J Physiol 585.1 Temperature-jump force generation in muscle 273 last step (detachment) occurs faster than in isometric – hence, a decreased occupancy of a later stage may lead to a simpler (single exponential) T-jump response. Increase of the rate with shortening velocity is expected from a faster cycle leading to an increase of pre-stroke crossbridges. The difference between 40–50 s−1 in isometric phase 2b and ∼20–30 s−1 in shortening is not large (∼2-fold), but accounting exactly for such a difference is difﬁcult; moreover, results in Fig. 3B suggest that, at faster shortening velocities, the rate of T-jump force rise may be even higher than in isometric, but this needs to be conﬁrmed experimentally. The slightly higher phase 2b rate in the isometric case is probably due to the slow ﬂow through the cycle, so that the reverse steps in the cycle would contribute more to a relaxation resulting from a rapid perturbation (e.g. T-jump). Another possibility is that while a T-jump in isometric conditions does not change the number of attached heads (Coupland et al. 2005 and references therein), a T-jump during shortening is accompanied by recruitment of new heads; at the same absolute shortening velocity, the number of attached heads would be greater at a higher temperature because V 0 is larger at higher temperature and hence the shortening after the T-jump occurs at a lower relative velocity. On the basis of similar temperature dependence (Figs 7 and 8), however, the T-jump-induced tension rise in shortening muscle seems to contain an endothermic step, similar to phase 2b in isometric muscle. Therefore, a clear ﬁnding here is that the endothermic force generation process is evident in isometric muscle, it is pronounced during steady muscle shortening, but it is depressed or inhibited, during muscle lengthening. T-jumps during steady muscle shortening and lengthening In principle, the active muscle force in an isometric contraction is maintained by an ‘equilibrium’ between low and high force, attached crossbridge states (acto-myosin motor conformations), so that a small rapid length release (a shortening, inducing negative strain on attached crossbridges) will reduce force and perturb the equilibrium leading to a force generation (Huxley & Simmons, 1971). Conversely, a small rapid stretch (a lengthening, inducing positive strain on crossbridges) would also perturb the equilibrium but lead to an inhibition/reversal of the force generation. In other words, the acto-myosin conformational change (the power stroke) leading to muscle force generation is more likely to occur when exposed to negative strain than when exposed to positive strain. From the above considerations, and that a T-jump is thought to perturb an early molecular step so that the T-jump tension response can be used as a ‘signature’ C 2007 The Authors. Journal compilation C 2007 The Physiological Society of the acto-myosin ATPase cycle in active muscle, our ﬁndings naturally lead to the following important conclusion. The enhanced endothermic force generation, signalling the force-generating conformational change of the motor, suggests that the ATPase cycle proceeds more readily during steady muscle shortening – as found in studies on muscle energetics (see references in Woledge et al. 1985; Smith et al. 2005); indeed, the need to have a fast crossbridge cycling rate (a high detachment rate) during shortening was well recognized in crossbridge modelling (Huxley, 1957). Conversely, it appears that, during lengthening, the myosin motor fails to undergo the force-generating transition and, hence, the crossbridge/acto-myosin ATPase cycle becomes short-circuited, before phosphate release Figure 8. Temperature dependence of the T-jump tension response during shortening Pooled data are from 4 ﬁbres (as in Fig. 6B) in each of which the tension response to a 3◦C T-jump, when shortening at the same velocity (range 0.06–0.09 L0 s−1 – in different experiments), was examined at different initial temperatures; the initial temperature was increased and clamped by the Peltier modules ﬁxed to the trough wall. Lower panel, the mean (± S.E.M.) amplitude of T-jump tension rise is plotted as a percentage of the post-T-jump tension (left ordinate) against the reciprocal absolute temperature on the abscissa; the dashed curve through the points is ﬁtted by eye. The relative amplitude decreases with increase of temperature. Upper panel, an Arrhenius plot; logarithm of the rate of tension rise (right ordinate) versus reciprocal absolute temperature. The open symbols represent the mean (± S.E.M.) values for the rate of T-jump-induced tension rise during shortening and the continuous line is the calculated linear regression (between the individual log rates, n = 36, and 1/T, P < 0.001) and the dotted lines denote its 95% conﬁdence limits; the slope corresponds to a Q10 of 2.64. The ﬁlled symbols and the crosses, respectively, represent the fast (phase 2b) and the slow (phase 3) rates from T-jumps in the isometric state, from one of the experiment; the Q10 values are 3 (fast) and 1.6 (slow), similar to the previous ﬁndings (Ranatunga, 1996).