Some advances in integrative muscle physiology.

Integrative muscle physiology has evolved from black box correlations to an understanding of how muscular systems are designed at the molecular level. This paper traces some of the obstacles facing integrative muscle physiology and some of the intellectual and technological breakthroughs which led to the field's development. The ability to determine (1) which fiber types are active, (2) over what sarcomere lengths and velocities they shorten during locomotion and (3) their respective force-velocity relationships, enabled us to show that many muscular systems are designed so that muscles operate at optimal myofilament overlap and at optimal V/Vmax (where maximum power is generated). The ability to impose the in vivo length change and stimulation pattern on isolated muscle has further showed that fish muscle has a relatively slow relaxation rate, and thus rather than generating maximum power during swimming, the muscle appears designed to generate power efficiently. By contrast, during the single shot jump, frog muscle remains maximally activated during shortening and generates maximum power. Recently biophysical techniques have shown that relaxation rate can be altered during evolution by changing (1) Ca2+ transient duration; (2) Ca(2+)-troponin kinetics, and (3) crossbridge kinetics. New technologies will soon enable us to better appreciate how different animal designs evolved.