Figure 4. Characteristics of T-jump induced tension rise during steady shortening.

Pooled data from nine fibres in each of which tension responses to a 3–4°C T-jump were examined over a range of shortening velocities at 8–9°C, as in Fig. 1. A single exponential curve was fitted to the post-T-jump tension rise to extract the rate of tension rise (1/τ₂ (time constant)) and the amplitude. The mean (±s.e.m., n= 5–18) data are plotted against shortening velocity as in Fig. 3. A, rate of tension rise. Filled symbols show the rate of T-jump induced tension rise, where the dashed line is the fitted linear regression to the pool of original data (excluding values for isometric; r > 0.7, n= 95). The rate increases approximately linearly with increase of shortening velocity, reaching >200 s⁻¹ at ∼V_max. Assuming monophasic tension rise, the isometric value is 22 (±2.5) s⁻¹ (filled symbol on the ordinate); phase 2b from biphasic analysis was ∼55 s⁻¹ (× on the ordinate and short-dashed horizontal line). For comparison, the open symbols show the data for the rate of tension decline during ramp shortening as in Fig. 3, but plotted as means (±s.e.m.). B, normalised amplitude of tension rise. The amplitude is plotted as a ratio of the post T-jump tension during steady shortening; the horizontal dashed line denotes the isometric value and the dotted curve through the points is fitted by eye. With increase of shortening velocity, the amplitude increases steeply at low velocities (as in our previous study) and then increases more slowly but remains above the isometric value at higher velocities. C, extent of shortening during tension rise. The extent of shortening was calculated using velocity (L₀ s⁻¹) and time constant (τ₂) from curve fit to tension rise, as (velocity × 3τ₂). It is an approximate estimate of ∼95% of the T-jump tension rise. The extent of shortening increases with velocity and remains at ∼1.6%L₀ (dotted line, fitted by eye) at velocities > −0.25 L₀ s⁻¹.