For more than 50 years, it has been known that vertebrates engage in a wide range of motor activities and that they possess muscle types with a similarly large range of contractile properties. However, only during the past 15 years has it been shown experimentally that the contractile properties of muscle fibers are well adjusted to their in vivo function. Arriving at this conclusion has required an integrative approach, that is, comparing measurements of muscle fiber properties with measurements of fiber use during normal motor activity. Because the muscles of mammals (and humans) generally are heterogenous in fiber type, this makes it technically very difficult to measure either the contractile properties of different fiber types or their use during normal motor activity. Therefore, many of the advances in the understanding of the design and function of vertebrate muscular systems have come from work on lower vertebrates. Fish, because of the anatomic separation of different muscle fiber types, have provided a key experimental model on which much of what is known about muscle design has been determined. Frogs, because of the near homogeneity of their large extensor muscles used during jumping, also provide an important model which will, in the near future, serve as the first platform where molecular properties of muscle (calcium and cross-bridge kinetics) can be related to whole body movement in a meaningful and predictive manner.