The Biphasic Force–Velocity Relationship in Frog Muscle Fibres and its Evaluation in Terms of Cross‐Bridge Function

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Abstract
The relationship between force and velocity of shortening was studied during fused tetani of single fibres isolated from the anterior tibialis muscle of Rana temporaria (1.5–3.3°C; sarcomere length, 2.20 mm). Stiffness was measured as the change in force that occurred in response to a 4 kHz length oscillation of the fibre. The results confirmed the existence of two distinct curvatures of the force–velocity relationship located on either side of a breakpoint in the high‐force, low‐velocity range. Reduction of the isometric force (P0) to 83.4 ± 1.7% (mean ±s.e.m., n= 5) of the control value by dantrolene did not affect the relative shape of the force–velocity relationship. The breakpoint between the two curvatures was located at 75.9 ± 0.9% of P0 and 11.4 ± 0.6% of maximum velocity of shortening (Vmax) in control Ringer solution and at 75.6 ± 0.7% of P0 and 12.2 ± 0.7% of Vmax in the presence of dantrolene. These results provide evidence that the transition between the two curvatures of the forcevelcity relationship is primarily related to the speed of shortening, not to the actual force within the fibre. The instantaneous stiffness varied with the speed of shortening forming a biphasic relationship with a breakpoint near 0.15 Vmax and 0.8 P0, respectively. The force/stiffness ratio (probably reflecting the average force per cross‐bridge), increased with force during shortening. The increase of the force/stiffness ratio with force was less steep at forces exceeding 0.8 P0 than below this point. A four‐state cross‐bridge model (described in the Appendix) was used to evaluate the experimental results. The model reproduces with great precision the characteristic features of the force–stiffness–velocity relationships recorded in intact muscle fibres.