Other than Olympic race walkers, people generally find it more comfortable to run than walk when they start moving at around 2 meters per second - about 4.5 miles per hour.

North Carolina State University biomedical engineers Dr. Gregory Sawicki and Dr. Dominic Farris have discovered why: At 2 meters per second, running makes better use of an important calf muscle than walking, and therefore is a much more efficient use of the muscle's - and the body's - energy.

Published online this week in Proceedings of the National Academy of Sciences, the results stem from a first-of-its-kind study combining ultrasound imaging, high-speed motion-capture techniques and a force-measuring treadmill to examine a key calf muscle and how it behaves when people walk and run.

The study used ultrasound imaging in a unique way: A small ultrasound probe fastened to the back of the leg showed in real time the adjustments made by the muscle as study subjects walked and ran at various speeds.

The high-speed images revealed that the medial gastrocnemius muscle, a major calf muscle that attaches to the Achilles tendon, can be likened to a "clutch" that engages early in the stride, holding one end of the tendon while the body's energy is transferred to stretch it. Later, the Achilles - the long, elastic tendon that runs down the back of the lower leg - springs into action by releasing the stored energy in a rapid recoil to help move you.

The study showed that the muscle "speeds up," or changes its length more and more rapidly as people walk faster and faster, but in doing so provides less and less power. Working harder and providing less power means less overall muscle efficiency.

When people break into a run at about 2 meters per second, however, the study showed that the muscle "slows down," or changes its length more slowly, providing more power while working less rigorously, thereby increasing its efficiency.

"The ultrasound imaging technique allows you to separate out the movement of the muscles in the lower leg and has not been used before in this context," Farris says.

The finding sheds light on why speed walking is generally confined to the Olympics: muscles must work too inefficiently to speed walk, so the body turns to running in order to increase efficiency and comfort, and to conserve energy.

"The muscle can't catch up to the speed of the gait as you walk faster and faster," Sawicki says. "But when you shift the gait and transition from a walk to a run, that same muscle becomes almost static and doesn't seem to change its behavior very much as you run faster and faster, although we didn't test the muscle at sprinting rates."

The research could help inform the best ways of building assistive or prosthetic devices for humans, or help strength and conditioning professionals assist people who have had spinal-cord injury or a stroke, Sawicki and Farris say.

The researchers are part of NC State's Human PoWeR (Physiology of Wearable Robotics) Lab, directed by Sawicki. The joint Department of Biomedical Engineering is part of NC State's College of Engineering and the University of North Carolina-Chapel Hill's School of Medicine.

"Human medial gastrocnemius force-velocity behaviour shifts with locomotion speed and gait" by Dominic James Farris and Gregory S. Sawicki, North Carolina State University - Proceedings of the National Academy of Sciences.