Hence, if the passive tension is great enough to affect force production ( De Tombe and ter Keurs, 1992), it may cause a thermal dependence of the qualitative relationship between ATP use and muscle power output.
This passive tension decreases with increasing temperature because muscle becomes less viscous ( Mutungi and Ranatunga, 1998). The resistance of skeletal muscle to length changes comprises viscous and elastic components of the sarcomere that are independent from crossbridge formation ( De Tombe and ter Keurs 1992 Fukuda et al., 2005 Mutungi and Ranatunga, 1998 Granzier and Wang, 1993). It is possible that the relationship between ATP use and power output can change with temperature. However, it is as yet unresolved whether temperature alters the energetics of muscle performing work. Variation in environmental temperature affects both locomotor and muscle performance ( Garland et al., 1990 James, 2013). Our data demonstrate the potential energetic benefits of warming up muscle before activity, which is seen in diverse groups of animals such as bees, which warm flight muscle before take-off, and humans performing warm ups before exercise.
To maintain activity across a range of temperature, animals must increase ATP production or face an allocation trade-off at lower temperatures. The metabolic cost of muscle performance and activity therefore increased with a decrease in temperature. Muscle therefore consumed significantly more oxygen at 15☌ for a given work output than at 25☌, and plastic responses did not modify this thermodynamic effect. Oxygen consumption of isolated muscle at rest did not change with test temperature, but oxygen consumption while muscle was performing work was significantly higher at 15☌ than at 25☌, regardless of acclimation conditions. Isolated muscle produced greater tetanus force, and faster isometric force generation and relaxation, and generated more work loop power at 25☌ than at 15☌ acute test temperature.
However, acclimation temperature did not affect isolated gastrocnemius muscle biomechanics.
Cold-acclimated frogs had greater sprint speed at 15☌ than warm-acclimated animals. To account for temperature variation at different time scales, we considered the interaction between acclimation for 4 weeks (to 15 or 25☌) and acute exposure to these temperatures. Here, we tested the hypothesis that ATP use increases at lower temperatures for a given power output in Xenopus laevis. However, the viscosity and stiffness of muscle increases with a decrease in temperature, which means that more ATP may be required to achieve a given work output. It is expected, therefore, that there is strong selection to maximise muscle power output for a given rate of ATP use. Metabolic energy (ATP) supply to muscle is essential to support activity and behaviour.
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