When most of us think of stretching, we're imagining at a minimum jogging, and probably something more like downhill skiing or sprints. But when University of Michigan researchers Nicole Lockhart and Susan Brooks talk stretching, their real interest is how to condition older folks' muscles so they'll eventually be willing to do even a little exercise to garner all the benefits that will follow.
"The elderly are far more susceptible to contraction-induced injury," notes Lockhart, lead author in two related papers being presented in American Physiological Society sessions at Experimental Biology in San Francisco. "Sometimes just by normal activity or a sudden movement a leg will jut out too far and they'll suffer a minor injury, but they'll be wary of further damage," she said.
Protect those muscles, as minor injuries may be cumulative
Brooks, her adviser, added: "We think that cumulative muscle injury may contribute to the loss of muscle mass as we grow old. So protecting muscles at all times is a good thing. And understanding how stretching increases resistance to injury will really help to do this."
The team had previously shown that stretching decreased muscle injury in mice when stretches were performed anywhere from one hour to 14 days (yes days) prior to exercise. But they didn't know why. What is known is that while stretching muscles produce nitric oxide (NO), a common signaling molecule. NO increases blood flow and decreases force during submaximal contractions, and also can modulate inflammation.
NO protects without stretching; but without NO, stretching doesn't seem to protect
So they tested whether the anti-inflammatory effects of NO were involved in the protection provided by stretching. And the results were: mixed. Mice were given substances that either increased or inhibited NO production. They found that increasing NO reduced inflammation and other measures of injury following exercise by half – even without prior stretching. On the other hand, when NO production was restricted, stretching an hour before exercise didn't reduce injury at all.
They also tested whether low level inflammation seen after stretching somehow primes muscles to decrease inflammation following subsequent damaging exercise. They found that when an antibody was administered that reduced the inflammation induced by stretching, no protection following subsequent exercise was observed.
Insights, but not yet answers on mechanisms of protection
"The results are somewhat contradictory," Brooks offers, "because first nitric oxide appeared to be important by inhibiting inflammation, but our second experiment showed that if you prevent inflammation you don't get the protection afforded by stretching. Nevertheless, while translating animal studies to human athletes, or elderly humans for that matter, is difficult, these studies do provide important insights into how different modes of training reduce muscle injury."
*Paper presentations: "NO is necessary and sufficient for the protection from contraction-induced injury provided by passive-stretch-conditioning," 12:30 p.m.- 3 p.m. Sunday April 2, APS Physiology Exercise Responses and Training Section 237.19/board #C773. Research was by Nicole C. Lockhart and Susan V. Brooks, Department of Physiology and Institute of Gerontology, University of Michigan, Ann Arbor.
And, "The infiltration of neutrophils following passive-stretch-conditioning is required for protection from contraction-induced injury": 237.20/board #C774.
Funding: National Institute on Aging/NIH.
Editor's Note: For further information or to schedule an interview with a member of the research team, please contact Mayer Resnick at the APS newsroom @ 415.905.1024 (March 31-April 5); or 301.332.4402 (cell) or 301.634.7209 (office), mresnick@the-aps.org; or Christine Guilfoy at 978.290.2400 (cell) or 301.634.7253 (office).
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