body weight support (BWS), and recumbent stepping using a commercially available exercise machine. Cross correlation of upper and lower limb electromyography patterns between conditions revealed high correlations for most muscles. A principal component analysis revealed that the first factor accounted for more muscle activation signal content during recumbent stepping (81%) than during walking (70%). This indicates that the motor pattern during walking is more complex than during stepping. Cross correlation analysis found a high correlation between factors for recumbent stepping and walking (R = 0.54), though not as high as the correlation between factors for walking at 0% BWS and walking at 50% BWS (R = 0.68). There were substantial differences in joint kinematics between walking and recumbent stepping, most notably in hip, elbow, and shoulder motions. These results suggest that although the two tasks have different kinematic patterns, recumbent stepping relies on similar neural networks as walking. Individuals with neurological impairments may be able to improve walking ability from recumbent stepping practice given similarities in neural control between the two tasks.     

控制犬狂犬病是消除我国人间狂犬病的关键

中国人间狂犬病自1996年以来一直持续上升,2007年报告3293个狂犬病死亡病例,为1996年狂犬病死亡病例数（159）的20．7倍。狂犬病的持续上升引起了国家有关部门的高度重视。为了尽快控制住狂犬病的流行,卫生部、农业部和中国疾控中心于2005-2009年先后发布了一系列预防控制狂犬病的指导性文件、规范和标准。     

Joint-level kinetic redundancy is exploited to control limb-level forces during human hopping

Compensatory mechanisms can take advantage of neuromechanical redundancy to meet global task goals in spite of local injuries or perturbations. We hypothesized that joint-level kinetic redundancy is also exploited during intact, unperturbed human locomotion to accomplish limb-level force goals. The limb-level force goals of hopping in place at a constant frequency are minimizing cycle-to-cycle variance of vertical ground reaction force and varying horizontal (fore-aft) ground reaction force to make backward and forward corrections in position from cycle to cycle. Uncontrolled Manifold analysis of joint torque variance showed that hoppers exploited redundancy to minimize vertical force variance at landing, mid-stance, and takeoff, and to vary horizontal force at landing and takeoff. Timing fluctuations, however, increased vertical force variance. We conclude that joint torque variance is not random noise, but has functional relevance and is purposefully structured to meet specific locomotor goals. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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.     

Individual characteristics of human walking mechanics

 Twenty-four subjects walked at different speeds (V) from 0.4 to 2.6 m s^–1, while motion and ground reaction forces were recorded in 3-D space. The total mechanical energy of each body segment was computed as the sum of the gravitational potential, translation and rotation kinetic energies. Energy profiles reveal that there are inter-individual differences, particularly at moderate and fast V. In some subjects, the energy excursions are less pronounced, and tend to evolve out of phase at the lower limbs and trunk. As a consequence, there is a better transfer of energy between the trunk and the leg segments, resulting in smaller oscillations of the net energy of the whole body. There is a threefold variation of the rate of increment of lnP _u (the mass-specific mean absolute power) with lnV across subjects. We show that this variability cannot be simply explained on the basis of the different biomechanical characteristics of the subjects, but that it depends on the different kinematic strategies. Subjects differ in their ability to minimize energy oscillations of their body segments and to transfer mechanical energy between the trunk and the limbs. Individual characteristics of the mechanical energy expenditure were correlated with the corresponding kinematic characteristics. The changes of the elevation angles of the lower limb segments covary along a plane in all subjects. Plane orientation (quantified by the direction cosine of the normal with the thigh axis, u _3t ) at any V is not the same in all subjects, but correlates with the net power output: smaller values of u _3t tend to be associated with smaller values of P _u , and vice versa. Received: 13 November 1997 / Accepted: 20 March 1998     

Feedback control system for walking in man

A computer control stimulation system is described which has been successfully tested by allowing a paraplegic subject to stand and walk through closed loop control. This system is a Z80 microprocessor system with eight channels of analog to digital and 16 channels of digital to analog control. Programming is written in CPM and works quite successfully for maintaining lower body postural control in paraplegics. Further expansion of this system would enable a feedback control system for multidirectional walking in man.     

Turning strategies during human walking.

