Abstract
Bipedal locomotion is a basic motor activity that requires simultaneous control of multiple muscles. Physiological experiments suggest that the nervous system controls bipedal locomotion efficiently by using motor modules of synergistic muscle activations. If these modules were merged, abnormal locomotion patterns would be realized as observed in patients with neural impairments such as chronic stroke. However, sub-acute patients have been reported not to show such merged motor modules. Therefore, in this study, we examined what conditions in the nervous system merges motor modules. we built a two-dimensional bipedal locomotion model that included a musculoskeletal model with 7 segments and 18 muscles, a neural system with a hierarchical central pattern generator (CPG), and various feedback inputs from reflex organs. The CPG generated synergistic muscle activations comprising 5 motor modules to produce locomotion phases. Our model succeeded to acquire stable locomotion by using the motor modules and reflexes. Next, we examined how a pathological condition altered motor modules. Specifically, we weakened neural inputs to muscles on one leg to simulate a stroke condition. Immediately after the simulated stroke, the model did not walk. Then, internal parameters were modified to recover stable locomotion. We refitted either (a) reflex parameters or (b) CPG parameters to compensate the locomotion by adapting (a) reflexes or (b) the controller. Stable locomotion was recovered in both conditions. However the motor modules were merged only in (b). These results suggest that light or sub-acute stroke patients, who can compensate stable locomotion by just adapting reflexes, would not show merge of motor modules, whereas severe or chronic patients, who needed to adapt the controller for compensation, would show the merge, as consistent with experimental findings.
Highlights
Bipedal locomotion is a basic motor activity
The rectus femoris (RF) produced the peak of the muscle activities in the middle of the gait cycle in the measured data
The model successfully acquired the stable walking again, and the activity patterns of motor modules were unaffected against the manipulation, there were some marked differences in muscle activity patterns
Summary
Several studies suggest that animal locomotion is controlled by central pattern generators (CPGs) in the spinal cord (Grillner, 1975). CPGs provide a rhythmic motor activity across multiple muscles in a coordinated manner in both space and time (Guertin, 2009). They generate coordinated flexor-extensor muscles’ activity, and adapt gait patterns to environmental changes by using sensory feedback. The motor modules hypothesis (Dominici et al, 2011; Lacquaniti et al, 2012) proposes that the motor system groups muscles into a smaller number of functional modules based on CPGs. Physiological experiments suggest that human locomotion may involve the motor modules of synergistic muscle activations.
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