Effect of contraction force and knee joint angle on the spatial representation of soleus activity using high-density surface EMG

The meaningful use of surface electromyographic signals (sEMG) is to find an electrode position and orientation in which the sEMG signals can be detected reliably. This becomes more challenging when muscles with pinnate fiber architecture are investigated. In this study, the effects of contraction force and knee inclination on the spatial representation of the soleus muscle activity on the skin surface have been investigated by using two-dimensional electrode grids. Four differently oriented bipolar leads have been calculated to identify not only a proper electrode location but also an adequate orientation of the bipolar lead. Relative measures have been introduced to compare changes in the spatial RMS distribution. It has been shown that in the case of the soleus muscle, bipolar electrodes should be placed on the lateral side. Additionally, the location of the electrodes should be rather proximal than distal, and the orientation of the bipolar lead should be 45° to the lateral side with respect to a line connecting the insertion of the Achilles tendon and the junction between both gastrocnemius heads. Our results have been used to identify adequate electrode locations and orientations in a muscle with such a complex architecture like the soleus muscle. Additionally, new parameters have been introduced, helping to analyze the resulting information about the spatial activation pattern in the soleus muscle.