Slow- and fast-contracting skeletal muscle fibers have more similar cellular and molecular contractile function at 37°C than at 25°C in older adults

As human skeletal muscle cellular and molecular contractile properties are temperature-sensitive, the ability to perform experiments at body temperature (~37°C) may lead to a better understanding of their in vivo responses and potentially their effects upon whole-muscle and whole-body performance. We quantified molecular (myosin-actin cross-bridge mechanics and kinetics) and cellular (specific tension; force divided by cross-sectional area) function in slow-contracting myosin heavy chain (MHC) I and fast-contracting MHC IIA fibers from older adults (n=13, 8 female) at 37°C and compared these to results at 25°C. MHC I fibers were more temperature-sensitive than MHC IIA fibers, showing greater increases in cross-bridge kinetics (MHC I: 4.9-8.7x; IIA: 4x) and number or stiffness of strongly-bound cross-bridges (MHC I: 86%; IIA: 34%), leading to increased specific tension in MHC I (19%), with no change in MHC IIA fibers. The expected relationship between fiber force and size (cross-sectional area, CSA) was stronger at 37°C in both fiber types, explaining 80-82% of the variance compared to 51-52% at 25°C. Specific tension was unchanged with size at 37°C in both fiber types, showing that force increases proportionally with CSA, which may be due to the increased number or stiffness of strongly-bound cross-bridges at this temperature. At 25°C, specific tension decreased with size in agreement with previous experiments. Overall, MHC I and IIA fibers at body temperature (37°C) became more analogous, including similar specific tension and closer cross-bridge kinetics, and force production was more strongly correlated with fiber size compared to a non-physiological temperature.