Motor control strategies can be described by muscle synergies, a model of functional muscle recruitment to perform a movement. However, stability of muscle synergies during locomotion has not yet been investigated. The objective of this work was the evaluation of the stability of muscle synergies while walking at normal (NS) and fast (FS) speed. Each walking condition was tested during a prolonged session lasting 5 minutes on five healthy subjects. After data processing with statistical gait analysis, 168±29 valid strides in NS and 181±48 in FS were obtained. They were aggregated in subgroups, with 10 strides each. Muscle synergies were extracted for all subgroups with non-negative matrix factorization. On the average, 6 synergies were suitable to reconstruct the original electromyographic signal. They were functionally correlated to the activities of propulsion, trunk stability, limb deceleration at the end of swing, forefoot control, and limb stiffening for initial contact stability. To compare muscle synergy stability over time, a similarity measurement was carried out. This showed that from 1 to 3 synergies were unstable in NS. As for the FS condition, only one subject showed unstable synergies, corresponding to the hip stabilizing synergy.
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