Spatial variation of surface electromyography amplitude and force relationships of proximal, middle, and distal sites of vastus lateralis muscle at various knee joint angles.

Surface electromyography (sEMG) provides useful information to understand neuromuscular activity during various human movements. However, we need to pay attention to the interpretation of sEMG signals during dynamic movements because sEMG signals are affected by changes in the geometrical arrangement between the electrode and muscle fibers/innervation zone when the joint angle is changed. This study aimed to quantify the effect of changes in the joint angle on the sEMG amplitude and its spatial variation at various sites along a muscle. Thirteen male performed isometric knee extension at 20, 40, 60, 80, and 100 % of maximal voluntary contraction (MVC) and at inner knee joint angles of 80°, 100°, 120°, 140°, and 160°. High-density sEMG was recorded using 64-channel electrode grids at the proximal, middle, and distal sites of the vastus lateralis muscle and the sEMG amplitude was normalized to that at an 80°knee joint angle during MVC for each electrode. Mean values and standard deviations of the normalized sEMG amplitudes within each electrode grid were calculated. Significant effects of the electrode site, joint angle, and force levels were observed in these values. The distal site showed significantly lower mean values at 20-60 % MVC but not at 80-100 % MVC. The standard deviation was significantly greater at distal sites at 120°-160°of the knee joint angle and at 60-100 % MVC. These findings suggest that the sEMG amplitude in the vastus lateralis muscle may be less sensitive to geometrical changes when signals are detected at proximal and middle sites. Therefore, caution should be taken when assessing sEMG signals from the distal site, as they may be more susceptible to spatial variations due to joint angle and muscle contraction level.