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Muscle synergies are specific muscle groups activated in coordination to achieve a particular behavior, or perform a given task. In this work, we compared muscle synergies extracted from quiet bipedal stance surface electromyogram data, in different experimental conditions. We propose these synergies will provide information regarding variations in the strategies used by the Central Nervous System (CNS) for postural control in different situations. The analyzed conditions were: with and without an additional sensory cue (here represented by Light Touch); and standing on a stable versus an unstable surface (here represented by a FOAM pad). Synergies were extracted by means of the Non-Negative Matrix Factorization algorithm. We compared the Synergy Stability Index (SSI) and the Synergy Coordination Index (SCI) between each subject's trials in each condition. We observed that SCI was increased by the presence of Light Touch, while decreased by the unstable surface (FOAM pad). This indicates that, while an unstable surface reduces synergy coordination, the introduction of a sensory cue is able to improve it, thus reducing synergy space. These findings were even more evident when evaluating subjects with higher SSI, which has been associated with better balance performance.
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