At the Biomechatronics Group at the Massachusetts Institute of Technology, the physics of human walking are being explored to understand how to design and control leg prosthetics to more closely mimic the comfortable ease of the human gait. Historically, the earliest prosthetics bring to mind the solid and unyielding peg legs and hook hands on Captain Hook and his fellow seafarers, cumbersome and incapable of achieving any of the angles that a human joint can assume. Through studies of the skeletal-muscular system of cadavers and recent technological advances that allow for manufacturing of malleable and lightweight materials, today’s prosthetics are made up of many movable parts. It is often assumed that prosthetics are kind of like shoes, in that your height and leg-length are most important for a proper fit, the way that shoe size is dictated by the length and width of your foot. As it turns out, however, the flexibility of the tendons and muscles also play a large role in how each individual walks. In fact, if the lab models the human gait with factors including the tensile strength and stretch of each tendon and the reflexes in each muscle, they can account for the kinetics of human walking over various surfaces, without an autonomous control center, such as the brain, supplying any external information. Thus, walking doesn’t necessarily require conscious thought, which is why we can converse or eat or stare at our phone screens while we walk without too many horrific consequences.
If the design of a human leg has been fine-tuned for walking, then the individual quirks of our legs must factor into the equation. The length of our fibulas and tibias, the amount of cartilage and synovial fluid cushioning each joint, the flexibility of our Achilles tendons, and length and density of each muscle subtly impact the way that each of us takes a stride. Furthermore, our legs and feet assume movements that are developed by habit; hence, the turned-out stance of a ballet dancer or the swagger of a sailor. The way each person takes a step and shifts his or her weight from left foot to right foot is ultimately unique.
Although we often take our footfalls for granted, it is intriguing to consider how different the sound of each person’s walk can be. By simply listening, what can we determine about a person? What do a pair of shoes, slippers, or lack thereof tell us? What does the tempo of a step reveal?