Dynamic behavior of semitendinosus muscle during isometric knee flexions

The purpose of this study was to investigate semitendinosus muscle (ST) behavior during isometric knee flexions. Healthy male subjects were directed to exert ramp isometric knee flexion up to a maximum of eight knee flexion angles (0, 15, 30, 45, 60, 75, 90, 105 degrees). Displacement of the tendinous intersection (TI) in ST was measured using ultrasonographic images. TI moved proximally 15.8±4.9mm by maximum isometric contraction at 0 degrees, and the displacement of TI was significant at over 30% ramp maximum. Maximum knee flexion torque significantly correlated to displacement of TI at every knee angle, and decreased as the flexion angle increased. These results suggested that TI moved proximally in accordance with flexion torque, and demonstrated the unique behavior of ST, that is, the proximal part concentrically contracted, whereas the distal part eccentrically contracted.