Effect of sEMG electrode positioning on muscle synergies assessment during walking

The central nervous system coordinates the contraction of many muscles during human motion. A hypothesis currently accepted by the scientific community is that CNS does not control each muscle separately during a motor task (e.g. walking), but groups together the muscles which perform similar functions during the task. The complexity of the motor control is thus reduced through the selection of an adequate number of motor modules, called muscle synergies. Muscle synergies are studied using factorization techniques applied to the sEMG signals sampled from the muscles involved in the motor task. Usually, muscle activation is assessed using a bipolar sEMG detection using a pair of electrodes for each muscle. However, recent studies highlighted that different muscle regions, controlled by different nerve branches, may contribute to different mechanical actions. For example, in the rectus femoris muscle, the proximal part is mainly involved in hip flexion movements, whereas the distal one is involved in knee extension. This fact might lead to the dependency of the muscle synergies from the electrode positioning. The study objective is to verify whether the electrode positioning over the muscles affects the extracted synergies. Twenty healthy volunteers participated in the study. Subjects were asked to walk at different speeds on a treadmill while sEMG from lower limb muscles and basographic signals were recorded. For rectus femoris, vastus medialis, biceps femoris, semitendinosus, gastrocnemius, and soleus two different electrode placements were used. Muscle synergies were extracted using the Non-Negative Matrix Factorization (NNMF) algorithm. Results show, 4 out of 6 muscles, a statistical difference in the sEMG envelopes depending on the electrode positioning; However, this difference is not always recognised by the synergies, since the model tends to group muscle activities, hiding small differences in myoelectric activation. The impact of these results from the clinical point of view must be assessed on the basis of the clinical question.