Why We Swing
In the past, many biomechanical models of gait have omitted the arms. But as such models strive for greater realism, it has become more important to account for secondary movement by the arms.
Most people swing their arms when they walk. Indeed, like several characters in a classic Seinfeld episode, we’re surprised when they don’t. Yet we don’t really need to swing our arms in order to move forward, as we all know when we carry a box with both hands. So why do we swing our arms when we walk? A recent computational model by Jaeheung Park, PhD, a researcher at the Stanford Artificial Intelligence Laboratory at Stanford University, provides some insight.
Arm swinging, Park hypothesized, serves the same purpose as rotational friction—the friction between the foot and the ground that keeps our feet from turning in or out like windshield wipers. And his simulations, published in the Journal of Biomechanics in April 2008, confirmed that possibility.
In the past, many biomechanical models of gait have omitted the arms. But as such models strive for greater realism, it has become more important to account for secondary movement by the arms. One way to do that is to simulate the trajectories of the arms and joints. But Park took a different “task-oriented”approach adapted for human simulations from his thesis advisor’s work on industrial robots.
In his simulations, Park instructed the feet to perform a task—“walk”—but gave no instructions to the arms. Then he varied the amount of rotational friction between the foot and the ground. When the rotational friction forces experienced by the model’s foot were large enough to minimize body movement, the arms didn’t swing. They didn’t need to. But when the rotational friction at the foot was constrained to zero, the arms swung naturally in compensation. This was true for two different styles of walking—static (a kind of slow stagger where the center of mass is always over one foot or the other or both) and dynamic (a more realistic style at a normal human pace).
To Park, these results suggest that arm swinging helps us maintain our balance on slippery surfaces because it compensates for the absent rotational friction. In addition, it provides greater comfort, since the foot and consequently all the leg joints do less work.
“This paper has elucidated the relationship between arm swing and the support moment at the foot,” comments Marcus Pandy, PhD, chair of mechanical and biomedical engineering at the University of Melbourne, Australia. More work remains to be done, though, to understand the relationship between the foot’s role and “energy consumption during gait.” Pandy also notes that “it would be interesting to see how the joint torques predicted by the model compare with those obtained from experiments when humans walk at their preferred normal speeds.”
In the future, Park would like to explore whether arm swinging affects the speed of movement. Eventually, such work might provide more evidence that there is a good reason to swing.