Figure 3. Analysis of tension transients by curve fitting.

A, the tension transient (upper trace,) and the sarcomere length change (lower trace) induced by a stretch of 0.5 % L_o in a muscle fibre (+25 mm P_i). A bi-exponential curve fitted to the initial part of the tension record is superimposed on the tension trace. Note that the curve deviates from the later part of the tension record. Curve fitting allowed extraction of two rate constants and amplitudes (for phases 2a - fast and 2b - slow) to define the T₁ to T₂ region in all transients. B, the calculated rates (s⁻¹) for phases 2a and 2b are plotted as in Fig. 2D. Given the large scatter, rate 2a (smaller symbols) remains similar with (filled symbols) and without added P_i (open symbols), particularly with length releases. The fitted regressions (r > 0.46, n > 57) (continuous line, controls) indicate that log rate (2a) changes monotonically; i.e. log rate 2a decreases with decrease of length step amplitude in releases and with increase in amplitude in stretches. The change, however, is less with added P_i. Values of rate 2b (larger symbols) from length releases are similar between control and +P_i, whereas they are clearly different from stretches. Phase 2b rate is more or less constant with increase of stretch amplitude in the controls but it is higher and shows a decrease with increase of stretch amplitude in the presence of 25 mm P_i. C and D, the amplitudes of phase 2a (C) and phase 2b (D) components plotted as ratios of the steady tension (T₀). The amplitudes of both components increase with amplitude of the length perturbation (abscissa), both for length releases (plotted on a positive ordinate) and for stretches (on a negative ordinate). In general, the relative amplitudes remain similar with (filled symbols) and without added P_i (open symbols); however, amplitude with P_i for phase 2a in stretches (C) is somewhat higher than in the controls.