Letter To The Editor

We read with great interest the editorial from Drs. Casaburi and Porszasz, providing a thoughtful discussion on the significance of the endurance time to constant work rate cycling exercise (1). We agree with the authors that the interpretation of this exercise parameter is tricky, because it is dependant upon several physiological and psychological variables.

In their editorial, Drs. Casaburi and Porszasz suggest that the endurance time to a fixed fraction of peak power output cannot be used to compare exercise performance between individuals because it is subjected to technical artifacts related to its design. Specifically, they argued that a lower endurance time found in women with COPD compared to men with the same disease (2) does not imply that women are physically disadvantaged compared to men.

Is the endurance time to a fixed fraction of peak power output not a good reflection of physical fitness for endurance exercise task? (1) Athletes would certainly refute this statement knowing that the ability to sustain a high fraction of peak exercise capacity for a long period of time is predictive of great physical performance (3). Could there be something wrong about Drs. Casaburi and Porszasz analysis? We would like to respectfully explain why we believe that this might be the case.

First, part of their reasoning is based on a figure describing the average power/duration relationship of a group of patients with COPD and healthy subjects. From this figure, it can be calculated that patients with COPD could sustain peak exercise performance (80 Watts) for over 6 minutes! This does not correspond to common clinical experience, and it suggests that this figure may be misleading when trying to make assumptions about the relationship between peak exercise performance and endurance time to constant work rate exercise.

The figure would also predict that individuals with greater peak exercise capacity should have a lower endurance time when exercising at a fixed fraction of peak power output. Again this does not correspond to what we see in patients with COPD. To illustrate this point, we use data from two large databases of exercise testing in COPD (4, 5) to show that peak power output correlates positively with the endurance time; individuals with larger peak exercise capacity during incremental cycling exercise testing also exhibit a larger endurance time to constant work rate cycling exercise. For example, patients with COPD that were in the upper tercile of peak exercise capacity had a peak power output of 93 ± 36 Watts and an endurance time at 80% peak power output of 890 ± 250 seconds. In comparison, patients in the lower tercile of peak exercise capacity (72 ± 23 watts) had an endurance time of 300 ± 37 seconds. It is thus apparent that the capacity to tolerate greater work rate is also associated with better endurance to constant work rate exercise.

When interpreting the observed differences between women and men with COPD in their tolerance to constant work rate cycling exercise, it is also important to consider that the cycling exercises were performed at the very same fraction of their respective peak VO₂, indicating similar metabolic stress between the two genders (2).

We thus propose that the reported differences in endurance time to constant workrate cycling exercise in women versus men with COPD (2) cannot be solely attributed to some technical artifacts with the constant work rate exercise testing. We still maintain that this difference in the capacity to endure constant workrate cycling exercise reflects poorer physical performance in women with COPD.