The importance of comparative physiology: mechanisms, diversity and adaptation in skeletal muscle physiology and mechanics.

Skeletal muscle powers animal movement, making it an important determinant of fitness. The classic excitation-contraction coupling, sliding-filament and crossbridge theories are thought to describe the processes of muscle activation and the generation of force, work and power. Here, we review how the comparative, realistic muscle physiology typified by Journal of Experimental Biology over the last 100 years has supported and refuted these theories. We examine variation in the contraction rates and force-length and force-velocity relationships predicted by these theories across diverse muscles, and explore what has been learnt from the use of workloop and force-controlled techniques that attempt to replicate aspects of in vivo muscle function. We suggest inclusion of features of muscle contraction not explained by classic theories in our routine characterization of muscles, and the use of phylogenetic comparative methods to allow exploration of the effects of factors such as evolutionary history, ecology, behavior and size on muscle physiology and mechanics. We hope that these future directions will improve our understanding of the mechanisms of muscle contraction, allow us to better characterize the variation in muscle performance possible, and enable us to infer adaptation.