No abstract

This experiment investigates the interaction of different sensory cues in the control of propulsive forces in human gait which in turn allow the body's forward progression to be regulated. The aim of this work was to determine how optic flow and leg-somatosensory feedback interact in this control. We therefore determined whether the responses to sinusoidal perturbations of optic flow were accentuated when leg-somatosensory feedback was modified by varying the support resistance. Subjects walked on a treadmill which was driven by their own locomotor activity (1) with a sinusoidal variation of optic flow velocity, (2) with a sinusoidal variation of support resistance which modified leg-somatosensory information and (3) with both visual and leg-somatosensory modification at different frequencies. The response of the subject was measured as changes in speed and propulsive power. The response to sinusoidal perturbations of optic flow was found to be increased and time delayed when visual perturbations are coupled with support perturbations in comparison with the response observed with visual perturbations only. This result shows the influence of leg-somatosensory feedback on the weighting of optic flow. Inversely, it was also found that the motor response to support perturbation was different when the flow was congruent (i.e., corresponding to the subject's virtual speed) and when it was not. This latter result shows the influence of optic flow on the weighting of leg-somatosensory feedback. The interaction between optic flow and leg-somatosensory feedback argues in favor of a multimodal sensory control of propulsive forces. This multimodal sensory control would be based on all the sensory feedback and all their mutual sensorial interaction. Therefore, the modification of one sensory input modifies not only this input but also the integration of the other inputs.

The intentional control of stride length is a fundamental basis for the adaptation of the stride to environmental constraints (obstacle avoidance, for example). Controlling the propulsive forces during the stance and/or controlling the pendular movement of the oscillating leg constitute the two potential and non-exclusive mechanisms underlying intentional stride length modulation. The present experiment was conducted in order to determine if these two mechanisms contribute to voluntary length modulation and, if so, how they cooperate according to whether the subject has to lengthen or shorten a stride and how these mechanisms are implemented at the neuromuscular level. Subjects had to produce a temporarily modulated stride of the same length, but originating from two different initial steady-states: one from shorter stride length and one from longer stride length. We found that the shortening was essentially realized by a swing-duration decrease (an increased activity in the hip extensor--biceps femoris--during the swing of the ipsilaterally shortened stride stopped the pendular leg movement earlier). The lengthening was realized by two mechanisms: (1) an increase in the propulsive forces (via an increased activity of the ankle extensor muscles--soleus--and the hip extensors--biceps femoris--from the stance of the ipsilaterally modulated stride, which was prolonged during the following stance of the contralateral leg), and (2) an increase in swing duration on the ipsilateral leg (an increased activity in hip and ankle flexors--rectus femoris and tibialis anterior--maintained the ipsilateral leg in flexion during the lengthened swing so that the foot landed later). In this experiment, the subjects were faced with a spatial constraint of the same magnitude in the direction of stride lengthening and stride shortening. However, under these conditions, subjects used a different balance between swing control (that directly modifies the foot trajectory without affecting the trajectory of the head-arm-trunk system) and/or the control of propulsive forces (that indirectly influences foot trajectory by modifying the trajectory of the head-arm-trunk system). In the first case, this concerns a voluntary control of gesture produced by the legs and usually implicated in the locomotor pointing; in the second case, this concerns a voluntary control of propulsive forces.