Methodological issues in the assessment of motor control during single-leg stance

In the study of muscle synergies during the maintenance of single-leg stance (SLS) there are methodological issues that must be taken into account before performing the synergy extraction. In particular, it is important to distinguish between epochs of surface electromyographic (sEMG) signals corresponding to a 'good' balance control during the SLS test, from those characterized by an 'excessive' body sway. The aim of this work is to assess the robustness in the segmentation and selection of sEMG signal epochs to be chosen as input for the synergy extraction algorithm. The robustness is evaluated in terms of: 1) consistency of the number of muscle synergies, and 2) weight vector correlation. Our results show that the same number of muscle synergies and similar weight vectors are obtained, independently from the threshold chosen to build the segmentation mask. The methodology proposed may help the interpretation of muscle synergies in SLS test.