Powered knees, microprocessor joints, and lightweight materials have sharply reduced the energy cost of prosthetic legs. Yet even with those enhancements, amputees expend up to 40 percent more energy on every step than able-bodied ambulators.
Why does the inefficiency persist? According to Daniel Renjewski, a mechanical engineer who’s spent the last decade trying to replicate the human gait in bipedal robots, it’s because researchers have discounted an important power source in gait biomechanics: the foot.
In the December 2022 issue of Physical Review E (a journal of the American Physics Society), Renjewski and his colleagues presented a new biomechanical model which identifies the foot as the key to understanding the dynamics of walking. Under their theory, the foot plays a dual role. It absorbs impact and regulates balance as our heel hits the ground, which has long been considered its core function. But the foot also amplifies and leverages torque as we vault forward, to a much greater degree than reflected in previous models.
“The large mechanical advantage of the foot yields a clear efficiency benefit,” the paper notes. The extra juice minimizes the burden on ankles, knees, and hips, enabling us to walk with exceptional stamina—a critical trait for ancient humans who had to track prey over great distances.
“Our ancestors did something called endurance hunting, where animals run away from you and you just walk after them,” Renjewski said recently on NPR’s Weekend Edition. “Running is more inefficient, so if I walk after you instead of running, you will spend more energy.”
In other words, slow and steady really does win the race. Gait efficiency enabled our species to chase down swifter animals through sheer persistence, giving humans an indispensable evolutionary advantage. Yet even the most advanced prosthetic legs don’t fully capture this benefit, and that’s largely because they don’t optimize the foot’s role in gait dynamics, Renjewski believes. Until they do, they’ll continue to impose a sizeable energy cost on the amputees who wear them.
“If we look at prosthetics today, they definitely require more energy than if we walk on our natural two legs,” Renjewski told Weekend Edition. “So when it comes to remobilization of humans, you want to know what the mechanics of walking are, and you really want to implement them in [prosthetic] devices.”
To find the full paper online, visit journals.aps.org/pre.