The consequences of fibre heterogeneity on the force-velocity relation of skeletal muscle

The consequences of fibre heterogeneity on the collective force-velocity properties of bundles of parallel fibres were examined in a simulation model. The model was tested by comparing the actual force-velocity curve of a bundle of three fibres, each of which had been individually characterized, with the force-velocity curve predicted by the model for the bundle based on the individual fibre properties. The predicted and measured force-velocity curves were in excellent agreement. The curvature of the force-velocity relation for a muscle, as indicated by a/P0 in Hill's (1938) hyperbolic equation, increases with increasing heterogeneity in the maximum shortening velocities (Vmax(i] of the individual fibres in the muscle. In a muscle that is heterogeneous with respect to Vmax(i), the maximum shortening velocity determined by the slack test method (V0) can be expected to represent the fastest fibre(s) in the muscle. The maximum velocity of shortening (Vm), determined by extrapolation from a hyperbola that is fitted to force-velocity data at finite loads, is substantially lower than V0. The difference in estimates of V0 and Vm is a function of: (i) the degree of heterogeneity of the muscle with respect to Vmax(i) and the curvature of the force-velocity relationship of the individual fibres, and (ii) the force range used to establish the hyperbola from which Vm is derived. The ratio of Vm to V0 can be used as an index to estimate the degree of variability in the maximum velocity of shortening among individual fibres in a muscle.