Washington, Jan 6 (IANS) People feel more comfortable running rather than walking when they start moving at around two metres per second or about 4.5 miles per hour.

North Carolina State University biomedical engineers Gregory Sawicki and Dominic Farris have discovered why: At two metres 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.

The results comes from the first 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 journal Proceedings of the National Academy of Sciences reports.

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, according to a Carolina statement.

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.

When people break into a run at about two metres 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.