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.
