Phosphate and acidosis cause fiber-type specific changes to cellular and molecular contractile mechanics at 37°C in skeletal muscle from older adults

Intracellular accumulation of hydrogen ions (H ⁺ ) and inorganic phosphate (P _i ) have temperature-dependent effects on single fiber contractile function between 10-30°C. In vivo , human skeletal muscle temperatures range between 35-38°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 is 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 fibers from vastus lateralis muscle of 13 older adults (8 females), under 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 fiber types was reduced only under fatigue conditions (20-26%). MHC I fibers had slower cross-bridge kinetics with fewer or less stiff strongly-bound myosin-actin cross-bridges in high P _i , low pH, and fatigue. In contrast, fatigued MHC IIA fibers had faster cross-bridge kinetics with increased myofilament and/or cross-bridge viscosity. Single fiber oscillatory work was reduced in both fiber types when P _i or pH alone was altered. However, fatigue conditions returned oscillatory work values toward control through alterations to cross-bridge kinetics in MHC I fibers and changes to work absorption and production processes in MHC IIA fibers. These findings quantify fiber-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 .