During each step of human walking, the swing foot passes close to the ground with a small but (usually) non-zero clearance. The foot can occasionally scuff against the ground, with some risk of stumbling or tripping. The risk might be mitigated simply by lifting the foot higher, but presumably at increased effort, of unknown amount. Perhaps the normally preferred ground clearance is a trade-off between competing costs, one for lifting the foot higher and one for scuffing it. We tested this by measuring the metabolic energy cost of lifting and scuffing the foot, treating these apparently dissimilar behaviors as part of a single continuum, where scuffing is a form of negative foot lift. We measured young, healthy adults (N=9) lifting or scuffing the foot by various amounts mid-swing during treadmill walking, and observed substantial costs, each well capable of doubling the net metabolic rate for normal walking (gross cost minus that for standing). In relative terms, the cost for scuffing increased over twice as steeply as that for lifting. That relative difference means that the expected value of cost, which takes into account movement variability, occurs at a non-zero mean clearance, approximately matching the preferred clearance we observed. Energy cost alone is only a lower bound on the overall disadvantages of inadvertent ground contact, but it is sufficient to show how human behavior may be determined not only by the separate costs of different trade-offs but also by movement variability, which can influence the average cost actually experienced in practice.
The authors declare no competing or financial interests.
A.R.W. and A.D.K. conceived the study and drafted the manuscript. A.R.W. designed and carried out the experiments and data analyses. Both authors read and approved the final manuscript.
This work was supported in part by the Office of Naval Research (ETOWL program), National Institutes of Health (AG030815), and US Department of Defense (W81XWH-09-2-0142, National Defense Science & Engineering Graduate Fellowship Program). Deposited in PMC for release after 12 months.
- Received January 15, 2016.
- Accepted July 22, 2016.
- © 2016. Published by The Company of Biologists Ltd