J Physiol 585.1 Temperature-jump force generation in muscle 275 indicates that the tension response to a T-jump and to a ramp shortening may represent separate events. This ﬁnding, which deserves to be experimentally addressed in more detail in the future, would be consistent with the general thesis (see above) that a T-jump enhances an early step, the force generation step, in the crossbridge cycle whereas shortening enhances (post-stroke) crossbridge detachment, a step near the end of the cycle. Figure 5 shows that a T-jump induces a constant amplitude tension rise at various velocities. A basically similar observation was made in our previous studies where we examined the tension response to a standard T-jump when ﬁbre tension was decreased with inorganic phosphate (Fig. 2A in Ranatunga, 1999) and increased with MgADP (Fig. 2A in Coupland et al. 2005). Taken together, these ﬁndings imply that endothermic force generation is an isolated step, it is not directly coupled to phosphate release or to ADP release and, as the present results show, that it is isolated from the shortening-induced crossbridge detachment step (rachet). (ii) Simulations. Using a minimal, ﬁve-step, crossbridge/ AM-ATPase kinetic scheme (similar to Scheme 1) that included a two-step phosphate release, where step 1 (only) generates force and is temperature sensitive (endothermic), and a subsequent slow, two-step MgADP release, we could qualitatively simulate many of the temperature-dependent features of isometric force (see Coupland et al. 2005). Therefore, we attempted to use such kinetic modelling simply to see whether simulations would, at least, show some of the features during shortening; no detailed analyses were carried out, however, since appropriate simulation of the present ﬁndings would indeed require speciﬁc consideration being given to the strain-sensitivity of various steps in the crossbridge cycle. Figure 9A shows simulated (see Methods for detail) tension responses to a small T-jump at ∼10◦C where pre-T-jump force is reduced to steady state, by simulated shortening (increased k+4). The T-jump-induced tension rise is faster as the force is decreased by higher shortening velocities (traces from top to bottom), as found in the experiments (Fig. 3B); also, the absolute amplitude of the T-jump tension rise remains similar (Fig. 5). Figure 9B shows simulated T-jump tension responses at different temperatures when the system was simulated to be in steady shortening at one velocity; the traces show that pre-T-jump force, during shortening, is increased and the T-jump force rises faster, as the temperature is raised – as found experimentally (Figs 7 and 8), and similar to isometric force (Coupland et al. 2005). The model in Scheme 1 does not identify the detached states separately but, when [ATP] is high as in muscle, changes in occupancy of [AM] (in the scheme) would indicate changes in occupancy of detached states (M.ATP, M.ADP.Pi); on that basis and compared with the isometric case, the occupancy C 2007 The Authors. Journal compilation C 2007 The Physiological Society of detached states increases when shortening is simulated. Thus, when force is reduced to ∼50% P0 by shortening at ∼10◦C, the summed occupancies of all AM.XX states except [AM] is decreased to ∼70% of the isometric value. 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