The mechanisms involved in rapidly turning during human walking were studied. Subjects were asked to walk at a comfortable speed and to turn toward the instructed direction as soon as they felt an electrical stimulus to the superficial peroneal nerve. Stimuli were presented repeatedly at random over 10- to 15-min periods of walking for turning in both directions. Electromyograms (EMGs), joint angular movements of the right leg, and forces under both feet were recorded. The step cycle was divided into 16 parts, and the responses to stimuli in each part were analyzed separately. Two turning strategies were used, depending on which leg was placed in front for braking. For example, to turn to the right when the right foot was placed in front, subjects generally altered direction by spinning the body around the right foot (spin turn). To turn left when the right foot was in front, subjects shifted weight to the right leg, externally rotated the left hip, stepped onto the left leg, and continued turning until the right leg stepped in the new direction (step turn). The step turn is easy and stable because the base of support during the turn is much wider than in the spin turn, so some subjects used it in all parts of the cycle. Initially, the deceleration of walking is similar to a rapid stopping task, which has been previously examined. The deceleration mechanism involves a sequence of distal-to-proximal activation of muscles on one side of the body (soleus, biceps femoris, and erector spinae). This pattern is similar to the "ankle strategy" used in postural control during forward sway. The control of foot placement in the swing leg and muscle activities for rotating the trunk in the stance leg occurred within a step after the cue. The action of ankle inverters and elevation of the pelvis by activity of gluteus medius may contribute to the control of trunk rotation. This activity was closely related to the timing of the opposite foot strike, independent of the part of the step cycle when the stimulus was applied. In most subjects, the turn was completed without resetting the underlying walking rhythm. This first EMG analysis of rapid turning shows how common strategies for postural sway and stopping can be combined with one of two turning strategies. This simplifies the complex task of turning at a random time in the step cycle.     

How is the normal locomotor program modified to produce backward walking?

Summary The modifications occurring in the movement and muscle activity patterns of the leg when changing from forward to backward walking were studied in five healthy subjects during walking on a motor driven treadmill. Movements were recorded with a Selspot optoelectronic system and muscle activity with electromyography using surface electrodes. The movement trajectories of the leg in forward and backward walking essentially mirrored each other, even though the movements occurred in the reversed direction. The angular displacements at the hip, knee and ankle joints showed similar overall magnitude and pattern in the two situations. Most of the investigated muscles changed their pattern of activity in relation to the different movement phases. At the ankle, there was a switch between flexors and extensors with flexor activation during support in backward walking. The bursts of activity in knee extensors were prolonged and shifted to the main part of the support phase. In the hip extensors, the activity periods retained their positions relative to the leg movements, but changed function due to the reversed direction of movement. Thus, drastic changes occur in the normal locomotor program to produce a reversal of leg movements and propulsion backwards.     

Humans robustly adhere to dynamic walking principles by harnessing motor abundance to control forces

Human walking dynamics are typically framed in the context of mechanics and energetics rather than in the context of neuromuscular control. Dynamic walking principles describe one helpful theoretical approach to characterize efficient human walking mechanics over many steps. These principles do not, however, address how such walking is controlled step-by-step despite small perturbations from natural variability. Our purpose was to identify neuromechanical control strategies used to achieve consistent and robust locomotion despite natural step-to-step force variability. We used the uncontrolled manifold concept to test whether human walkers select combinations of leading and trailing leg-forces that generate equivalent net-force trajectories during step-to-step transitions. Subjects selected leading and trailing leg-force combinations that generated consistent vertical net-force during step-to-step transitions. We conclude that vertical net-force is an implicit neuromechanical goal of human walking whose trajectory is stabilized for consistent step-to-step transitions, which agrees with the principles of dynamic walking. In contrast, inter-leg-force combinations modulated anterior–posterior net-force trajectories with each step to maintain constant walking speed, indicating that a consistent anterior–posterior net-force trajectory is not an implicit goal of walking. For a more complete picture of hierarchical locomotor control, we also tested whether each individual leg-force trajectory was stabilized through the selection of leg-force equivalent joint-torque combinations. The observed consistent vertical net-force trajectory was achieved primarily through the selection of joint-torque combinations that modulated trailing leg-force during step-to-step transitions. We conclude that humans achieve robust walking by harnessing inherent motor abundance of the joints and legs to maintain consistent step-by-step walking performance.