The relation between velocity of shortening and the tension‐length curve of skeletal muscle

Many different types of apparatus have been used in the past by physiologists to study the active shortening of muscle. Isotonic, auxotonic and inertia levers control the force applied to the muscle; other types of apparatus apply a definite pattern of shortening. Fortunately, the wide range of results obtained in these different experimental arrangements can be interpreted in terms of a single property of muscle-that the force of contraction is a function of the velocity of shortening. The isotonic force-velocity curve of striated muscle was first studied by Fenn & Marsh (1935): three years later Hill derived-the equation: (P+a) (V+b)=constant=(Po+a)b (1) (a and b are constants, P0 the isometric tension), which describes with remarkable accuracy the force-velocity relation in muscles of the frog (Hill, 1938); tortoise (retractor penis, Katz, 1939; ilio-fibularis, Abbott, 1953); man (Ralston, Polissar, Inman, Close & Feinstein, 1949; Wilkie, 1950); snail (Abbott, 1953); ray (Wilkie, 1952, unpublished), and Mytilus (Abbott & Lowy, in preparation). The isometric tension developed by a muscle depends on the length at which the muscle is stimulated, being greatest when the muscle has about the same length that it had in the body. Hill's equation can only be applied in the region of this maximum, where the variation of P0 with length is slight; for P0 appears in the equation as a constant. In the intact vertebrate body, muscle length changes are more or less limited to this maximal region. The experiments described in this paper were carried out in order to examine the relation between force and velocity in other regions of the tension-length curve.