By Steve McCaw

Part of Biomechanics For Dummies Cheat Sheet

Humans have been running for millions of years. Large forces are produced at the foot–ground interface when running. The force from the ground stops the downward motion and slows the forward motion of the runner during the first half of ground contact, and then propels the runner upward and forward into the next running step during the second half of ground contact. Larger forces are produced to run faster and when running on harder surfaces, like concrete or asphalt (as opposed to softer surfaces, such as grass or dirt).

The foot is a structural marvel because of its anatomy. The 26 bones of the foot are aligned in two arches: one extending the length of the foot (the longitudinal arch) and the other traversing across the foot (the transverse arch). The arches are supported by muscles and ligaments. The foot’s anatomy allows it not only to serve as a flexible lever to help absorb energy during the first half of ground contact, but also to become a rigid lever to push the body into the next step during the second half of ground contact.

During the first half of ground contact, the foot pronates, a combination of inward rotation along the length of the foot (eversion), upward rotation of the foot toward the shin (dorsiflexion), and outward rotation of the foot relative to the tibia (external rotation). Muscles pulling on the foot act eccentrically (pull while getting longer) to control the rate and extent of pronation. The second half of ground contact is a reversal of the pronation. During this phase, the foot supinates, a combination of outward rotation along the length of the foot (inversion), downward rotation of the foot away from the shin (plantarflexion), and inward rotation of the foot relative to the shin (internal rotation). Muscles pulling on the foot act concentrically (pull while getting shorter) to cause the supination.

Pronation is a critical part of absorbing energy, and supination is a critical part of generating energy, and the two actions of the foot are coordinated with the flexion and extension occurring at the knee when running. The amount of pronation and supination differs among individuals, because of differences in skeletal structure, muscle strength and endurance, and running style.

Running shoes provide an interface between a runner’s feet and the ground. A main purpose of shoes is to protect a runner from the dangers on the ground surface like sharp rocks, jagged pavement, or broken glass. A tough material called the outsole on the bottom of the shoe provides this protection. The rest of the shoe is a manmade attempt to improve on the evolutionary design of the foot itself by increasing energy absorption (a feature called cushioning) and controlling the pronation and supination of the foot (a feature called stability).

There is a major trade-off in creating a shoe to provide both cushioning and stability: A shoe with more cushioning provides less stability, and a shoe with more stability provides less cushioning. This tradeoff results from the materials used to make the shoe and how they’re put together.

No one shoe is ideal for everyone. If you currently run in shoes that are comfortable and you’ve been injury free, buy another pair just like them when it’s time to replace your shoes. (Better yet, buy several pairs at the same time, because shoe manufacturers have the tendency to replace their current models with “newer and better” models every year or two.) When you first start using a new pair of shoes, don’t make big changes in the distance, speed, or terrain you run on for at least a few training sessions. You want to make sure you maintain a consistent running routine so that if you develop pain, you know for sure it’s the shoe causing the problem, not the fact that your routine has changed.