Contraction specific changes in passive torque in human skeletal muscle.

The present investigation examined passive torque and electromyographic response in human skeletal muscle during passive static stretch within 60 s after maximal repetitive eccentric and concentric contractions. Passive torque (Nm) offered by the hamstring muscle group was measured during passive knee extension in a modified dynamometer in 10 subjects. The distal thigh was elevated to 0.52 rad from horizontal and the backrest was positioned at 1.57 rad. The lever arm moved the leg passively at 0.09 rad s-1 from a starting position of 1.48 rad below horizontal to the final position where it remained stationary for 90 s. Gross electrical activity of the human hamstring muscle group was recorded simultaneously. The effect of concentric or eccentric contraction was tested on separate days. Two stretch manoeuvres with a 45 min hiatus were administered on a control and experimental side. The experimental side performed 40 maximal effort repetitive concentric or eccentric hamstring muscle contractions at 1.05 rad s-1 prior to the second stretch. Passive torque during the 90 s stretch declined 30-35% on the experimental and control side in all stretches, P < 0.001, without a significant effect of prior contraction mode. Passive peak and final torques were lower on the experimental side, P < 0.01 after concentric contractions. Passive peak and final torques remained unchanged after eccentric contractions on the experimental side. The low level EMG response of the hamstring muscle during the stretch was unchanged after maximal repetitive concentric or eccentric contractions. These data demonstrate acute contraction specific alteration in passive torque in human skeletal muscle, which cannot be accounted for by EMG activity. Furthermore, the lack of difference on the control side implies that one 90 s stretch has no effect on passive torque of the muscle 45 min later.