At what point does it become "easier" for us to run rather than to walk? Many of you reading this might be thinking; "right, umm never" ... but surprising research out of NC State’s Human PoWeR (Physiology of Wearable Robotics) Lab proves that the muscles of the body might be helping make that decision for us.

NC State University biomedical engineers Dr. Gregory Sawicki and Dr. Dominic Farris have discovered that around 4.5 miles per hour, 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.

The results from this unique study are published online this week in Proceedings of the National Academy of Sciences.

Using a unique ultrasound imaging technique, a force-measuring treadmill, high-speed motion-capture - and a few willing "runners" - the team examined a key calf muscle and compared it's behavior when people ran versus walked.

A small ultrasound probe fastened to the back of participants leg showed in real time the adjustments made by the muscle as study subjects walked and ran at various speeds.

“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 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 (4.5 mph), 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 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.

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