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First published online January 25, 2005
Journal of Experimental Biology 208, 439-445 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01408

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Mechanics and energetics of swinging the human leg

Jiro Doke^1,*, J. Maxwell Donelan² and Arthur D. Kuo¹

¹ Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109-2125 USA
² Department of Physiology, University of Alberta, Edmonton, AB T6G 2H7 Canada

^* Author for correspondence (e-mail: jdokeh{at}umich.edu)

Accepted 25 November 2004

We measured how much metabolic energy is expended to swing a human leg. A previous dynamical model of walking predicted that increasing metabolic costs for walking with step length and step frequency trade-off against each other to determine the optimum step combination at a given speed. Simple pendulum dynamics indicate that the cost of walking at high step frequencies could be associated with driving the legs back and forth relative to the body, at a rate increasing approximately with the fourth power of frequency, possibly due to the low economy of producing muscle force for short durations. A similar cost would be expected for isolated swinging of a leg at faster than its natural frequency. We constructed an apparatus to measure work performed on the leg, and measured metabolic cost as human subjects (N=12) swung one leg at frequencies 0.5-1.1 Hz and fixed amplitude. Rate of mechanical work ranged from 0.02-0.27 W kg^-1 over these frequencies. Net metabolic rate for leg swinging (subtracting that for quiet standing) increased from 0.41-2.10 W kg^-1, approximately with the fourth power of frequency (R²=0.92) and in proportion to a hypothesized cost of force production for short durations. The costs of producing force and work could account for the increase. In a crude comparison, moving the legs back and forth at a typical stride frequency of 0.9 Hz, might consume about one-third of the net energy (2.8±0.8 W kg^-1) needed for walking at 1.3 m s^-1.

Key words: metabolic energy, locomotion, biomechanics, muscle, force production

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