Force-velocity relations of nine load-moving skeletal muscles.

The relationship between maximal velocity and load was studied in nine muscles of the cat's hind limb using a technique in which the initial and final muscle lengths are determined by equilibrium of a suspended mass and the muscle's passive and active forces elicited by tetanic stimulation. The maximal velocities of shortening during contraction under each of various loads was used to fit a Hill model using the least-squares method. It was shown that different muscles varied significantly in their ability to generate maximal velocity over a range of loads. The tibialis anterior muscle generate the highest velocity (28.4 cms-1), whereas the tibialis posterior generated the lowest maximal velocity (4.2 cms-1). In general, muscles with predominantly fast twitch fibres and with the largest elongation/shortening range displaced the load at the highest velocities, as compared with muscles with predominantly slow twitch and short excursion range which respond with low velocities. The a/P0 ratio of Hill's equation, which defines the curvature of the force velocity, also varied widely, being most monotonic (0.927) for the soleus and the steepest (0.067) for the extensor digitorum longus, further suggesting that fibre composition is also highly influential on the force-velocity relations of the muscle.