Quantifying intervention-related improvements in exercise tolerance.

Among the benefits expected to result from a therapeutic intervention in patients with impaired systemic functioning is an increase in exercise tolerance. For this a constant high-intensity work rate has been shown to provide a more sensitive index of improvement than the maximum work rate, or oxygen uptake, on a symptom-limited incremental test. However, the extremely large variability of the improvement in this particular index of tolerance undermines the ability to make general inferences for the underlying functional improvement. We argue that this is a necessary consequence of the particular work rate chosen for the test and the change in the parameters of the subject's hyperbolic power-duration relationship for that form of exercise: its "critical power" and "curvature constant". Without knowledge of these features, any absolute or per cent increase in tolerance time to a single constant-load exercise bout must be interpreted with caution regarding the physiological benefit(s) that have accrued from the intervention.