Force rate dependency of metabolic cost during cyclic muscle contraction

Muscle fibres consume metabolic energy to deliver force and do mechanical work. Less well characterized is a separate metabolic cost for cyclic force production, that is not included in current models for metabolic cost. We therefore characterized the metabolic cost for cyclic muscle force, hypothesized to increase with the frequency of force production. Six healthy participants exerted isometric knee torque of fixed sinusoidal amplitude at frequencies from 0.5 to 2.5 Hz, while seated in a knee dynamometer chair. Metabolic power, obtained from respirometry, increased with 0.25 W/kg/Hz (95% CI: 0.17 – 0.33 W/kg/Hz). This increase was not explained by force-time integral, nor by muscle fibre mechanical work done. The increase in metabolic cost was explained by the time-derivative of force (i.e. “force rate"), which is thought to demand calcium pumping in muscle. A force rate dependent metabolic cost may contribute substantially to tasks that entail cyclic force production, for example