Abstract
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 . KEY POINTS SUMMARY: Working skeletal muscle increases intracellular concentrations of hydrogen ion and inorganic phosphate, leading to fatigue, or loss of force-generating capacityTemperature plays a well-established role in the muscle response to hydrogen ion and/or inorganic phosphate accumulation, but has not previously been studied at human body temperature (37°C)At 37°C, reduced force generation only occurs when high phosphate and hydrogen ions are combined, not when changed individuallyIn slow-contracting fibers, fatigue slowed myosin-actin cross-bridge kinetics and reduced the number or stiffness of strongly-bound cross-bridges. In fast-contracting fibers, fatigue increased myosin-actin cross-bridge kinetics and increased myofilament viscosity.The distinct responses by fiber type to fatigue provides new insight into its mechanisms and advances our understanding of the whole muscle and body responses to fatigue.