Abstract
The regulation of walking speed is easily achieved. However, the central nervous system (CNS) must coordinate numerous muscles in order to achieve a smooth and continuous control of walking speed. To control walking speed appropriately, the CNS may need to utilize a simplified system for the control of numerous muscles. Previous studies have revealed that the CNS may control walking via muscle synergies that simplify the control of muscles by modularly organizing several muscles. We hypothesized that the CNS controls the walking speed by flexibly modulating activation of muscle synergies within one gait cycle. Then, we investigated how the activation of muscle synergies depend on walking speeds using the center of activity (CoA) that indicates the center of the distribution of activation timing within one gait cycle. Ten healthy men walked on a treadmill at 14 different walking speeds. We measured the surface electromyograms (EMGs) and kinematic data. Muscle synergies were extracted using non-negative matrix factorization. Then, we calculated the CoA of each muscle synergy. We observed that the CoA of each specific synergy would shift as the walking speed changed. The CoA that was mainly activated during the heel contact phase (C1) and the activation that contributed to the double support phase (C3) shifted to the earlier phase as the walking speed increased, whereas the CoA that produced swing initiation motion (C4) and the activation that related to the late-swing phase (C5) shifted to the later phase. This shifting of the CoA indicates that the CNS controls intensive activation of muscle synergies during the regulation of walking speed. In addition, shifting the CoA might be associated with changes in kinematics or kinetics depending on the walking speed. We concluded that the CNS flexibly controls the activation of muscle synergies in regulation of walking speed.
Highlights
Healthy adults can adjust their walking speed in accordance with a given situation
Because the activations that produced forward propulsion (C2) were invariant, the center of activity (CoA) of the C2 was invariant with changes in the walking speed. These main findings indicate that the CoA that contributed to body support (C1), swing initiation (C3), swing phase (C4) and leg deceleration (C5) shifted as the walking speed changed
We observed that the CoAs that contributed to body support (C1), swing initiation (C3), swing phase (C4) and leg deceleration (C5) shifted as the walking speed changed
Summary
Healthy adults can adjust their walking speed in accordance with a given situation. Some muscle activities change non-linearly with an increasing walking speed as shown in the following literature. A detailed description follows below; the peak amplitude of hip extensor muscle activity in the mid-stance phase at extremely slow walking speeds is larger than at normal walking speeds (den Otter et al, 2004). This finding suggests that the muscle activity involved in single leg support largely contributes to slow walking speeds. The muscle activity changes non-linearly as the walking speed increases or decreases. The central nervous system (CNS) needs to construct a control system that solves the redundant degrees of freedom during walking
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