The basis of differences in thermodynamic efficiency among skeletal muscles.

Muscles convert chemical free energy into mechanical work. The energy conversion occurs in 2 steps. First, free energy obtained from oxidation of metabolic substrates (ΔGS ) is transferred to ATP and, second, free energy from ATP hydrolysis (ΔGATP ) is converted into work by myosin cross-bridges. The fraction of ΔGS transferred to ATP is called mitochondrial efficiency (ηM ) and the fraction of ΔGATP converted into work is called cross-bridge efficiency (ηCB ). 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 ηCB whereas ηM is similar (~80%) in all the muscles assessed. There is an inverse, linear relationship between maximum normalised power output and ηCB ; 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 (ηCB  = 45%) produce close to the maximum possible work a cross-bridge can perform in a single attachment cycle.