J Physiol 585.1 Temperature-jump force generation in muscle 267 The other features were as described before; brieﬂy, the forward rate constant of Step 1 (k+1) is temperature-sensitive (endothermic) force generation. Step 2 is rapid Pi release and steps 3 and 4 represent the two-step ADP release (slow in isomertic muscle). Step 5 includes all the steps after ADP release that are necessary to re-prime a crossbridge for the next cycle including the M.ADP.Pi↔M.ATP cleavage step after detachment, as indicated, but not separately identiﬁed in the model; the overall rate through this route is low (k+5, ∼10 s−1; see He et al. 2000) and was forward biased. The states AM∗.ADP.Pi, AM∗ADP and AM∗ .ADP were taken to be equal-force bearing states (F) and the sum of their fractional occupancy is taken as force (see Coupland et al. 2005 for other details). The linear kinetic Scheme 1 above was solved by the matrix method using Mathcad 2000 Professional software (Mathsoft) as previously described (Gutfreund & Ranatunga, 1999). After a steady state in the occupancy is reached, the method was used to simulate a T-jump relaxation by increasing k+1 with a Q10 of 4 as previously described (see Ranatunga, 1999; Coupland et al. 2005) and the approach to the new steady state obtained. Scheme 1 The data presented here are from experiments on 16 single muscle ﬁbres; their mean (± s.e.m.) isometric tension (P0) at ∼9◦C was 192 (± 16) kN m−2, the ﬁbre length (L0) was 1.33 (± 0.08) mm; in data presentation, the velocity and length changes will be given in ﬁbre length (L0) and their relation to half-sarcomere length will be considered later. Results Tension responses to T-jumps during ramp shortening and lengthening The tension response induced by a standard laser T-jump of ∼3◦C was examined in maximally Ca2+-activated muscle ﬁbres, when they were held isometric and when they were steadily shortening or lengthening at different constant velocities; Fig. 1 shows tension responses from one experiment. Figure 1A and B show that during ramp lengthening, the tension rises rapidly towards a level of about twice the isometric tension (P0) and a T-jump produces no further tension rise; a small instantaneous drop in tension was sometimes seen (as in Fig. 1B), indicative of thermal expansion in the ﬁbre (see Ranatunga, 1996). Figure 1C and D show that, during C 2007 The Authors. Journal compilation C 2007 The Physiological Society steady shortening, the tension decreases to a level lower than P0, the tension level reached being lower at the higher velocity (D); in either case, a T-jump produces a marked tension rise with a characteristic time course. As reported in previous studies, a T-jump induces a rise in tension to a new steady level when the ﬁbre is held in isometric (Fig. 1E). The superimposed tension traces shown in Fig. 2A and B illustrate the type of analyses carried out to characterize the T-jump tension responses. In isometric state, the post-T-jump tension trace could be ﬁtted with a double exponential curve, as reported in previous studies (see Ranatunga, 1999), yielding the time constant and the amplitude of two components (referred to as fast, phase 2b and slow, phase 3; see Ranatunga et al. 2002); at the post-T-jump temperature of 11–12◦C, their reciprocal time constants were ∼50 s−1 (phase 2b) and 5–10 s−1 (phase 3). Since, the pre-T-jump tension had reached an approximate steady level during ramp shortening, the post-T-jump tension rise could be characterized, as in the isometric case, by curve ﬁtting; however, the T-jump tension rise during shortening required only a single exponential, with a reciprocal time constant of 20–30 s−1 at the velocities shown in Fig. 2. During ramp lengthening, the tension rose rapidly initially and continued to rise slowly afterwards, as has been observed previously (see Pinniger et al. 2006). The fact that a T-jump does not lead to a net tension rise was, nevertheless, clear when tension records at the same velocity, with and without a T-jump, were examined (Fig. 2A); this was not experimentally proﬁtable, however, when the aim was to collect data from a ﬁbre at a range of different velocities. Therefore, whether a T-jump induced a net tension change was determined by ﬁtting a single exponential curve each to the pre-T-jump tension record and to the post-T-jump tension record (see Fig. 2B) and determining the difference in the extrapolated tensions at the ramp-end. The velocity dependence of the T-jump tension response The amplitude of tension change and the rate of the exponential components, derived by curve ﬁtting to the T-jump-induced tension responses, were obtained at different velocities in six ﬁbres and the data are shown in Fig. 3. Figure 3A shows that, plotted as a ratio of the post-T-jump tension, the amplitude of the T-jump-induced tension rise in shortening ﬁbres is correlated with velocity (ﬁlled circles, P < 0.05); the amplitude of tension change during lengthening is not signiﬁcantly different from zero and is not correlated with velocity (P > 0.05). Figure 3B (ﬁlled circles) shows that the rate of T-jump-induced tension rise during steady shortening increases with velocity (P < 0.001). For completeness, the data for the post-T-jump tension rise