ABSTRACT
Terrestrial locomotion on legs is energetically expensive. Compared with cycling, or with locomotion in swimming or flying animals, walking and running are highly uneconomical. Legged gaits that minimise mechanical work have previously been identified and broadly match walking and running at appropriate speeds. Furthermore, the ‘cost of muscle force’ approaches are effective in relating locomotion kinetics to metabolic cost. However, few accounts have been made for why animals deviate from either work-minimising or muscle-force-minimising strategies. Also, there is no current mechanistic account for the scaling of locomotion kinetics with animal size and speed. Here, we report measurements of ground reaction forces in walking children and adult humans, and their stance durations during running. We find that many aspects of gait kinetics and kinematics scale with speed and size in a manner that is consistent with minimising muscle activation required for the more demanding between mechanical work and power: spreading the duration of muscle action reduces activation requirements for power, at the cost of greater work demands. Mechanical work is relatively more demanding for larger bipeds – adult humans – accounting for their symmetrical M-shaped vertical force traces in walking, and relatively brief stance durations in running compared with smaller bipeds – children. The gaits of small children, and the greater deviation of their mechanics from work-minimising strategies, may be understood as appropriate for their scale, not merely as immature, incompletely developed and energetically sub-optimal versions of adult gaits.
FOOTNOTES
Competing interests
The authors declare no competing or financial interests.
Author contributions
T.Y.H. and J.R.U. performed the measurements and theoretical development; T.Y.H. led the kinetic and kinematic analyses; J.R.U. developed the numerical models. Both authors gave final approval for publication.
Funding
This work was funded by the Biotechnology and Biological Sciences Research Council [BB/G021627/1] and a Wellcome Trust Fellowship to J.R.U. [095061/Z/10/Z]. Deposited in PMC for immediate release.
Supplementary material
Supplementary material available online at http://jeb.biologists.org/lookup/suppl/doi:10.1242/jeb.122135/-/DC1
- © 2015. Published by The Company of Biologists Ltd
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