CARD, G.*; WEYAND, P.; BIEWENER, A.A.: Changes in Effective Mechanical Advantage of the Human Limb during Sprint Acceleration

How do people go fast? Previous studies have examined the role of leg muscle force in relation to energy cost during running at various steady speeds, but few have addressed how the leg muscles contribute to accelerating the body to attain those speeds. We examined humans sprinting from a crouch to top speed. Using a runway with an imbedded force plate and synchronized video data, we were able to determine the effective mechanical advantage (EMA) of the limb at different steps of each subject’s sprint acceleration to top speed. EMA is defined as the ratio of limb muscle force to ground reaction force integrated over the time of limb contact with the ground. It was measured at each of the leg joints (hip, knee, ankle) and then averaged for an overall measure. Data from the nine runners measured indicate that, as opposed to steady state running in which EMA does not change with speed, EMA increases slightly as the subject increases speed by accelerating in the first several steps of a sprint. EMA distribution between joints also changes during acceleration, with hip EMA starting low and increasing while knee EMA starts high and decreases. These results imply that during rapid acceleration, more of the force is supplied by hip extensor muscles whereas during steady state running, these muscles have diminished importance and supporting force is generated more by knee extensors. Understanding muscle mechanics during acceleration thus has implications for fiber type distribution at different joints and for better quantifying the mechanics and energetics of free-ranging animals who rarely move at a steady rate.