Phosphate and acidosis cause fibre‐typespecific changes to cellular and molecular contractile mechanics at 37°C in skeletal muscle from older adults

Abstract Intracellular accumulation of hydrogen ions (H + ) and inorganic phosphate (P i ) has temperature‐dependent effects on single‐fibre contractile function between 10°C and 30°C. In vivo, human skeletal muscle temperatures range between 35‐39°C, and although contractile function is highly dependent on temperature, the effects of fatigue‐inducing H + and P i on contractile mechanics at 37°C are unknown. Using sinusoidal analysis, the independent and combined effects of these metabolites on cellular and molecular contractile function were determined at 37°C in slow‐contracting myosin heavy chain (MHC) I and fast‐contracting MHC IIA fibres from vastus lateralis muscle of 13 older adults (8 females), in four conditions: maximal calcium activation (‘control’; 5 mM P i , pH 7.0), high P i (30 mM), low pH (6.2) and fatigue (30 mM P i and pH 6.2). Specific tension (force/cross‐sectional area, mN/mm 2 ) in both fibre types was reduced only in the fatigue condition (20%–26%). MHC I fibres had slower cross‐bridge kinetics, with fewer or less stiff strongly bound myosin–actin cross‐bridges in high P i , low pH and fatigue. The rate of myosin force production was slowest in low pH and fatigue conditions, whereas the myosin detachment rate was most altered by low pH alone. This indicates that during fatigue, high P i moderates the slowing of cross‐bridge detachment owing to low pH. In contrast, fatigued MHC IIA fibres had faster cross‐bridge kinetics with increased myofilament viscosity. These findings quantify fibre type‐specific mechanical and kinetic mechanisms of fatigue in human skeletal muscle at 37°C, thus advancing our understanding of metabolite‐based muscle fatigue in vivo.