Energetic cost of producing cyclic muscle force, rather than work, to swing the human leg

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Abstract
We compared two hypothesized energetic costs for swinging the human leg quickly. The first cost is to perform mechanical work on the leg, and the second is to produce muscle force cyclically at high frequencies. Substantial metabolic energy is expended to perform isolated leg swinging, especially at rates greater than the leg's natural pendular frequency. To determine whether the production of muscle force contributes to this cost, we measured oxygen consumption in human subjects (N=6) performing isolated swinging of the leg at frequencies 0.7-1.1 Hz. Amplitude of swing was varied as a function of frequency so that the rate of positive mechanical work performed on the leg remained fixed. We expected that the cost of producing force would increase, in contrast to the cost of performing work. The results showed that average rate of positive mechanical work performed on the leg remained nearly constant as a function of frequency, at 0.073±0.014 W kg-1. Net metabolic rate, however, increased by 53%, from 0.66 W kg-1 to 1.01 W kg-1. Work may be performed on the leg and with a proportional metabolic cost, but it cannot explain the substantial increases observed here. Metabolic energy expenditure appears to increase in proportion with muscle force or torque, and in inverse proportion to duration of force. This energetic cost may be associated with cyclical calcium transport, where rate-limiting of crossbridge attachments may require greater sarcoplasmic calcium concentration at high frequencies of leg swinging to produce the same amplitude of muscle force. It may also be relevant to moving the legs back and forth relative to the body during walking.