Story URL: http://news.medill.northwestern.edu/chicago/news.aspx?id=189508
Story Retrieval Date: 5/19/2013 9:44:24 AM CST
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Courtesy of Nicolai Konow
Researchers used wild turkeys to study how tendons act as shock absorbers during impact, protecting the fragile muscles from injury.
Tendons take the heat
by Sarah Eberspacher(2)
Sep 28, 2011
Sarah Eberspacher / MEDILL
A downhill hike can result in sore leg muscles a day or two later.
Sarah Eberspacher / MEDILL
Each step down involves a lengthening of muscle that absorbs energy and turns it into heat. The resulting pain is called late-onset muscle damage that reseearchers are learning how to treat.
The study, published Tuesday in “Proceedings of the Royal Society,” used the wild turkeys because their leg anatomy is surprisingly similar to humans, said Thomas Roberts, a researcher on the project in Brown University’s Department of Ecology and Evolutionary Biology.
“It’s not the obvious choice, but they actually have a number of features that make them good to work with,” he said. “They’re large enough, they’re great runners, they’re easily trainable - and they have calcified tendons.”
Those tendons – the connective tissue bridging bone and muscle - are key to the study, because researchers utilized micro-sonar sensors embedded in the calf muscle of each turkey. The turkeys were dropped in special harnesses from a height of 1.5 meters. A slow-motion camera recorded changes to the turkey’s leg configuration throughout the full motion of the landing.
With impact, movement is decelerated by the legs flexing, and the lengthening of muscles actually absorb energy instead of producing it, converting that energy into metabolic heat, explained Nicolai Konow, lead author of the study.
While the assumption would be that humans – and turkeys - have leg muscles predisposed to stop them, in reality those fascicles are composed of small subunits of two types of fibers arranged in series. The fibers move parallel to each other when the muscle contracts. During lengthening contractions, those fibers move away from each other, which can cause microscopic-level damage.
“If you’re a very energetic person used to doing mountain hikes, you may come back and experience muscle pain a couple of days later,” Konow said. “That’s what we call late-onset muscle damage, which isn’t from walking up the hill but from walking down. Each step down involves a lengthening of muscle that absorbs energy and turns it into heat.”
Athletes such as long jumpers or ballet dancers should take note, but so too should those who are aging, because how legs make impact with the ground is critically important. Landing “stiff-legged” means the tendons have less lengthening space and are unable to take in as much of the energy.
The result can be higher muscle forces posing higher risk to the fragile fascicles and other parts of an aging body. Knowing more about how those tendons and muscles interact could have positive payoffs for rehabilitation practices in the future, Roberts said.
“Therapies could be designed to target not just muscle function but connective tissue function as well,” he said. “Conditions like cerebral palsy or muscular dystrophy both ultimately increase the stiffness of the connective tissue that holds muscle together. We don’t know yet how much that contributes to those disorders, but I think these kinds of studies suggest it’s worth looking at whether those connective tissue pathologies might be important.”
The research showed the fascicles actively resist the initial harmful lengthening, Konow said. Simple deduction means if the muscle is being lengthened, but the fascicles aren’t initially lengthening, another part must be involved: the tendon. The muscles are the motors that power movement, or as shown in this study, they absorb energy when a person needs to stop moving.
“This explains why the fascicles remain relatively safe,” he said. “After most of the energy has been taken into the tendon, little sensory cells tell the muscles, ‘Hey! Force has peaked and is going down now, so it’s safe to lengthen and dissipate that energy.”
