There are many factors that may play a role, says University of Alabama at Birmingham exercise physiologist Gary Hunter, Ph.D., and tendon length may be an important one. In findings just published in Medicine & Science in Sports and Exercise, a journal of the American College of Sports Medicine, Hunter shows that a longer Achilles tendon leads to greater energy efficiency in running, which in turn might enable better running performance.
“Longer Achilles tendons appear to generate more power because they stretch more,” said Hunter, a professor in the UAB Schools of Education and Health Professions. “It’s like a rubber band; the longer the stretch, the more force that can be generated to provide forward velocity while running.”
When describing running, Hunter says muscles and tendons work together in a muscle/tendon complex. During the landing phase of running and jumping — as the foot hits the ground — the tendons, particularly the Achilles tendon, stretch. Longer tendons have more capacity to stretch than shorter ones. During the push-off phase while running, energy from the stretched Achilles tendon (similar to the elastic energy developed in a stretched rubber band) adds to the force generated by the contracting muscle. The addition of this elastic energy during the push-off, added to the energy created by the muscles’ own contractions, means more potential for speed.
“Let’s imagine two runners whose muscles need X-amount of force to run a certain speed,” he explains. “The runner with the longer Achilles tendon gets more of that needed force from the tendon’s elastic recoil, requiring the muscle to generate less force and use less energy while running at that speed. Theoretically, a shorter tendon doesn’t have as much potential to stretch and thus supply additional force.”
In previous studies, Hunter showed that longer Achilles tendons also produced more energy efficiency in walking. The current study examined 21 long-distance runners, and evaluated them at running speeds of six and seven miles per hour.
“We found a strong correlation between tendon length and running economy, or energy expenditure, in our subjects at both the six and seven mph running speeds and at a walking pace of three mph,” said Hunter. “Coupled with our previous studies, I’m convinced that tendon length contributes to running efficiency and probably performance.”
Unfortunately, Hunter says, aspiring athletes just can’t grow a longer tendon in hopes of running faster. Hunter’s previous research indicates that ethnic groups such as African-Americans tend to have longer limbs and shorter calf muscles and thus longer Achilles tendons than Caucasians, which may be a contributing factor to why some African-Americans seem to excel in sports involving running.
The study was funded by the UAB Department of Human Studies. Research collaborators include John McCarthy, William Ogard, Marcas Bamman, Jan Den Hollander, Tamilane Blaudeau and Bradley Newcomer, of UAB, and Konstantina Katsoulis, University of Toronto and David Wood, California State Polytechnic University, San Luis Obispo, Calif.