Isometric slow- and fast-contracting skeletal muscle fibres have more similar cellular and molecular properties 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 on whole-muscle and whole-body performance. We utilized sinusoidal analysis to quantify molecular (myosin-actin cross-bridge mechanics and kinetics) and cellular (specific tension; force divided by cross-sectional area) functions in slow-contracting myosin heavy chain (MHC) I and fast-contracting MHC IIA fibres from older adults (N = 13, 8 female) at 37°C and compared these to results at 25°C. MHC I fibres were more temperature-sensitive than MHC IIA fibres, showing greater increases in cross-bridge kinetics (MHC I: 6.7-9.4-fold; IIA: 4.7-4.8-fold) and number or stiffness of strongly bound cross-bridges (MHC I: 62%-77%; IIA: 31%-32%), leading to increased specific tension in MHC I (19%), but no change in MHC IIA fibres. The relationship between force and size (cross-sectional area (CSA)) was stronger at 37°C in both fibre types, explaining 80%-82% of the variance compared to 51%-52% at 25°C. Specific tension was not associated with size at 37°C in both fibre types, indicating force increases proportionally with CSA, potentially due to the increased number or stiffness of strongly bound cross-bridges at this temperature. At 25°C, specific tension decreased as size increased, in agreement with previous experiments. Overall MHC I and IIA fibres at body temperature and at near-isometric conditions became more analogous, including similar specific tensions and closer cross-bridge kinetics, and force production being more strongly correlated with fibre size compared to a subphysiological temperature. KEY POINTS: Although skeletal muscle function is highly sensitive to temperature, human single-fibre studies have been conducted only at ≤ 30°C. Small-amplitude sinusoidal perturbations were utilized to elucidate mechanisms of single-fibre force production at human physiological temperature (37°C). We found that functional differences in slow-contracting myosin heavy chain (MHC I) and fast-contracting MHC IIA fibres observed at 25°C were less apparent at 37°C, as force, cross-bridge kinetics and strongly bound cross-bridges increased more in MHC I fibres than MHC IIA fibres with the increase in temperature. These results indicate that, given the different sensitivity of each fibre type to changes in temperature, functional assessments of muscle should be conducted at 37°C to better translate to in vivo conditions.