A New View of How Leg Muscles Operate on the Run

Abstract

Leg muscles are generally viewed as motors, contracting and relaxing to drive legs up, forward, and down. But new results reported on page 1113 challenge the long-standing assumption that muscles always do actual work, shortening against resistance to propel an animal forward. By directly monitoring a leg muscle in turkeys running on a treadmill, researchers have now found that the muscle hardly contracts at all, at least when the animals run on level ground. Instead, it shortens slightly before the foot is planted to exert the force needed to keep the tendon attaching the muscle to the heel stretched while the foot is on the ground, thus storing energy for the next stride. As a result, the muscle does very little work.