Non‐Hyperbolic Force‐Velocity Relationship in Single Muscle Fibres1

Abstract

Single fibers (16) from frog semitendinosus muscle were studied with particular attention to the high-force portion of the curve. The force-velocity curve was hyperbolic except for a reversal of curvature near 80% measured isometric tension (Po). Rectangular hyperbolas fitted (linear, least-squares method) these data well only when values below 0.78 Po were considered. Extrapolation of these hyperbolas above 0.78 Po gave predicted isometric tensions .**GRAPHIC**. which averaged 32 .+-. 6% above the measured Po values. Hill''s constants (1.84.degree. C) for these hyperbolas were: .**GRAPHIC**. = 0.177 .+-. 0.021, b = 0.329 .+-. 0.035 M.L./s, Vmax = 1.91 .+-. 0.074 M.L./s. The reversal of curvature persisted when force-velocity data were obtained using: 1 or 60 min response intervals, afterloaded isotonic responses, grid stimulation, electrically induced contractures and bundles of fibers. The reversal of curvature diminished when force-velocity data were obtained from slightly stretched fibers (about 2.3 .mu.m sarcomere length as compared to 2.1 .mu.m in the control). Sarcomere length redistributions did not account for the non-hyperbolic force-velocity relation. The behavior based on the geometry of the contractile filament lattice was discussed.