266 K. W. Ranatunga and others J Physiol 585.1 under the microscope and an experiment was terminated when evidence of ﬁbre damage was observed. The outputs from the force transducer, the motor (ﬁbre length), the thermocouple and, in some experiments, the diffractometer (sarcomere length change) were examined on two digital cathode ray oscilloscopes and digital voltmeters and, using a CED micro-1401 (Mk II) laboratory interface and Signal 2/3 software (Cambridge Electronic Design Ltd, Cambridge, UK), stored in a PC based computer. The sampling rate was 15 kHz and the duration of a sweep examined was 1 s. In some experiments, the position of the ﬁrst order He–Ne laser diffraction was also monitored by means of a position detector and the output of this diffractometer was used to monitor sarcomere length change during imposed ﬁbre length changes; the analyses will be made with regard to ﬁbre length (L0) and the corresponding sarcomere length will be brieﬂy mentioned. A continuous record of tension and temperature was also made on a chart recorder. The force measurements and curve ﬁtting to tension records were made using Signal software and further analyses were made using Fig.P software (Biosoft, Great Shelford, Cambridgeshire, UK). Simulating T-jump transients To qualitatively model the changes in T-jump-induced force responses in shortening ﬁbres, we used the same 5-step scheme for the crossbridge/AM–ATPase cycle as used previously for isometric muscle ﬁbres (Coupland et al. 2005); the aim was to see whether the basic trends observed in experiments can be seen in such simulations. The scheme is given below (Scheme 1) and shortening at a certain velocity was simulated by increasing k+4 (i.e. the ADP release step – see He et al. 2000). Figure 2. Analyses of the T-jump tension responses A, ﬁve superimposed tension records (middle panel) and corresponding length records (bottom panel) from an experiment. The ﬁbre was held isometric and maximally Ca2+ activated at ∼9◦C and, when the active tension had reached a plateau, a T-jump of ∼3◦C (schematically shown in the top panel) was applied. The tension trace marked ‘isometric’ in middle panel was so obtained; the tension rises to a new steady level following the T-jump and a double exponential curve is ﬁtted (dotted line on the right-hand of the trace) to the post-T-jump tension trace. Once the temperature was clamped again at 8–9◦C, it was stretched at a constant velocity of ∼50 nm hs−1 (hs, half-sarcomere) (top length record in the bottom panel) to obtain the two tension traces marked ‘lengthening’ (one without and the other with a standard T-jump). Note that, during lengthening, the tension rose towards a higher steady level (∼2.2 Po) and that the T-jump does not lead to a net increase of tension; the instantaneous drop in tension due to T-jump is more obvious in lengthening ﬁbre and this may indicate expansion in non-crossbridge elements (Ranatunga, 1996). The procedure was then repeated when the ﬁbre was shortening at a similar velocity (lower length record in the bottom panel) to obtain bottom two tension traces marked ‘shortening’. The tension decreased towards a steady level below the isometric tension (∼0.5 Po) during steady shortening, but the standard T-jump induced a pronounced tension rise; the 300 ms duration of the post T-jump tension trace is ﬁtted with a single exponential function. B, traces from another ﬁbre where the presentation is similar to A, but separate tension records without T-jump were not made. During shortening, the tension immediately before T-jump represents steady tension level at the pre-T-jump temperature. For lengthening, single exponential curve was ﬁtted to the pre-T-jump tension trace and to the post-T-jump trace (dotted lines through the trace) and, by extrapolation to the ramp end, the tension change induced by a T-jump was determined. C 2007 The Authors. Journal compilation C 2007 The Physiological Society