The phalangeal portion of the forefoot is extremely short relative to body mass in humans. This derived pedal proportion is thought to have evolved in the context of committed bipedalism, but the benefits of shorter toes for walking and/or running have not been tested previously. Here, we propose a biomechanical model of toe function in bipedal locomotion that suggests that shorter pedal phalanges improve locomotor performance by decreasing digital flexor force production and mechanical work, which might ultimately reduce the metabolic cost of flexor force production during bipedal locomotion. We tested this model using kinematic, force and plantar pressure data collected from a human sample representing normal variation in toe length (N=25). The effect of toe length on peak digital flexor forces, impulses and work outputs was evaluated during barefoot walking and running using partial correlations and multiple regression analysis, controlling for the effects of body mass, whole-foot and phalangeal contact times and toe-out angle. Our results suggest that there is no significant increase in digital flexor output associated with longer toes in walking. In running, however, multiple regression analyses based on the sample suggest that increasing average relative toe length by as little as 20% doubles peak digital flexor impulses and mechanical work, probably also increasing the metabolic cost of generating these forces. The increased mechanical cost associated with long toes in running suggests that modern human forefoot proportions might have been selected for in the context of the evolution of endurance running.
We are grateful to the subjects who participated in this study. We thank Alan Tomasko for his assistance with data collection, David Raichlen for help early on with data processing and two anonymous reviewers for suggestions on improving the manuscript. Funding was provided by a Leakey Foundation Research Grant and an NSERC Postgraduate Scholarship to C.R., by the American School for Prehistoric Research and the Department of Anthropology at Harvard University, and by NSF (BCS 044033 to D.E.L.).
↵* Present address: Department of Cell Biology and Anatomy, University of Calgary, G503, 3330 Hospital Drive, NW Calgary, Alberta, T2N 4N1 Canada