Figure 5. Velocity dependence of the tension responses.

A, the tension responses (upper traces) to ramp stretches (lower traces) at a range of velocities (0.1–4.0 L₀ s⁻¹) from one bundle are shown; middle traces show displacement of a marker placed on the surface of the fibre bundle (segment length responses). The rate of tension development and the rate of tension decay increased with stretch velocity; the tension decayed to a similar level (P₃ above the isometric tension, the lowest tension record) when measured 500 ms after the end of the ramp. Note, the tension trace at the slowest stretch velocity has been truncated before reaching this point. B, velocity dependence of P_k (▴), P₃ (▾) and P₁ (○) from one fibre bundle. The P_k tension increased with velocity towards a plateau, P₃ was relatively insensitive to velocity and P₁ increased in proportion to velocity with a slope of 6.97 kN m⁻² per L₀ s⁻¹. The dashed line represents the curve fit to P_k using eqn (1) in the text. A better fit (continuous line) was obtained using eqn (2), where V_c= 1.46 L₀ s⁻¹, P_L= 100 kN m⁻², and P₃*= 73.45 kN m⁻². In this example, P₃* is approximately fivefold greater than the measured P₃ tension. C, velocity dependence of P_k−P₃ (▪) and P_k−P₃* (□). For P_k−P₃ data, the dashed curve represents the fit of eqn (1) and the continuous curve of eqn (2), where V_c= 1.30 L₀ s⁻¹, P_L= 111 kN m⁻², and P₃*= 44.71 kN m⁻². Note that P_k−P₃* data and the fitted curve (pure viscoelastic equation, eqn (2) with P₃*= 0) characterize the velocity dependence of force in the crossbridge component only. Note the intercept on the P_k axis (Fig. 5B) is P₃* and on the P_k−P₃ axis (Fig. 5C) is P₃*−P₃.