J Physiol 585.1 Temperature-jump force generation in muscle 269 Figure 4C shows, from the T-jump experiments, the pooled ‘steady’ tension data for different velocities of shortening (ﬁlled symbols) and of lengthening (open symbols), at 8–9◦C (pre-T-jump temperature), where the curves were ﬁtted by eye. The velocity range is zero (isometric) to ∼0.17 L0 s−1, or ∼0.15 V 0, for shortening and lengthening. The data show that the tension increases rapidly with an increase of lengthening velocity, whereas the tension decreases more slowly with an increase of shortening velocity. The data examination also showed that the shortening velocity range nearly covers the increase with velocity (the ascending limb) of the bell-shaped, mechanical power (force × velocity) versus shortening velocity curve. The tension during lengthening was estimated here as the tension at the ramp-end (ramp amplitude was ∼5% L0; Fig. 2). However, after an initial rapid rise, the tension continued to rise slowly (at ∼1 P0 s−1 at 0.1 L0 s−1) during a ramp stretch; hence, to be consistent with previous studies (see Getz et al. 1998; Pinniger et al. 2006), the tension at the transition from the initial (fast) phase to the late (slow) phase of tension rise was also estimated, thus removing the slow tension rise contribution; with that analysis, the force level reached, at the transition and with an increase of lengthening velocity, was ∼2 × P0, less than in Fig. 4C. A slow continued tension change (decline) was seen during ramp shortening (see Roots et al. 2007) but its amplitude, in the present experiments, remained small (< 0.2 P0 s−1); nevertheless, this uncertainty may have contributed to some of the scatter in the amplitude data analysis in Fig. 3A. Figure 4. The force–velocity relation at low temperatures (8–9◦C) A, tension responses (upper) to rapid (∼1 ms) length steps of different amplitudes (lower) to induce ﬁbre slack (horizontal dotted line); the duration of slack time (time delay to tension rise) was measured. B, a plot of the length step amplitude versus time delay; the slope of the ﬁtted linear regression gives an unloaded velocity (V0) of ∼1.0 L0 s−1. C, from the T-jump experiments, the velocity dependence of steady force at 8–9◦C (the pre-T-jump temperature). The data are from 6 ﬁbres, as in Fig. 3 – but include measurements without T-jumps, in each of which data were obtained for both shortening (ﬁlled) and lengthening (open) limbs; a symbol represents the mean (± S.E.M.) of 5–8 measurements (n = 68 for shortening and n = 46 for lengthening) and the line through the data was ﬁtted by eye. Note that the force during lengthening is estimated as tension at the ramp-end (ramp amplitude was ∼5% L0) – see Fig. 2 legend. D, the mean V0 (– S.E.M., diamond) from 6 ﬁbres and the force shortening velocity data from two of them (circles and squares). An approximate Hill curve is ﬁtted with an a/P0 ratio of 0.05 (see text). C 2007 The Authors. Journal compilation C 2007 The Physiological Society