ScienceDaily (Jan. 5, 2012) — 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.
