Selected variables were measured to examine differences in overground and treadmill running at sprinting speeds of maximal effort. Five college-varsity sprinters volunteered to run 100-yd sprints in both overground and treadmill running conditions. After a minimum of 10 training sessions on the treadmill, the subjects were filmed (75 fps) sprinting 100 yd and expired respiratory gases were collected during an 18-min recovery period. The oxygen debt of the overground condition, means = 47.86 ml X kg-1, was 36% greater than the treadmill running condition, means = 30.64 ml X kg-1. Regardless of individual running style, the major biomechanical differences between treadmill and overground running conditions occurred during the support phase and were observed in the supporting leg. During treadmill running, the leg of the supporting lower extremity was less erect at contact (means = 83.9 vs 88.3 deg) and moved through a greater range of motion (means = 60.6 vs 54.5 deg) with a faster overall angular velocity (means = 566.36 vs 478.07 deg X s-1). The thigh of the supporting lower extremity was more erect at contact (means = 67.1 vs 61.1 deg) and moved with a slower overall angular velocity (means = 435.14 vs 528.77 deg X s-1). Data suggest that the moving treadmill belt reduces the energy requirements of the runner by bringing the supporting leg back under the body during the support phase of running.