The Force-Load-Velocity Relation and the Viscous-Like Force in the Frog Skeletal Muscle

The load-velocity relations were determined at various contractile forces in the small bundle dissected from frog semitendinosus muscle.1) The tension-extension curve of the bundle was nearly the same as that of single fibers.2) The load-velocity curves of shortening muscle obtained at the standard length L0 fit HILL'S hyperbolic equation at any contractile force in the partially activated muscle, and the dynamic constants were a/P0=0.25 and b/L0=0.9/sec at 10°C. The viscous-like force Fv at a given velocity increased linearly with increasing contractile force F. These results were valid in the length region between 0.8 and 1.2 L0.3) The load-velocity curves of lengthening muscle were also hyperbolic at any contractile force and Fv was also proportional to F, unless the velocity exceeded 1.0 L0/sec. The dynamic constants were a'/P0=0.4 and b'/L0=0.85 /sec, i. e., a'/P0 was 1.6 times larger than a/P0.4) HILL's force-velocity equation was generalized to the force (F)-load (P)-velocity (v) equation (P+A)(v+b)=b (F+A), A=aF/P0; or the force-Fv-velocity equation Fv=(F/P0)(P0+a) v/(v+b).5) The value of Fv on lengthening was 1.4 times larger than that on shortening under the same contractile force and velocity.6) These force-load-velocity equations were valid not only during steady contractile force but also for any instance during the change in contractile force.7) The significance of Fv, or force-dependent viscosity, is discussed with respect to the sliding filament theory.