Proposed complementary effects of myosin and regulatory proteins on the temperature dependence of sarcomeric activation in fast skeletal versus cardiac muscle. Left: Regulation of the thin filament state by Ca²⁺ (top) or heating (bottom). Ca²⁺ binding to TnC shifts the thin filament state from off to on (top). Heating induces Ca²⁺-independent thermal activation of thin filaments via partial dissociation of Tm–Tn from F-actin (or weakening of the binding of Tm–Tn to F-actin), resulting in sliding movements of thin filaments in an in vitro motility assay (sliding velocity faster with fast skeletal Tm–Tn on fast skeletal myosin [red line] than with cardiac Tm–Tn on cardiac myosin [blue line]; see Figs. 2 and 3). Right: Complementary relationship between temperature and Tm–Tn regulation (left) or ATPase cycle of myosin (right) that causes Ca²⁺-independent thermal activation observed in the present study. Curved arrows indicate the rates of attachment or detachment of myosin to actin. The present as well as previous studies (Chandy et al., 1999; Houmeida et al., 2010; Risi et al., 2017; Heeley et al., 2002, 2006, 2019) suggest that the inhibitory effect of Tm–Tn on the thin filament state (i.e., Tm–Tn regulation) against a rise of temperature is weaker in cardiac muscle (blue line) than in fast skeletal muscle (red line). Therefore, an increase in temperature to within the body temperature range activates thin filaments to a greater magnitude in fast skeletal muscle (high Q₁₀) than in cardiac muscle (low Q₁₀). The ATPase cycle of fast skeletal myosin (red line) is faster than that of β-cardiac myosin (blue line) (Walklate et al., 2016), which results in a relative loss of recruitable heads in fast skeletal muscle at temperatures lower than body temperature. Accordingly, an increase in temperature to the body temperature range activates myosin to a greater magnitude in cardiac muscle (high Q₁₀) than in fast skeletal muscle (low Q₁₀). This way, the temperature dependence of striated muscle contraction is regulated by Tm–Tn and myosin in a complemental manner.