Abstract
The need to move over uneven terrain is a daily challenge. In order to face unexpected perturbations due to changes in the morphology of the terrain, the central nervous system must flexibly modify its control strategies. We analysed the local dynamic stability and the modular organisation of muscle activation (muscle synergies) during walking and running on an even- and an uneven-surface treadmill. We hypothesized a reduced stability during uneven-surface locomotion and a reorganisation of the modular control. We found a decreased stability when switching from even- to uneven-surface locomotion (p < 0.001 in walking, p = 0.001 in running). Moreover, we observed a substantial modification of the time-dependent muscle activation patterns (motor primitives) despite a general conservation of the time-independent coefficients (motor modules). The motor primitives were considerably wider in the uneven-surface condition. Specifically, the widening was significant in both the early (+40.5%, p < 0.001) and late swing (+7.7%, p = 0.040) phase in walking and in the weight acceptance (+13.6%, p = 0.006) and propulsion (+6.0%, p = 0.041) phase in running. This widening highlighted an increased motor output’s robustness (i.e. ability to cope with errors) when dealing with the unexpected perturbations. Our results confirmed the hypothesis that humans adjust their motor control strategies’ timing to deal with unsteady locomotion.
Highlights
The need to move over uneven terrain is a daily challenge
We evaluated the centre of activity (CoA) and full width at half maximum (FWHM) for the resulting curves of the extracted spinal maps and motor primitives in both conditions and types of locomotion
We hypothesized a decrease in the dynamic stability and a transfer from an accurate to a more robust motor control during US locomotion
Summary
The need to move over uneven terrain is a daily challenge. In order to face unexpected perturbations due to changes in the morphology of the terrain, the central nervous system must flexibly modify its control strategies. The widening was significant in both the early (+40.5%, p < 0.001) and late swing (+7.7%, p = 0.040) phase in walking and in the weight acceptance (+13.6%, p = 0.006) and propulsion (+6.0%, p = 0.041) phase in running This widening highlighted an increased motor output’s robustness (i.e. ability to cope with errors) when dealing with the unexpected perturbations. The same amount of basic activation patterns could be found in patients with spinal cord injury and in healthy participants at different speeds and gravitational loads[47] Synergies similar to those found in humans at a spinal[47] or muscular level can be observed in the motor cortex of the primate and cat[48,49]. We used an unsupervised learning method called non-negative matrix factorisation (NMF)[50] for reducing the high dimensional EMG input into a small number of synergies
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