In chapters 18 to 20 we have been very largely concerned with the properties of vertebrate twitch skeletal muscle, exemplified by the sartorius of the frog. But it must be realized that the frog sartorius represents only one of a considerable variety of types of muscle, and a rather specialized one at that.

Muscles vary greatly in the time characteristics of their contractions. Some have much higher velocities of shortening than others; the muscles of the hare contract faster than do those of the tortoise. Muscles involved in sound production often contract more rapidly than do the locomotor muscles. Even similar muscles from similar animals have higher contraction speeds in small animals than in large animals; compare the wing-beat frequencies of a sparrow and a pelican (Hill, 1950b). These differences are correlated with differences in the ATPase activities of the myosin extracted from the various muscles (Bárány, 1967), and these in turn with differences in the amino acid sequences of the myosin and other myofibrillar molecules (Schiaffino & Reggiani, 1996).

The way the contractile machinery is organized is not uniform, although all muscles work via actin–myosin interactions involving the splitting of ATP. Striated muscles from different animal phyla may have different arrangements of the thick and thin filaments. Not all muscles are striated. The arrangement of myosin molecules in the thick filaments varies. Some muscles contain extra proteins such as paramyosin in their filaments.