BROWN U. (US) — Tendons in the legs act as shock absorbers, offering protection at the moment of impact with muscles stepping up less than a second later to absorb the remaining energy.
The tendons’ role is crucial, researchers write in the journal Proceedings of the Royal Society B, because they help protect the fascicles—fibrous bundles in skeletally connecting muscles—against damage from the rapid burst of energy and power generated by the impact.
“Something has to take up the slack, and it falls to the tendon,” says Nicolai Konow, postdoctoral researcher at Brown University and lead author on the paper. “The good news is it’s elastic enough to do that.”
The research underscores the critical role that tendons play as shock absorbers in activity involving skeletal muscles, which could refine the development of synthetic tendons and improve rehabilitative practices after tendon reconstructive surgery.
The findings may cross into biomimetics, used to make two-legged robot locomotion more similar to human locomotion, for example. It could also help in athletic training.
“We used to think that all of the motion of the body could be explained just from the action of our muscle motors,” says co-author Thomas Roberts, associate professor of biology, specializing in animal movement. “It is becoming increasingly apparent that springy tendons are a big part of what makes us go.”
To determine how muscles and tendons work in tandem, the researchers studied turkeys, whose legs have a muscle-tendon structure similar to humans and whose walking posture (with the legs under the body) largely mimics our own.
The researchers outfitted turkeys with special sonar sensors embedded in a calf muscle that recorded changes in muscle fascicle length at 1,000 times per second as the turkey landed from a jump. Other devices measured the force on the muscle from landings, while a slow-motion video camera caught the changes in leg configuration upon landing to understand how muscles and tendons were flexed and stretched.
The researchers focused on two main periods.
One, called the force rise, began when the turkey landed. The researchers were surprised to see that muscle fascicle length remained essentially the same. The jolt of the impact was being absorbed somewhere else: in the tendon, Konow says.
The researchers believe there is a biological reason for this. To protect themselves from the energy generated at the moment of landing, the fascicles leave the impact to the tendons, which stretch like a spring. In fact, electrical pulses recorded in the fascicles indicate the muscle steeled itself against the landing even before the turkey jumped. “The muscle is pre-activating prior to impact, to increase stiffness, to resist being forcibly lengthened,” Konow says.
Following the jolt of landing comes period called force decay. During force decay, the tendon recoils to its original length, releasing the energy it had accepted during the landing.
“That means the tendon is shunting energy to something that’s lengthening and that’s the fascicles. The fascicles are sensing that the force (from impact) is going down and it’s safer to lengthen to absorb the energy,” says Konow.
The researchers documented an approximately 20 percent lengthening of the fascicles during the force decay period. “We can say that in real ways, the muscle has a safety net with the tendon there and protecting it,” Konow said.
Emanuel Azizi, a former postdoctoral researcher at Brown now at the University of California–Irvine, contributed to the paper that was funded by a National Institutes of Health grant to Roberts.
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