Double-hyperbolic nature of the force-velocity relation in frog skeletal muscle

Load-clamp recordings were performed on single fibres of the anterior tibialis muscle of Rana temporaria to further characterize the relationship between force and velocity of shortening. With the techniques used the speed of shortening could be recorded at different loads both from the fibre as a whole and from individual segments (ca. 0.5 mm in length) along the intact fibre. In accordance with previous observations the force-velocity relation was found to have a break point near 78% of the measured isometric force, P0. The results furthermore demonstrate that the force-velocity relation is composed of two hyperbolic functions, above and below the break point, respectively. The hyperbola in the high-force region has a more pronounced curvature than that observed at low and intermediate loads. An increase in sarcomere length from 2.10 to 2.60 microns makes both portions of the force-velocity relation less curved and reduces, to a great extent, the difference in curvature between the two hyperbolas. Similar effects are produced by raising the tonicity of the extracellular medium. Addition of caffeine (0.5 mM), or changing the temperature within the range 1-11 degrees C, does not affect the shape of any portion of the force-velocity relation. The fact that the above features of the force-velocity relation appear in very short segments as well as in the whole fibre suggests that the double-hyperbolic shape of the force-velocity curve does represent the contractile behaviour at sarcomere level. The system behaves as if a fraction of myosin cross-bridges were neutralized, or abolished (leading to a relative decrease in force), as the speed of filament sliding is reduced below approximately 10% of the maximum.