Greater distal excitation of the biceps femoris long head reflects proximodistal differences in motor unit action potential properties

Evidence on regional muscle excitation within hamstrings remains controversial, primarily because information derived solely from surface electromyograms (sEMG) amplitude does not necessarily provide an accurate estimate of neural drive to the muscle. To address this limitation, this study investigated whether there are proximodistal differences in motor unit properties of the biceps femoris long head during isometric hip extension and knee flexion tasks. Seventeen resistance-trained males performed isometric knee flexion and hip extension tasks at 20 % and 40 % of maximal voluntary contraction. High-density sEMG were recorded from proximal and distal regions of the biceps femoris long head and decomposed into individual motor units. Central motor unit and action potential properties were analyzed. Bipolar sEMG amplitude was also calculated for each region to simulate traditional sEMG measurements. Bipolar sEMG amplitude, motor unit action potential amplitude and conduction velocity were significantly greater in the distal region during both tasks. In contrast, no proximodistal differences were observed in central motor unit properties. These findings suggest that increased bipolar sEMG amplitude in the distal region of the biceps femoris long head is driven by motor unit action potential properties rather than differences in central modulation, likely influenced by intra-muscular variations in muscle mechanics and geometry.