Figure 3. Phosphocreatine depletion rates during ischaemic exercise.

A, the observed time course of phosphocreatine (•) together with lines showing the results of fitting to monoexponential and biexponential kinetics. The monoexponential fit is eqn (24) with Δ[PCr]= 30 ± 4 mmol l⁻¹, and k = 0.7± 0.1 min⁻¹ (corresponding to t_1/2 = 1.4 ± 0.5 min). The biexponential fit is eqn (27) with Δ[PCr]_A = 6 ± 1 mmol l⁻¹, Δ[PCr]_B = 30 ± 2 mmol l⁻¹, k_A = 3.7 ± 0.8 min⁻¹ (corresponding to t_1/2 = 0.5 ± 0.3 min) and k_B = 0.31 ± 0.04 min⁻¹ (corresponding to t_1/2 = 0.31 ± 0.04 min). B, the residuals (observed minus fitted) for these two fits, scaled to resting phosphocreatine concentration. C, the estimated initial rates of phosphocreatine depletion (expressed as a fraction of the adjusted value described in the text and in Fig. 6), measured in five different ways: from monoexponential (eqn (25)) and biexponential fits (eqn (28)), from the difference between the basal and first-exercise point as measured (eqn (22)), and ‘corrected’ to t = 0 using the monoexponential rate constant (eqn (23)), and by three-point linear regression, extrapolated to t = 0 (eqn (30)). D, the time course of phosphocreatine depletion rate calculated in three of these five ways: regression, two-point corrected and biexponential fit (eqns (23), (29) and (30)) (the monoexponential fit (eqn (26)) is obviously inappropriate). Error bars show s.e.m. (omitted in D).