Letter to the Editor Response

We appreciate the comments of our Canadian friends and hope that this exchange of views yields more light than heat. We’ve tried to isolate the issues of apparent disagreement and suggest how we might resolve them.

Part of the resolution may hinge on agreeing on more precise terminology. We can't see that it is useful to define “fitness for an endurance exercise task” as being able to tolerate a given fraction of peak exercise tolerance. “Fitness for an endurance exercise task” implies, we would think, ability to sustain a given imposed task, such as a given work rate…and therefore have some relation to something to be accomplished in real life. As compared with an individual with high peak exercise tolerance, an individual with a low peak exercise tolerance would be judged less fit to perform a given endurance task even the two were able to tolerate the same fraction of the peak exercise tolerance for the same period of time.

Defining “similar metabolic stress” also may be seen as troublesome. Basing “metabolic stress” on measures of metabolic substrate utilization or, perhaps, consequent oxygen utilization might be reasonable. Alternately, basing metabolic stress on the relation of the imposed work rate to the lactate threshold or critical power might make sense. However, anchoring metabolic stress to a fraction of peak work rate (or oxygen uptake) seems untenable. As the authors’ own data shows (1), different individuals and different subject groups respond to a given fraction of their different peak work rates with greatly differing metabolic rates and exercise endurance; defining this as a “similar metabolic stress” does not seem useful.

We feel that the construct we presented in the figure that accompanied our editorial (2) was a useful counterexample of a situation where the authors’ strategy of judging fitness for an endurance task would yield a logically incorrect result. We showed that a group of COPD patients with unquestionably poorer exercise tolerance than a group of healthy subjects was nonetheless able to exercise at the same high fraction of their peak work rate for a longer period of time. The authors’ strategy would judge these COPD subjects as more fit than the healthy subjects. The authors reason that the performance of these COPD patients somehow “contradicts clinical experience” because they were able to tolerate 80 Watts for 6 minutes, but we cannot see why this is so. First, our figure was composed based on the published data of Neder et al. (3) Second, one of the authors recently published results in mild COPD patients (4), in which a work rate averaging 99 Watts was tolerated for 8.2 minutes. But, more importantly, objecting that these responses are somehow unrepresentative of COPD patients in general seems to miss the point.

Are women with COPD “physically disadvantaged” compared to men? Well, on reflection, we would agree that, based on the data presented, it is reasonable to conclude that this is so. Compared to the men studied, on average, they are given a lower work rate to perform and, on average, they are able to tolerate this work rate for a shorter period of time. It seems inescapable that the women would have tolerated the same absolute work rate (the same task) for a shorter time. But we hope that we can also agree that the duration of exercise tolerated at a given percentage of peak work rate is a flawed measure of any reasonable definition of endurance exercise tolerance…and that its use for this purpose should be discouraged.

We find ourselves repeating that the ability to tolerate a given work rate is dictated by power-duration considerations; both the critical power and the shape factor (W’) are involved. Exercising at a given fraction of the peak work rate will yield different exercise durations among subjects and among groups because both critical power and W’ differ among subjects and among groups.

We shouldn't lose sight of the fact that the principal use of constant work rate exercise testing is to define the response to interventions (as opposed their use in cross-sectional studies, as discussed here). This testing modality is finding good use in multicenter clinical trials, having been shown to be a sensitive indicator of improvement in exercise endurance capability.