Spatiotemporal modulation of a fixed set of muscle synergies during unpredictable and predictable gait perturbations in older adults

Muscle synergies as functional low-dimensional building blocks of the neuromotor system regulate the activation patterns of muscle groups in a modular structure during locomotion. The purpose of the current study was to explore how older adults organize locomotor muscle synergies to counteract unpredictable and predictable gait perturbations during the perturbed and the recovery steps. Sixty-three healthy older adults (71.2 ± 5.2 years) participated in the study. Mediolateral and anteroposterior unpredictable and predictable perturbations during walking were introduced using a treadmill. Muscle synergies were extracted from the electromyographic activity of 13 lower limb muscles using Gaussian non-negative matrix factorization. The four basic synergies responsible for unperturbed walking (weight acceptance, propulsion, early swing and late swing) were preserved in all applied gait perturbations, yet their temporal recruitment and muscle contribution in each synergy were modified (p<0.05). These modifications were observed up to four recovery steps and were more pronounced (p<0.05) following unpredictable perturbations. The recruitment of the four basic walking synergies in the perturbed and recovery gait cycles indicates a robust neuromotor control of locomotion by using activation patterns of a few and well-known muscle synergies with specific adjustments within the synergies. The selection of pre-existing muscle synergies while adjusting the time of their recruitment during challenging locomotor conditions may facilitate the effectiveness to deal with perturbations and promote the transfer of adaptation between different kinds of perturbations. Summary statement The flexible recruitment of the four and well-known muscle synergies responsible for unperturbed walking during unpredictable and predictable gait perturbations indicates an effective way to counteract locomotor perturbations, where fast reactive responses are necessary to maintain postural stability.