Fig. 4. Charts to show the methodology of biphasic modelling using broken stick analysis. (A) V_O2 data for four tufted ducks were smoothed and plotted on a logarithmic axis. The break point was arbitrarily chosen at 6 s. Linear regression was performed up to and including the break point (Linear phase 1) and also on all remaining data points (Linear phase 2). Regression parameters were then used to determine the point of inflection (C) where the two lines intersect. (B) V_O2 data for four tufted ducks were regressed onto the two regression lines shown (biphasic and linear). The biphasic regression line was calculated as shown in Fig. 4A. Commonly in this type of analysis the biphasic point of inflection (C) was not identical to the selected break point.