Changes in passive tension after stretch of unexercised and eccentrically exercised human plantarflexor muscles.

The study measured the effect of stretch on passive mechanical properties in unexercised and eccentrically exercised plantarflexor muscles, to obtain insight into how stretch might serve athletes as a warm-up strategy. Passive torque, voluntary contraction strength and muscle soreness were measured before and after a large amplitude stretch given before and after a period of eccentric exercise and at 0, 1, 2 and 24 h later. Stretch of the unexercised muscle led to a 20% fall in passive torque which recovered within an hour. About 40% of the fall could be recovered immediately with a voluntary contraction. After eccentric exercise there was a rise in passive torque by 20% at 2 h post-exercise. This rise was postulated to result from an injury contracture in muscle fibres damaged by the exercise. It was accompanied by a fall in maximum voluntary torque and the development of muscle soreness at 24 h. Stretch of the exercised muscle led to a fall in passive torque and rise in pain threshold. It is proposed that in response to a stretch there is a fall in passive tension in the muscle due to stable cross-bridges in sarcomeres which could be recovered with a voluntary contraction and an additional component attributable to the elastic filament, titin. The size of the fall was not significantly different between exercised and unexercised muscle. These observations provide a physiological basis for the effects of passive stretches on skeletal muscle and help to explain why they are used as a popular warm-up strategy.