The basis of differences in thermodynamic efficiency among skeletal muscles

Summary Muscles convert chemical free energy into mechanical work. The energy conversion occurs in 2 steps. First, free energy obtained from oxidation of metabolic substrates (Δ G S ) is transferred to ATP and, second, free energy from ATP hydrolysis (Δ G ATP ) is converted into work by myosin cross‐bridges. The fraction of Δ G S transferred to ATP is called mitochondrial efficiency (η M ) and the fraction of Δ G ATP converted into work is called cross‐bridge efficiency (η CB ). Overall cross‐bridge efficiency varies among muscles from ~20% and 35% and the analysis presented in the current studies shows that this variation is largely due to differences in η CB whereas η M is similar (~80%) in all the muscles assessed. There is an inverse, linear relationship between maximum normalised power output and η CB ; that is, more efficient muscles tend to be less powerful than less efficient muscles. It is proposed that differences in cross‐bridge efficiency reflect the extent to which cross‐bridges traverse the force‐length relationship for attached cross‐bridges. In this framework, cross‐bridges from tortoise muscle (η CB = 45%) produce close to the maximum possible work a cross‐bridge can perform in a single attachment cycle.