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Thu Apr 25, 1:49 PM ET
By E. J. Mundell

NEW ORLEANS (Reuters Health) - While most of us have been taught to stretch before vigorous exercise, in recent years some fitness experts have expressed doubt as to its actual value in shielding muscle from harm.


But now, studies performed at the cellular level in mice suggest that "stretching, at least in our model, is protective against muscle injury," said researcher Dr. Timothy J. Koh of the University of Michigan, Ann Arbor. He presented the findings here Wednesday at the annual Experimental Biology 2002 conference.

Speaking with Reuters Health, Koh explained that, until recently, there had been little hard scientific evidence that the pre-workout stretch had real protective value. "There had been a lot of anecdotal stuff in humans with people that stretch," he said. "Some people said it worked, while other people didn't think so."

He and co-researcher Dr. Francis Pizza of the University of Toledo, Ohio, wondered if stretching might somehow induce inflammation. No one had yet investigated such a link, Koh said, because with simple stretching the musculature shows no overt sign of damage--such as soreness or swelling--that might indicate inflammation. Still, "we took a gamble and tried to look at that," he said.

To do so, they hooked tiny stimulators to single muscles in the hind limbs of young, adult, anesthetized mice. In one experiment, the researchers stimulated the muscle as if it was performing an "eccentric contraction"--vigorous, unfamiliar and soreness-inducing exercise akin to a human walking downhill. In another experiment, the mouse muscle was stimulated to perform gentler "passive stretching" that mimicked the kind of stretching humans might do when working on various muscle groups at the gym.

The researchers then closely examined the mouse muscle under laboratory conditions for signs of inflammatory immune system activity.

The result? Even 2 weeks after the experiment, passive stretching was shown to have "provided some protection" against injury, Koh said. Injury-preventing immune system inflammatory cells called neutrophils were seen in large numbers among the passive-stretching muscle fibers, indicating a protective effect. While the level of immune cell activity in the stretched muscle was slightly less than two-thirds that observed in muscle that had undergone the tougher, "eccentric contraction" workout, Koh said it remained significant.

How and why undamaged, casually stretched muscle 'calls out' to healing immune system neutrophils remains a mystery, Koh said. "In this situation we really don't know what's attracting them," he said, "because there is no overt injury, so you wouldn't expect them to be there. It's clear that the muscle cells must be releasing some agent in response to passive stretch that is attracting the cells there."

Whatever the mechanism, the findings suggest that what our physical education teachers taught us might be true after all--a good stretch primes the immune system for the tougher workout to come. According to Koh, follow-up study is also suggesting that the protective benefits of stretching may be greatest if performed just prior to vigorous exercise, as usually happens in the real world (news - Y! TV).

Stretching may protect the elderly as effectively as it does the young, Koh added. Studies using the musculature of 24-month-old mice--the equivalent of 70-year-old humans--"found that they are protected just as well with passive stretching" as young mice, he said.