T. J. van der Zee, K. K. Lemaire, and A. J. “Knoek” van Soest
The metabolic cost per unit force is generally thought to increase with the mechanical work done by the muscle fibres. It is currently unclear how the metabolic cost of doing alternating positive/negative muscle fibre mechanical work relates to the metabolic cost of doing zero muscle fibre mechanical work at similar muscle force. The current study aimed at investigating this issue by comparing in vivo metabolic power between a dynamic and an isometric near-constant force production task. In both tasks, participants performed periodic movement about the knee joint in the gravitational field. Therefore, net external mechanical work was constrained to be zero. The tasks mainly differed from each other in average positive knee joint mechanical power, which was 4.3 (0.5) W per leg during the dynamic task and 0.1 (0.1) W per leg during the isometric task. Knee extension torque was near-constant around 15.2 (1.7) N·m during the dynamic task and around 15.7 (1.7) N·m during the isometric task. Due to near-constant knee extension torque, quadriceps tendon length presumably was near-constant during both tasks. Therefore, knee joint mechanical work was predominantly done by the muscle fibres in both tasks. Average metabolic power was 3.22 (0.46) W·kg–1 during the dynamic task and 2.13 (0.36) W·kg–1 during the isometric task. Because tasks differed mainly in the amount of positive muscle fibre mechanical work, these results imply that the metabolic cost of near-constant force production in vivo at zero net mechanical work can be reduced through minimizing positive muscle fibre mechanical work.