Effect of joint position on the electromyographic activity of the semitendinosus muscle.

The semitendinosus (ST) muscle has a tendinous intersection within the muscle belly that separates the ST muscle into distinct proximal and distal compartments. Thus far, no study has compared the electromyographic (EMG) activities between the proximal and distal compartments of the human ST muscle. This study aimed to investigate the intramuscular EMG activity patterns of the proximal and distal compartments of the ST muscle by altering the hip and knee joint positions. The study population comprised eight healthy male volunteers. They performed ramp isometric knee flexion tasks form the relaxed state to the maximal voluntary contraction (MVC) state with (1) the hip and knee at 90 degrees and 0 degrees, respectively (90-0 position), (2) both the hip and knee at 00 (0-0 position), and (3) the hip and knee at 0 degrees and 90 degrees, respectively (0-90 position). Fine-wire electrodes were inserted into the proximal and distal compartments of the ST muscle and the individual EMG activities were recorded. In the 90-0 position, the EMG activity of the distal compartment was higher than that of the proximal compartment at 60%, 80%, and 90% MVC. Moreover, in the 0-90 position, the EMG activity of the proximal compartment was higher than that of the distal compartment at 60% MVC. These results indicated that the lengthened or shortened muscle conditions induced regional differences in the EMG activity patterns, while the two compartments showed equivalent activity when the muscle length was moderate.