Passive stiffness of the biarticular hamstring muscles could acutely decrease after eccentric-only resistance exercise, but not after passive muscle lengthening: Effects of exercise load.

This study aimed to investigate the acute changes in the passive stiffness of biarticular hamstring muscles after passive stretching and eccentric-only resistance exercise performed at different loads. Thirteen healthy young male participants performed four exercise sessions (on separate days) that comprised passive knee extension (0% of maximal eccentric torque) and eccentric-only knee flexion at different loads (25%, 50% and 75%). Maximal knee joint range of motion, passive torque, shear moduli of the biarticular hamstring muscles, and maximal isometric torque were measured before, 5 min, and 30 min after completing each session. The shear moduli of the biarticular hamstring muscles did not significantly change after passive knee extension, whereas they significantly decreased immediately after eccentric-only knee flexion at one or more loads ranging from 25% to 75%. Significant immediate decreases in the shear modulus were observed in the biceps femoris long head at 25%, 50%, and 75%; in the semitendinosus at 50%; and in the semimembranosus at 50% and 75%. Eccentric contraction appears to have a greater acute effect than passive lengthening in acutely decreasing the passive stiffness of the biarticular hamstring muscles, and the exercise load required for acute decrease in passive stiffness is muscle-dependent under the present experimental conditions.