Posted: May 8, 2005

Athletics: Stretch!

From Lore of Running-4th Edition by Timothy Noakes

Stretching is not something that most runners do willingly. Runners who somehow manage to squeeze in 1 or 2 hours of running a day never seem quite able to find the additional 5 to 10 minutes needed for adequate stretching. There are a variety of reasons for this. First, most inflexible runners are unconvinced that stretching is beneficial. Secondly, we are ignorant of what is involved. Third, experience has taught us that stretching hurts; and finally, we are haunted by the suspicion that we may be doing it wrong anyway. There are now some excellent references that review all aspects of stretching (Anderson 1975; Uram 1980; Beaulieu 1981; Shellock and Prentice 1985; Alter 1996; 1998; Shrier and Gossal 2000). The information presented here is a synthesis of the ideas in these books.

Muscles have a complicated mechanism that prevents them from ever being damaged by overstretching. Muscles contain tiny stretch receptors that are attached to the working parts of the muscle--the muscle fibers. When a muscle is stretched suddenly, the degree of stretch is sensed by the stretch receptors. They then send messages back, via the spinal cord, to the nerves that control the contraction of the muscles in which the receptors lie (that is, the stretched muscles). As the intensity of the stretch increases, the stretch receptors begin to fire more rapidly and more strongly. Ultimately, these impulses exceed a certain threshold, and the stretched muscle contracts and shortens, preventing the muscles from being overstretched.

A general rule is that the intensity of the muscle contraction induced by a stretch reflex varies with the rapidity with which the stretch is applied. The faster the stretch is applied, the more powerful the contraction it evokes. Another important stretch reflex, the inverse stretch reflex, performs an exactly opposite function to that of the conventional stretch reflex. The receptors of this reflex are not embedded in the muscle fibers but are situated in the muscle tendons. These receptors are sensitive to the tension present in the muscle tendons.

As the muscle contracts, the tension in the tendon rises, and the tension receptors are activated. As in the conventional stretch reflex, these receptors send messages to the nerves controlling the contraction of the muscles in which these receptors lie. But in contrast to the conventional reflex, when these tension receptors in the inverse stretch reflex are activated, they inhibit the contraction of the relevant muscle. Therefore, the inverse stretch reflex provides a protective mechanism that prevents a muscle from contracting so strongly that it ruptures its own tendons. If the tendons sense that the muscle is contracting too powerfully, the tendon receptors cause the contraction to be "switched off." One theory holds that it is the inhibition of this reflex during prolonged exercise that contributes to cramping during exercise (see chapter 15).

How does stretching increase flexibility? When muscles are stretched, they exhibit two characteristics of a viscoelastic substance--creep and stress relaxation. Thus, unlike an elastic band that stretches immediately to a final length when a constant force is applied, the length of stretched muscles increases gradually with time (creeps). This is similar to any viscous substance, such as molasses. Stress relaxation occurs when the tone of the muscle suddenly falls after a period of constant stretching at the same length.

Stretching acutely reduces muscle stiffness by decreasing viscoelasticity and by reducing muscle tone during the stretch. If muscles are stretched for 30 seconds per day, these acute changes are retained and enhanced so that ranges of motion allowed by the stretched muscles are increased.

Posted with permission from Human Kinetics Publishers, Inc.