The metabolic cost of force generation.

INTRODUCTION The purpose of this study was to provide support, based on a review of existing data, for a general relationship between metabolic cost and force generated. There are confounding factors that can affect metabolic cost, including muscle contraction type (isometric, eccentric, or concentric), length, and speed as well as fiber type (e.g., fast or slow) and moment arm distances. Despite these factors, empirical relationships for metabolic cost have been found that transcend species and movements. METHODS We revisited the various equations that have been proposed to relate metabolic rate with mass, velocity, and step contact time during running and found that metabolic rate was proportional to the external force generated and the number of steps per unit time. This relationship was in agreement with a previously proposed hypothesis that the metabolic cost to generate a single application of a unit external force is a constant. RESULTS Data from the literature were collected for a number of different activities and species to support the hypothesis. Running quadrupedal and bipedal species, as well as human cycling, cross-country skiing, running (forward, backward, on an incline, and against a horizontal force), and arm activities (running, cycling, and ski poling), all had a constant metabolic cost per unit external force per application. CONCLUSION The proportionality constant varied with activity, possibly reflecting differences in the aspects of muscular contraction, fiber types, or mechanical advantage in each activity. It is speculated that a more general relation could be obtained if biomechanical analyses to account for other factors, such as contraction length, were included.