WASHINGTON (AP) - Common ideas about stretches may stretch the truth, researchers say.

"Stretching recommendations are clouded by misconceptions and conflicting research reports," said a report in a medical journal, The Physician and Sportsmedicine.

Stretching is supposed to reduce the risk of injury, relieve pain from exercise and improve performance. But the benefits are only partly supported by the evidence, the experts said.

Exactly what to stretch, for how long, and why are still up for scientific debate, said Drs. Ian Shrier and Kav Gossal of Montreal. New research is challenging commonly held ideas, they said.

The doctors checked the medical database Medline and the sports science database SPORTDiscus for articles on stretching to help muscles relax and lengthen after being tightened in a contraction.

The researchers found that, in general, only one post-exercise stretch of 15 to 30 seconds per muscle group is needed for most people, although some people or muscle groups may need more. There seems to be a limit - after the fourth or fifth stretch, stretching seems to make muscles tighter instead of more relaxed, they said.

Another expert finds some fault with that, however. Research suggests that 3 to 5 stretches of 30 seconds are optimal, said Lynn Millar of Andrews University, Berrien Springs, Mich. Larger muscle groups such as the hamstrings in the legs benefit more from repeated stretches than do smaller muscles such as the biceps in the arm, she said.

If the stretches are intended to relieve post-exercise aches, additional stretching may provide greater benefits, the article reported. Research indicates that part of the benefit of stretching is in the mind, by increasing stretch tolerance "patients feel less pain for the same force applied to the muscle," the doctors said.

The idea is that stretching reduces the perception of pain, although how this could work is not known, Shrier said. And, although the benefit has been shown in research on healthy tissue, people with injuries probably benefit as well, he said.

The benefit in pain relief might more than offset the cost in stretch-tightened muscles, making greater stretching worthwhile, they said.

Stretching before exercise, a common way to prepare for sports or exercise, isn't as valuable as exercisers think, the article reported. An active warmup such as a jog is better, it said. For instance, a warmup run has been shown to decrease stiffness in the calf muscle, although not in the hamstrings, it said. And warmup has been shown to decrease injuries.

Studies have found that an active warmup followed by stretching increases range of motion better than stretching alone does, the doctors said. But they feel the major value is in the warmup. They warn athletes that the added stretching has not been shown to reduce the injury rate.

"If injury prevention is the primary objective (e.g. recreational athletes who consider performance a secondary issue), and the range of motion necessary for an activity is not extreme, the evidence suggests that athletes should drop the stretching before exercise and increase warmup," the article said.

This may not be the case for competitive athletes, Millar said. Some preliminary research has found soccer players reduced injury risks by stretching, although the number of players studied was too small to draw a firm conclusion, she said.

Static stretching - moving slowly into a position and holding it - is generally considered to be less risky than dynamic stretching, in which the athlete stretches while moving. But the risk is probably overstated, because dynamic stretches still tax the athlete far less than the activity for which the athlete is preparing, the article said.

And although textbooks commonly recommend static over dynamic stretching, "there isn't any research to support the idea that there could be more injuries," Millar said. Her feeling is that dynamic stretches may benefit competitive athletes because the movements will be similar to movements they would use in their sports.

As for the best type of stretch, the researchers leave that to the athlete. "Individuals should determine a strategy for themselves by simply holding a stretch until no additional benefit is obtained," they said.

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