A team of University of Wisconsin-Madison researchers has discovered the muscles in our lower legs work much like a bicycle shifting through gears during walking. As your ankle speeds up during each step, your nervous system appears to hand off the work from one muscle to another, recruiting muscles with progressively smaller lever arms to generate an efficient push-off.
The finding, published in the journal Proceedings of the National Academy of Sciences, reveals a previously unrecognized strategy the body uses to produce power at the ankle during walking.
“The calf muscles that go into the Achilles tendon are the ones that everybody always thinks about and cares about in research because they are the big force producers in your lower leg for plantarflexion,” says Jack Martin, a research scientist in UW-Madison’s Departments of Mechanical Engineering and Orthopedics & Rehabilitation. “But there are other muscles that can contribute. And what this study shows is that they’re actually contributing a lot more than we thought previously.”
The discovery began with something that didn’t make sense. Martin and his colleagues were analyzing muscle forces measured with shear wave tensiometers, small wearable sensors invented during Martin’s PhD research that estimate muscle force by measuring tiny waves traveling through tendons. While studying walking, the researchers noticed Achilles tendon force wasn’t perfectly synchronized with the overall torque produced about the ankle. Rather than dismissing the discrepancy, the researchers dug deeper to understand its source.
They found the calf muscles attached to the Achilles tendon don’t work alone. As ankle motion accelerates during push-off, smaller muscles such as the tibialis posterior and peroneus longus take on a much larger role than scientists had previously appreciated. The team’s mathematical model suggests this isn’t accidental. Instead, muscles are recruited in sequence according to their lever arms, much like a cyclist shifts into different gears as speed changes.
“What appears to be muscle redundancy is actually a functional evolutionary feature,” says Darryl Thelen, John Bollinger Chair of Mechanical Engineering and Bernard A. & Frances M. Weideman Professor at UW-Madison. “Our model predicts that muscles with larger lever arms are recruited first while ankle motion is relatively slow. As motion accelerates, muscles with smaller lever arms become increasingly important. This allows each muscle to operate closer to its optimal length and contraction speed, improving power production and overall efficiency.”
The discovery could change how researchers think about movement, injury recovery and even robotics. Achilles tendon injuries have become increasingly common, including among elite athletes. Understanding the unique roles played by different lower-leg muscles could inform targeted rehabilitation strategies tailored to distinct muscle roles. And applying the same principles to the design of powered prostheses, exoskeletons and legged robots by incorporating multiple coordinated actuators could improve efficiency and power production in these systems.
A few months after completing the research, Thelen received an unwelcome reminder of the importance of one of those “supporting” muscles. He strained his tibialis posterior, a deep muscle in the calf that the study identified as a key contributor during push-off.
“It literally hurt to walk for about three months,” he says.
Martin laughs: “You notice funny things after doing this work.”
The research was supported by the National Institutes of Health and the Department of Defense Clinical and Rehabilitative Medicine Research Program.
Co-authors include Lauren Welte, assistant professor of mechanical engineering at the University of Alberta and former postdoctoral fellow in the Thelen Lab, and Keith Knurr, assistant professor in the UW–Madison Department of Orthopedics & Rehabilitation.