272 K. W. Ranatunga and others J Physiol 585.1 show that the rate of the faster (phase 2b) exponential component also has a high Q10 (∼3) (see ﬁgure legend for details). The temperature dependence of tension during shortening would be inﬂuenced by a number of factors; the decrease in the relative amplitude with warming (Fig. 8, lower panel), in particular, is as expected from Fig. 3A. For instance, the relative velocity (V /V 0, at each temperature) is 0.05–0.08 V 0 at ∼10◦C, whereas due to an increase of V max on warming (Ranatunga, 1984), it would only be ∼0.02–0.04 V 0 at 20◦C; from Fig. 3A, the relative tension increment by T-jump would hence be lower. Discussion The present study extends our previous ﬁndings, on the effects of temperature and T-jumps on isometric muscle ﬁbres (Ranatunga, 1996), to shortening and lengthening muscle ﬁbres. We examine below whether our ﬁndings provide some insight into the underlying processes that contribute to the generation and maintenance of muscle force under different conditions. Endothermic force generation in muscle As summarized in the introduction, previous studies on isometric muscle ﬁbres have shown that, at ∼10◦C, a T-jump of 3–5◦C induces a tension rise to a new steady level (see Coupland et al. 2005); the tension rise consisted of two exponential components, phase 2b (fast) and phase 3 (slow), where phase 2b was thought to represent endothermic force generation in attached crossbridges; phase 2b rate increased markedly with warming (Q10 of 3; Ranatunga, 1996) whereas phase 3 rate was much less temperature sensitive. The endothermic force generation represents an early step (before the release of inorganic phosphate) in a Lymn & Taylor (1971) type of scheme for the acto-myosin ATPase pathway in muscle (see Ranatunga, 1999; Coupland et al. 2001). The occurrence of a single endothermic force generation step in the crossbridge cycle could basically account for the increase of steady isometric muscle force with warming, the sigmoidal relation between isometric force and temperature, and the effect of inorganic phosphate and of Mg-ADP on isometric force at different temperatures (see references in Coupland et al. 2005). Unlike in isometric muscle, the T-jump-induced tension transient in steadily shortening muscle consists of only one exponential component; its relative amplitude is more pronounced and the rate is intermediate between phase 2b and phase 3 rates of isometric muscle (Fig. 3). In the isometric case, the slow, phase 3, component is Pi insensitive and is probably due to a slow, later step in the cycle. During shortening, the Figure 7. T-jumps during steady shortening at different temperatures A, superimposed tension (upper panel) and length (lower panel) records from an experiment in which the response to a 3◦C T-jump (upward arrow), during same ramp shortening (velocity ∼0.06 V0), was recorded at a number of different temperatures; the post-T-jump temperature is given to the right of each trace. The isometric tension (i.e. tension before the downward arrow) and the tension during steady shortening are higher and the T-jump-induced tension rise, while shortening, becomes faster at the higher temperatures. B, the temperature dependence of the isometric tension (ﬁlled symbols) and of the tension during steady shortening (open symbols). Mean (± S.E.M.) data are from 4 ﬁbres in each of which tensions are plotted as a percentage of its isometric tension at the low temperature (7–9◦C); the abscissa is reciprocal absolute temperature (103 × 1/T) and is also labelled in ◦C. The total number of measurements was 42 for isometric (n = 2–5 per data point) and 72 for shortening (n = 8 per data point). The shortening (velocity range 0.05–0.08 V0 in different experiments) decreased the tension during steady shortening to ∼50% P0 at ∼9◦C. Note that in warming over this temperature range, the isometric tension increases ∼2-fold (as shown in previous studies, see references in Coupland et al. 2005) and the tension during steady shortening, within this velocity range, also shows a similar increase with warming. C 2007 The Authors. Journal compilation C 2007 The Physiological Society