The ground reaction forces at the foot and the shock transmitted through the body to the head when running on different surfaces has been presented. Although differences in the vertical force and acceleration were measured, they appear to be relatively small. It may be possible that the runner is subconsciously able to adjust the stiffness of his leg just prior to heel strike based upon his perception of the hardness of the surface. It is doubtful that differences of this small magnitude in vertical force would lead to a higher incidence of injury on a particular surface. More likely to be a causative factor might be the rapid transmission of the shock wave through the body on a harder surface, like concrete or asphalt, and the apparent limitation of the runner's ability to dampen the high-frequency shock waves as his speed increases.