It is well known that muscle contraction is temperature sensitive; in mammalian skeletal muscle, both the active force and shortening velocity decrease in cooling from 35°C to 10°C so that the maximal mechanical power output of muscle is reduced to ~5-10% at 10°C (Ranatunga, 1998). In order to identify the underlying mechanism(s), rapid temperature-jump (T-jump) technique has been used; T-jump experiments on maximally Ca-activated skinned fibres from rabbit psoas and tetanised intact fibres from a rat foot muscle (flexor hallucis brevis) have shown that active muscle force is endothermic, i.e. force increases as heat is absorbed in warming (see references in Coupland & Ranatunga, 2003). The main findings from such temperature-studies on isometric muscle were the following. Firstly, whereas the rigor force decreases linearly, the active force in muscle increases ~two-fold when the temperature is raised from ~10°C to high physiological (>30°C) temperatures. Force versus reciprocal absolute temperature is sigmoidal with indication of saturation at the physiological temperatures; the increase of active force on heating is not accompanied by a concomitant increase of stiffness and, hence, it is not due to an increase in the fraction of attached crossbridges (see references in Coupland & Ranatunga, 2003; Roots & Ranatunga, 2008). Secondly, a rapid T-jump (3-5°C in <0.2 ms) induces a bi-exponential rise (labelled phase 2b and phase 3) in force to a level as expected from steady state experiments; the (faster) phase 2b is identified as “endothermic force generation” in attached crossbridges; so the increase of force in warming is due to conversion of low-force to high force crossbridge states. Finally, on the basis of its sensitivity to inorganic phosphate (Pi), the endothermic force generation has been identified as a molecular step before Pi-release in the acto-myosin-ATPase cycle that is coupled to the crossbridge cycle (Ranatunga, 1999). Pi-induced force depression is temperature-sensitive, being less marked at higher temperatures (Coupland et al., 2001). On the basis of the well-known force-velocity relation, active muscle force decreases below the isometric level during steady shortening and increases above the isometric level during lengthening, in a velocity dependent manner. Therefore, in a recent study (Ranatunga et al., 2007), we examined endothermic force generation in shortening and lengthening muscle fibres. A muscle fibre was maximally Ca-activated at ~10°C and ramp shortening or lengthening of different velocities applied; when the force reached a steady level, a ~0.2 ms laser pulse (λ = 1.32μ) induced a T-jump of ~3°C in the fibre. The experiments showed that endothermic crossbridge force generation is enhanced during muscle shortening but inhibited during lengthening; a T-jump did not lead to a net tension rise during lengthening. Taking T-jump force generation as a signature of the acto-myosin-ATPase cycle, results suggest that the cycle operates faster during shortening but it is inhibited during lengthening; this would account for the well known Fenn effect that energy liberation in muscle is enhanced during shortening and inhibited during lengthening. In addition to providing some insight into the contractile mechanisms, the basic thesis that has emerged from temperature studies – such as the above and studies on different muscle types, including human muscles, is that active muscle force is endothermic (force rises on heating) but its exact underlying molecular mechanism remains to be elucidated. Given that temperature in peripheral muscles in the body changes readily depending on the ambient conditions and exercise, findings summarised above have some relevance with regard to muscle function. The fact that mechanical power-output is markedly temperature-sensitive, even at higher temperatures, indicates that temperature may be a determining factor for optimal in situ muscular performance (Ranatunga, 1998). The finding that active muscle force is endothermic and its sensitivity to products of ATP hydrolysis is decreased with warming suggests that effect of accumulation of products (e.g. Pi), that is thought to be a major determinant of muscle fatigue, would be less marked at the higher physiological temperatures; indirect evidence to that effect was produced in a recent study (see Roots et al., 2009).