J Physiol 585.1 Temperature-jump force generation in muscle 271 T-jump tension responses during shortening at a range of temperatures. Figure 7A shows tension responses recorded from a ﬁbre at four different temperatures; the ﬁbre was ﬁrst activated at 7–8◦C, and the temperature raised (and clamped) by the Peltier, thermo-electric module system in the trough. At each temperature, it was shortened at the same ramp velocity and a T-jump of 3◦C applied when tension during shortening was nearly steady. It is seen that the tension before the ramp (isometric) as well as the tension during shortening, before and after the T-jump, rise with increase of temperature. Figure 7B shows the pooled data (mean ± s.e.m.) for isometric tension (ﬁlled symbols) and for steady-shortening tension (open symbols), plotted against the reciprocal absolute temperature; the temperature dependence of isometric tension is essentially similar to previous ﬁndings, and the data also show that the tension during shortening, at about the same velocity, shows a similar temperature dependence. Figure 8 shows the pooled data for the rate (upper panel, open symbols) and the amplitude (lower panel) of the tension rise induced by a T-jump, during shortening (velocity range 0.06–0.09 L0 s−1 – in different experiments), at different initial temperatures; the abscissa is the post T-jump temperature, plotted as reciprocal absolute temperature and the rate is plotted on a logarithmic ordinate. In warming from 10 to 30◦C, the rate increases with a Q10 of 2.64 and the relative amplitude decreases sharply. The isometric ﬁbre data (ﬁlled circles), which are essentially similar to the more detailed data in previous studies (Ranatunga, 1996; Coupland et al. 2005), Figure 6. Tension response to a T-jump at different times during a ramp shortening A, 8 superimposed tension responses (upper panel) each with a standard T-jump of 3◦C applied at a certain time interval; the ﬁbre was activated to steady state at 9◦C, shortened at 0.06 L0 s−1 (lower panel). Note that the uppermost T-jump tension response (ninth) was isometric and the pre-T-jump isometric P0 level is indicated by the horizontal dashed line. B, tension data from two experiments. Upper panel shows the pre-T-jump tension (as a percentage of P0) plotted against time (dashed vertical line denotes the beginning of ramp shortening); tension decreases to a steady level at ∼500 ms. Lower panel shows the post-T-jump peak tension increase above the pre-T-jump level: the triangle shows the mean (± S.E.M., n = 7) T-jump-induced isometric tension increment. Note that the apparent T-jump-induced net increment of tension is small at short intervals but increases to a steady level at 500 ms (curves through the data points are ﬁtted by eye). C, the top trace is the isometric T-jump tension response, the middle trace is the ramp-shortening tension record without a T-jump; the lower traces are difference tension traces, obtained by subtracting the middle shortening tension record from each of seven shortening-tension records with a T-jump, applied (as in A) at various times during the ramp shortening. Note that a T-jump induces approximately the same tension rise; the one at the beginning of the ramp, however, seems more similar to the isometric (bi-component). C 2007 The Authors. Journal compilation C 2007 The Physiological Society