Dynamic versus passive stretches
Raphael Brandon reviews the benefits and effects of dynamic and static stretching on the muscles.
Dynamic and static stretches have very different effects, according to a new study. Researchers measured the effects of passive static and passive dynamic stretching on two biomechanical properties of the ankle joint muscle stiffness and force relaxation.
Muscle stiffness refers to the ratio between the change in muscle resistance and the change in muscle length. The more the muscle is stretched, the more resistance to the stretch is produced. But the lower the ratio, i.e. the less the stiffness, the easier it is to move through the range of motion.
Muscle stiffness is believed to be directly related to muscle injury risk, and so it is important to reduce muscle stiffness as part of a warm-up. Force relaxation refers to the decrease in peak force produced by the muscle when it is stretched to the end of its range. After holding a stretch for some time the peak force relaxes, which helps the muscle move further.
Force relaxation has also been related to injury risks and the maximum range of motion in a muscle.
McNair et al. (2001) involved 22 active and healthy subjects. A Kin Corn isokinetic dynamometer was used to stretch each subject's ankle joint. The joint was stretched into dorsiflexion, stretching the calf muscle. The Kin Corn machine also measured the forces in the joint that resulted from the stretching.
Four different conditions of stretching were tested:
To control the test conditions, subjects were instructed not to actively increase the stretch themselves. The results were as follows:
Thus it seems that there are clear differences in the effects of dynamic and static stretches. Only dynamic movement throughout the range of motion resulted in any reduction in muscle stiffness, an important factor in reducing injury risks. However the static stretches produced the greatest peak force relaxation effect.
This suggests that dynamic stretches, slow controlled movements through the full range of motion are the most appropriate exercises for warming up. By contrast, static stretches are more appropriate at the end of a workout to help relax the muscles and facilitate an improvement in maximum range of motion.
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