The reason why people generally tend to run rather than walk when they hit the speed of 2 meters per second has been discovered by scientists.
At this pace, running makes better use of an important calf muscle than walking, and thus is a much more efficient use of the muscle's and the body's energy, North Carolina State University biomedical engineers Dr. Gregory Sawicki and Dr. Dominic Farris has found.
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 said.
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 said.
"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," Sawicki added.
The study has been published in Proceedings of the National Academy of Sciences.