The ventilatory response to incremental exercise: is it one or two breakpoints?

The primary objective of this study was to determine if the ventilatory (&OV0312;E) breakpoint during incremental exercise to fatigue is best represented by a single breakpoint (BP1), double breakpoint (BP2), or an exponential function (EXP) exhibiting no BP. A secondary objective was to determine if visual assessment of the BP was valid and reliable. Subjects (n = 49) were both men (n = 12) and women (n = 37) who identified either running (n = 31) or triathlon (n = 18) as their primary sport. Subjects completed a maximal oxygen test on a motor-driven treadmill according to a modified Bruce protocol. The difference in mean square error (MSE) for BP1 (4.91 ± 3.35 L) was slightly but significantly (p < 0.05) greater than in MSE for BP2 (2.75 ± 1.7 L) for VE. Mean square error for EXP (7.85 ± 5.1 L) was significantly greater (p < 0.05) than BP1 and BP2. Visual assessment of VE on scatterplots produced poor validity, with a mean of 71.7% being misclassified compared to the computer model. Both interevaluator reliability (30.6% agreement) and intraevaluator reliability (59.7% agreement) were low. From the results of this study, it is concluded that (a) VE exhibits a BP2 model rather than BP1 or EXP and (b) visual assessment of BP is both invalid and unreliable.