Guissard, N., & Duchateau, J. (2004). Effect of static stretch training on neural and mechanical properties of the human plantar-flexor muscles. Muscle and Nerve, 29, 248-255.

To determine the contributions of neural and mechanical mechanisms to the limits in the range of motion about a joint, the effects of 30 sessions of static stretch training on the characteristics of the plantar-flexor muscles in 12 subjects were studied. Changes in the maximal ankle dorsiflexion and the torque produced during passive stretching at various ankle angles, as well as maximal voluntary contraction and electrically induced contractions, were recorded after 10, 20, and 30 sessions, and one month after the end of the training program.

Training caused a significant 30.8% increase in the maximal ankle dorsiflexion. The improved flexibility was associated with a decrease in muscle passive stiffness and, after the first 10 sessions only, with a small increase in passive torque at maximal dorsiflexion. The maximal voluntary contraction torque and the maximal rate of torque development were not affected by training. Changes in flexibility and passive stiffness were partially maintained 1 month after the training program, reflex activities had already returned to control levels.

Implication. Increased flexibility results mainly from reduced passive stiffness of the muscle-tendon unit and tonic reflex activity. The underlying neural and mechanical adaptation mechanisms, however, showed different time courses.

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