For something so routine, walking is shockingly complicated. Biomechanists break a single step into several phases: First there’s touchdown, when your heel strikes the floor. Next comes the single support phase, when you’re balancing on that leg. After that, you roll onto your toes for takeoff and your leg goes into a forward swing.
All of this contains a mystery. Researchers have long observed that when we walk, our planted leg bounces twice before swinging into the next step. That is, the knee bends and extends once when the foot first touches down, then again just before takeoff. That first bounce helps our foot absorb the impact of our weight as we hit the ground. But the function of the second bounce, a feature characteristic to human gait, has never been clear.
In a Physical Review E paper published last month, scientists at the University of Munich may have found an answer. By modeling the physical forces that drive our double bounce, they deduced that it’s an energy-saving technique for a species that has long prioritized endurance over speed—which may be a clue about why humans evolved such an odd gait. Now, they think their model can help improve prosthetic and robotic designs, and may even lend insight into the evolutionary pressures our ancestors faced.
“The foot is the key element here,” says Daniel Renjewski, a mechanical engineer who led the study. The human foot is, frankly, kind of an oddity in the animal kingdom. People have a 90-degree angle between the foot and the leg, he continues, but few other animals do. That means most animals walk on their tiptoes or the balls of their feet, while we walk heel-to-toe. Human feet are also relatively flat, and our legs are quite heavy, both of which make staying upright while propelling the body forward a mechanical challenge.
Our double-bounce walking pattern is distinct from the single bounce we enact when running, which is a motion that’s mostly airborne, says University of Munich sports scientist Susanne Lipfert, a study coauthor. While walking, the foot stays planted for up to 70 percent of a step cycle to help us stay balanced at slower speeds. But that comes with a tradeoff: less time to propel ourselves forward. Counterintuitively, that means your body has to work harder when walking to recirculate the leg into its next step. “It seems odd, at first glance, to aim for a gait that leaves very little time to swing your leg forward,” Renjewski says, because of how heavy our legs are: More mass requires more power.
So given all these challenges, how does humanity manage to get around? For years, even our mechanical understanding of how we walk has been limited, because trying to model what all of the muscles, tendons, and joints of the lower body are doing at any given time is an arduous—if not impossible—task. Renjewski’s team, however, discovered that the human walking gait could be reduced to a single equation, based on how the foot behaves during the double bounce.
To build their model, the researchers reduced the foot-leg system to just four joints at the hip, knee, ankle, and toes. Using data Lipfert collected as a graduate student—information about the forces and joint positions of 21 people videotaped while walking on a treadmill—they tried to describe the foot’s heel-to-toe stride as if it were a simple object rolling on the ground. That movement is easier to understand than trying to account for the entire anatomy of the foot.
The resulting model quantified two competing factors that influence how the foot moves: the force of the upper body keeping it anchored on the ground, and the torque of the ankle trying to rotate the leg into swing. As long as the force of the upper body is larger than the ankle’s torque, we stay upright. But, the team found, the longer this occurs, the harder the ankle works to overcome it—eventually loading up enough power to thrust the leg forward. And that’s the magic: a little last-minute snap from the ankle.
It’s like nature came up with a clever trick to circumvent the limits of human body design, Renjewski says. The foot stays planted as long as possible to keep us balanced. But the ankle takes advantage of that downtime, slowly building up energy for the eventual release. (Think of it like a catapult: a heavy mass—your upper body—holds the ankle down. The more it pulls the ankle back, the harder it snaps forward.) The team realized that the second bounce in our gait, when the knee bends just before the foot takes off, gives the ankle the final push it needs to fling the leg into the next step.
Renjewski says that walking this way would have given early humans an edge in persistence hunting—pursuing animals until they surrendered from fatigue. Our flat feet and heavy legs aren’t optimized to let us move as fast as four-legged sprinters, so it’s possible that our gait pattern evolved to grant us an advantage for distance, not speed. Because the second bounce catapults the leg from the ankle, rather than powering its swing from the hip, the motion uses a lot less energy, allowing our ancestors to stalk prey for hours or days without needing to recover.
“It’s a nice simplification of what you might think of as fairly complicated foot mechanics,” says Peter Adamczyk, a biomechanist at the University of Wisconsin-Madison who was not involved in the study. “They’ve essentially computed the way that the force from the rest of your body anchors the ankle against its own torque.” Adamczyk plans to investigate how this model relates to his own work in prosthetic foot design. (Currently, he’s studying how ankles stiffen and loosen for different movements, like running, incline walking, and climbing stairs. This will improve the design of devices that better mimic the natural adjustments human ankles make.)
And though he’s not a roboticist, Adamczyk also speculates that this could eliminate some of the less-than-human ways these machines try to locomote. “One way to control a robot is to think of it as a mass, and where you want that mass to go—then compute the positions, velocities, and accelerations needed to get it there,” he says. But many times, that result looks bizarre. There are infinite ways for a robot to bend its joints to get from point A to point B, but only a handful of them might seem human. Making a robot follow a model derived from our own gait would help winnow out some of the wonkier options.
So is the mystery of the double bounce closed? Renjewski thinks it is. He points out that nature usually takes the simplest route—unless pressured to do otherwise. Humans wouldn’t have evolved this complexity unless it conferred an advantage, he says: “It obviously gave our ancestors some extra benefit that was worth the effort.”
