Zelik, team discover hip, foot muscles more important to walking than previously thought

Karl Zelik (Vanderbilt University)

In his effort to develop better prosthetic limbs, Karl Zelik had to start with deciphering more clearly how muscles function in walking. His path not only led to a better way of quantifying human locomotion, but also to the discovery that muscles around the hip and in the foot are more important to walking than previously thought.

The traditional way to estimate muscle power during walking and running is with 3 degree-of-freedom (3DOF) inverse dynamics, a mathematical calculation of how muscles and tendons generate forces to rotate the body’s limbs. But when Zelik, an assistant professor of mechanical engineering at Vanderbilt University, applied those calculations to previously collected data, he realized there was a large discrepancy. Based on 3DOF estimates, total muscle power generated by the body was 25 percent too low, meaning these muscle estimates could not explain the kinetic and potential energy changes of the body over a stride cycle.

Basically, 25 percent of walking movement could not be explained biomechanically.

To understand why that was, Zelik and his team recruited 10 healthy subjects to walk at various speeds and placed reflective markers at 30 points on their bodies, using a motion-capturing system to record leg movements and an instrumented treadmill to record the forces their feet exerted on the ground.

“The research suggests there may be ways to improve mobility by providing more assistance at the hip …” — Karl Zelik, assistant professor of mechanical engineering

They then developed a new way to combine these motion and force data, using a 6 degree-of-freedom analysis of the hip, knee, ankle and foot. When they calculated the power generated by muscles and other biological tissues using this new method, it finally added up correctly to account for 100 percent of the body’s energy changes.

“When I initially found the gap, it was frustrating that so much of our science was built on measurements that didn’t add up properly,” Zelik said. “I had been stewing on this problem for three or four years before a fortunate encounter with people who had a different perspective, and combining our work helped us find the solution.”

They were Kota Z. Takahashi, a post-doctoral researcher who specializes in foot biomechanics, and Gregory S. Sawicki, a professor who specializes in muscle biomechanics, both of North Carolina State University. The team’s results were published in The Journal of Experimental Biology in March.

Their work also revealed that propulsive power generated by the hip is 50 percent greater than measured by conventional means.

“Many prostheses and exoskeletons have focused on assisting the ankle,” Zelik said. “The research suggests there may be ways to improve mobility by providing more assistance at the hip as well.”

Zelik said he was drawn to the topic of human locomotion growing up in a family of four athletic boys. He was a perennial medal-winner on the track and field team at Washington University, where he earned his bachelor’s and master’s degrees in biomedical engineering.


Heidi Hall, (615) 322-6614
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