There are many components that make up the muscular system and some show more than 1000-fold variation in nature. Why is there all this variation and by what rules does evolution set the value for a component in a particular muscular system? The results from my laboratory show that there are three simple rules or design goals by which evolution sets the values for these components. First, fibre gear ratio (Δ body movement/Δ sarcomere length) and myofilament lengths are adjusted such that the muscle fibres being used always operate at optimal myofilament overlap where near-maximal force is generated. Second, gear ratio and the maximum velocity of shortening (Vmax) are adjusted so that muscle fibres being used operate at the V/Vmax (where V is the velocity at which the muscle is shortening) where maximum power is generated and where experiments on isolated muscle suggest that efficiency is nearly optimal. Third, the rate of activation and the rate of relaxation are adjusted to be relatively slow, which may permit mechanical power to be produced with optimal efficiency. Thus the way animals produce both slow, low frequency movements and fast, high frequency movements, is by recruiting different fibre types with the appropriate gear ratio, Vmax, and kinetics of activation and relaxation.