Intentional Compensation for Selective Loading Affecting Human Gait Phases

The rat lumbosacral spinal cord adapts to robotic loading applied during stance

Load-related afferent information modifies the magnitude and timing of hindlimb muscle activity during stepping in decerebrate animals and spinal cord-injured humans and animals, suggesting that the spinal cord mediates load-related locomotor responses. In this study, we found that stepping on a treadmill by adult rats that received complete, midthoracic spinal cord transections as neonates could be altered by loading the hindlimbs using a pair of small robotic arms. The robotic arms applied a downward force to the lower shanks of the hindlimbs during the stance phase and measured the position of the lower shank during stepping. No external force was applied during the swing phase of the step. When applied bilaterally, this stance force field perturbed the hindlimb trajectories so that the ankle position was shifted downward during stance. In response to this perturbation, both the stance and step cycle durations decreased. During swing, the hindlimb initially accelerated toward the normal, unperturbed swing trajectory and then tracked the normal trajectory. Bilateral loading increased the magnitude of the medial gastrocnemius electromyographic (EMG) burst during stance and increased the amplitude of the semitendinosus and rectus femoris EMG bursts. When the force field was applied unilaterally, stance duration decreased in the loaded hindlimb, while swing duration was decreased in the contralateral hindlimb, thereby preserving interlimb coordination. These results demonstrate the feasibility of using robotic devices to mechanically modulate afferent input to the injured spinal cord during weight-supported locomotion. In addition, these results indicate that the lumbosacral spinal cord responds to load-related input applied to the lower shank during stance by modifying step timing and muscle activation patterns, while preserving normal swing kinematics and interlimb coordination.

Dynamic stabilization of rapid hexapedal locomotion

To stabilize locomotion, animals must generate forces appropriate to overcome the effects of perturbations and to maintain a desired speed or direction of movement. We studied the stabilizing mechanism employed by rapidly running insects by using a novel apparatus to perturb running cockroaches (Blaberus discoidalis). The apparatus used chemical propellants to accelerate a small projectile, generating reaction force impulses of less than 10 ms duration. The apparatus was mounted onto the thorax of the insect, oriented to propel the projectile laterally and loaded with propellant sufficient to cause a nearly tenfold increase in lateral velocity relative to maxima observed during unperturbed locomotion. Cockroaches were able to recover from these perturbations in 27±12 ms(mean ± S.D., N=9) when running on a high-friction substratum. Lateral velocity began to decrease 13±5 ms (mean ± S.D., N=11) following the start of a perturbation, a time comparable with the fastest reflexes measured in cockroaches. Cockroaches did not require step transitions to recover from lateral perturbations. Instead, they exhibited viscoelastic behavior in the lateral direction, with spring constants similar to those observed during unperturbed locomotion. The rapid onset of recovery from lateral perturbations supports the possibility that, during fast locomotion, intrinsic properties of the musculoskeletal system augment neural stabilization by reflexes.

The top down and bottom up mechanisms involved in the sudden awareness of low level sensorimotor behavior

Motor control can be achieved in the absence of awareness, even when performed intentionally. The aim of this study was to understand the mechanisms of the sudden awareness of our own movement. This was studied in locomotion because it is an automatic behavior which can be intentionally modulated. Subjects walked continuously with the instruction to maintain either a constant walking speed (compensation condition) or constant propulsive forces (no-intervention condition); they were sometimes faced with slow variations in resistance that they had to detect. The results show that: (1) the subject remains unaware of his force increase (in compensation) or his walking velocity decrease (in no-intervention) for a long time, although these modifications go largely beyond the variability range in which he is able to intentionally control his force (in no-intervention) or his velocity (in compensation) and (2) the detection of the resistance increase occurs at the same time in both conditions. We conclude that the sudden awareness of a movement pattern produced at a low level was found to emerge from the interaction between a top down mechanism where the intentional control of goal feedback delays the aware perception of the other sensory sources and a bottom up mechanism where high level mechanisms of sensorimotor integration come into play beyond a discrepancy threshold between different sensory information.

Joint-dependent mechanisms to adapt to an imbalance between flexion and extension forces in human gait

In human gait, alternating leg flexion/extension movements essentially require the production of extension muscle forces due to the large contribution of passive forces to leg flexion. In this experiment, we studied the adaptive capabilities of walking subjects constrained with elastic cords which further facilitated leg flexion and impeded leg extension. In order to walk, the subjects let the moments created by the elastic cords increase the ankle flexion during the whole cycle, which allowed them to reduce part of these moments. By contrast, at the knee level, they increased their extension muscle activity to compensate for the remaining constraint moments during the swing phase, which resulted in unchanged kinematics. Although neuromuscular locomotor synergy is often considered to control the lower limb as a unit, we showed here that different adaptive mechanisms can act at different joints of the same leg.

Steady-state fluctuations of human walking

In steady-state walking, fluctuations in space-time behavior are observed for normal adult subjects. In the present study, the intrinsic fluctuations of gait have been analyzed when walking on a subject-driven treadmill (with adjustable inertial forces). Furthermore, these intrinsic fluctuations have been compared with those observed in natural overground locomotion which involves a real subject's displacement and thus an optical flow. Four adult subjects participated in both experimental sessions. It was found that the frequency and amplitude of the instantaneous fluctuations of leg movement were weak and of equal magnitude with or without optical flow. This was also the case for instantaneous fluctuations in displacement speed. Secondly, a low-frequency fluctuation in walking speed was observed when no optical flow information was available to the subject. This fluctuation results from the addition of a series of leg-movement fluctuations, whose values are all either positive or negative. As the optical flow provides information about the displacement speed, it allows the subject to avoid such addition, and thus plays a role in maintaining steady leg movement. Theoretical models linking space-time behavior of rhythmic movement with stiffness strongly suggest that the observed low-frequency fluctuations in speed result from fluctuations in stiffness.