Discover recently got a peek at Adidas’s innovative adiSTAR Trail shoe. (It won’t go on sale until August.) The designers—as well as top biomechanists—explain what goes into a modern running shoe.
Photograph by Craig Cutler
Too little traction and the runner slips; too much and the runner sticks to the road, which could cause injury. The best nonslip materials, such as rubber, wear out quickly; more durable materials, like polyurethane, are too slippery. Running shoes like this men’s size 9 have two surfaces: an outside edge of soft rubber for pavement and harder thermoplastic urethane cleats for dirt.
Studies show that even a few extra ounces of shoe weight increase the workload considerably. Barefoot running decreases energy needs up to 5 percent depending on the surface. To keep shoes light (these weigh 12.75 ounces each), the upper skeletons are made with nonstretch synthetics and are covered with a flexible but abrasion-resistant plastic mesh.
Can any running shoe make you faster? No. But by absorbing shock, the elastomers in this shoe can make movement less fatiguing.
The end of the stride poses a greater risk of injury than anything else, argues Benno Nigg, professor of biomechanics at the University of Calgary in Alberta. During “toe-off,” the foot acts like a lever, putting ankle muscles and ligaments under tremendous tension—as much as seven times the body’s weight. So far, other biomechanists say, those forces haven’t been proved to cause injury.
Feet tend to pronate—roll inward and forward onto the ball. Too much may lead to knee, hip, and foot injuries. Here Adidas uses a new running-shoe technology inspired by the Mercedes SL series suspension systems. Inside the two-piece heel, dual curved polyimide plates slide sideways and backward as much as one-third of an inch before being snapped back into place by four stiff rubber springs. The motion decelerates the foot over 25 milliseconds, lessening pronation.
Eighty percent of runners land here, hitting a three- to four-square-inch area 800 or more times every mile with a force of up to three times their body weight. The pounding, which may cause tendinitis, stress fractures, shin splints, or knee pain, is a major concern of shoe designers. Adidas uses ethylene vinyl acetate, a polymeric foam, to absorb about 40 percent of the force.