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 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. Abstract Figure: