JOHNSEN, D.A.J.*; GRIFFIN, T.M.; WICKLER, S.J.; COGGER, E.A.; HOYT, D.F.: Does time of contact predict the metabolic cost of walking?
The metabolic rate during trotting and galloping is primarily determined by the rate of generating muscular force to support the body, which is inversely proportional to the time of contact (tc). Does this important generalization also apply to walking? We tested the hypothesis that the rate of oxygen consumption during a walk is proportional to 1/tc. If this is true, then the cost coefficient (the ratio of weight specific metabolic rate to the rate of force application) should be constant across speed. Four horses (average mass = 440 kg) walked on a treadmill at speeds ranging from 1.2 to 1.9 m/s. Time of contact was determined using accelerometers taped to the hind hoof. Oxygen consumption was measured using open flow respirometry in a separate study of three horses walking at speeds ranging from 0.5 to 1.8 m/s on the same treadmill. Using a second order polynomial fit to the oxygen consumption data, we calculated the cost coefficient at each speed where tc was measured. Our results showed a 43% linear increase in the cost coefficient (0.21 to 0.30 J/N) as speed increased in the walk from 1.2 to 1.9 m/s. Thus, time of contact explains nearly 60% of the increase in oxygen consumption across walking speed, compared to about 75% for trotting and galloping. At least two factors may explain the increase in the cost coefficient at faster walking speeds. First, muscles may shorten at a faster velocity at faster walking speeds because mechanical power requirements increase curvilinearly with speed. Second, the mechanical advantage of the limb extensor muscles may decrease at faster speeds, which would increase the force required to support body weight. Supported by NIH S06-GM53933 and NSF IBN-0073020.
