The application of ground force explains the energetic cost of running backward and forward.

We compared backward with forward running to test the idea that the application of ground force to support the weight of the body determines the energetic cost of running. We hypothesized that higher metabolic rates during backward versus forward running would be directly related to greater rates of ground force application and the volume of muscle activated to apply support forces to the ground. Four trained males ran backward and forward under steady-state conditions at eight treadmill speeds from 1.75 to 3.50 m x s(-1). Rates of oxygen uptake were measured to determine metabolic rates, and inverse periods of foot-ground contact (1/tc) were measured to estimate rates of ground force application. As expected, at all eight speeds, both metabolic rates and estimated rates of ground force application were greater for backward than for forward running. At the five slowest speeds, the differences in rates of ground force application were directly proportional to the differences in metabolic rates between modes (paired t-test, P<0.05), but at the three highest speeds, small but significant differences in proportionality were present in this relationship. At one of these three higher speeds (3.0 m x s(-1)), additional measurements to estimate muscle volumes were made using a non-invasive force plate/video technique. These measurements indicated that the volume of muscle active per unit of force applied to the ground was 10+/-3% greater when running backward than forward at this speed. The product of rates of ground force application and estimated muscle volumes predicted a difference in metabolic rate that was indistinguishable from the difference we measured (34+/-6% versus 35+/-6%; means +/- s.e.m., N=4). We conclude that metabolic rates during running are determined by rates of ground force application and the volume of muscle activated to apply support forces to the ground.