Potential role of passively increased muscle temperature on contractile function

Abstract

Declines in muscle force, power, and contractile function can be observed in older adults, clinical populations, inactive individuals, and injured athletes. Passive heating exposure (e.g., hot baths, sauna, or heated garments) has been used for health purposes, including skeletal muscle treatment. An acute increase in muscle temperature by passive heating can increase the voluntary rate of force development and electrically evoked contraction properties (i.e., time to peak twitch torque, half-relation time, and electromechanical delay). The improvements in the rate of force development and evoked contraction assessments with increased muscle temperature after passive heating reveal peripheral mechanisms' potential role in enhancing muscle contraction. This review aimed to summarise, discuss, and highlight the potential role of an acute passive heating stimulus on skeletal muscle cells to improve contractile function. These mechanisms include increased calcium kinetics (release/reuptake), calcium sensitivity, and increased intramuscular fluid.

Keywords: Calcium kinetics; Heat stress; Muscle fluid; Muscle strength; Muscle temperature; Neuromuscular function; Passive heating; Rate of force development.

Fig. 1

Conceptual diagram of increased Ca²⁺ kinetics (release/reuptake) triggered by a passive rise in muscle temperature. Arrows indicates the increased Ca²⁺ flow from sarcoplasmic reticulum trough ryanodine receptors (RyR) and transient receptor potential vanilloid 1 (Trpv1) channels to myoplasm. However, it is unclear if sarcoplasmic Ca²⁺ release is triggered by RyR and Trpv1 or Trpv1 alone. Myoplasm Ca²⁺ unblocks the sites between actin and myosin heads increasing the cross-bridge formations. Subsequently, the sarcoplasmic reticulum Ca²⁺ ATPase (SERCA) channel reuptakes the Ca²⁺ back to the sarcoplasmic reticulum

Fig. 2

a Representative cross-sectional positron-emission tomography (PET) blood flow image from the middle calf at normothermic (control) condition (muscle temperature at ~ 33.4 °C) and immediately after a local passive heating session (muscle temperature at ~ 37.4 °C). Image taken with permission from Heinonen et al. (2011). b A hypothetical schematic representation of changes in muscle fibre shape during contraction at thermoneutral and passively heated condition. The red circles denote the cross-sectional area of the muscle fibre, and the blue dots denote the intramuscular fluid accumulation. In a thermoneutral condition, during contraction, the muscle fibre tends to compress in the thickness direction changing the shape of the muscle expanding in the radial direction (i.e., width) (Eng et al. 2018). After a passive heating session, with increased muscle temperature and fluid, the incompressible nature of the fluid inside the cells increases the muscle cells’ pressure, decreasing muscle fibre deformation during contraction and creating a spring-like property