268 K. W. Ranatunga and others J Physiol 585.1 during steady lengthening are also shown in Fig. 3B (open circles). Since the amplitude is negligible (Fig. 3A), this is only an apparent rate and it increases slightly with velocity; the rate of tension rise determined by curve ﬁtting to the late part of the pre-T-jump tension trace (crosses) was not signiﬁcantly different from the post-T-jump rates (Student’s t test, P > 0.05). As found previously, the T-jump-induced tension rise in the isometric case contained two components; their mean (± s.e.m.) amplitude and rates (40–50 s−1 and 5–10 s−1 in the present experiments) are shown by open squares on the ordinates. Thus, the data in Fig. 3 show that the tension during steady lengthening is not changed by a T-jump, whereas the tension in shortening is enhanced by a T-jump – as found in previous studies on isometric ﬁbres. The force–velocity curve Determination of the steady-state force–velocity curve was not a main concern in this study; nevertheless, it would be useful to survey the force–velocity range covered in the above experiments, particularly in relation to the maximum or unloaded shortening velocity. In experiments on six ﬁbres, we determined the unloaded shortening velocity (V 0) at 8–9◦C, by the slack test method (Edman, 1979). Figure 4A shows tension responses to different amplitude length steps; the time to tension redevelopment at ﬁve different length step amplitudes was measured and V 0 was determined from the slope of a linear regression ﬁtted to such data (Fig. 4B). The mean (± s.e.m.) V 0 was 1.17 (±0.2) L0 s−1. Our previous studies (Ranatunga, 1984; Coupland et al. 2005) showed that V max of mammalian, intact and skinned, fast ﬁbres at 10◦C would be 1–2 L0 s−1 and several other studies on fast (type 2) skinned ﬁbres at 12◦C (Bottinelli et al. 1996; He et al. 1999, 2000; Linari et al. 2003a) also report V max and V 0 values ranging from ∼1 to 2.5 L0 s−1. Given the variability seen, a value of ∼1 L0 s−1 for V 0 for fast ﬁbres at 8–9◦C is consistent with those ﬁndings. In plots such as Fig. 4B, the intercept value on the ordinate was 0.025 ± 0.0025 L0 (mean ± s.e.m., n = 6); this is slightly higher than 0.021 L0 obtained in similar experiments on intact frog ﬁbre (Edman, 1979). Figure 3. The velocity dependence of the T-jump tension response Data from 6 ﬁbres in each of which tension responses to a standard T-jump (as in Figs 1 and 2) were collected at different velocities; velocity is plotted as L0 s−1 on the abscissa, with negative for shortening and positive for lengthening. A, amplitude of tension change: the net tension change after a T-jump (post-T-jump minus pre-T-jump tension) is plotted as a ratio of the post-T-jump tension. The T-jump-induced tension rise during shortening (ﬁlled circles) is correlated with velocity (P < 0.05); the mean (± S.E.M., n = 7 or 8 per point, total = 57) are plotted and the curve is ﬁtted by eye. During lengthening, the amplitude of tension change by a T-jump (open symbols) is not correlated with velocity (P > 0.05) and is not signiﬁcantly different from zero; individual data (n = 38) are plotted. Mean (S.E.M., n = 30) for isometric state is plotted at zero velocity (on the ordinate). B, rate of tension rise: a single exponential function was ﬁtted to deﬁne the post-T-jump tension trace in shortening and the reciprocal of the time constant (τ ) is plotted as a rate of tension rise on the ordinate; the rate (ﬁlled circles) is correlated with shortening velocity; mean (± S.E.M.) are shown where the line is the calculated linear regression for individual data (P < 0.001). A similar curve-ﬁtting procedure was also done on tension traces during lengthening, although the T-jump-induced tension amplitude was insigniﬁcant (Fig. 3A); the apparent rate of post-T-jump tension rise is very low (mean ∼5 s−1, open circles, individual data); the crosses show the rate of tension rise determined by curve ﬁt to the late part of pre-T-jump tension trace during lengthening, and they are not signiﬁcantly different from the post-T-jump rates (Students t test, P > 0.1). As found previously (Ranatunga, 1999), the T-jump-induced tension response in isometric case contained two components, with rates (reciprocal time constants from bi-exponential curve ﬁt) of 40–50 s−1 and 5–10 s−1 in the present experiments; the mean (S.E.M., n = 30) values are shown as squares at zero velocity. C 2007 The Authors. Journal compilation C 2007 The Physiological Society