Progressive irreversible airway obstruction is the defining characteristic of chronic obstructive pulmonary disease (COPD). However, people with COPD do not complain of airway obstruction but, more usually, about the associated restriction of physical activity. The disability resulting from COPD may also be compounded by peripheral muscle dysfunction that further limits exercise capacity. Traditionally, we have used tests of whole body exercise performance to reflect this process. However, as we learn more about COPD, it is revealed as a complex condition where apparently straightforward truths are neither pure nor simple. The relationship between airway function and exercise capacity is a good example of this complexity, since it may naively be expected that the decline in physical activity may mirror the progression of airway obstruction. Furthermore, it might also be reasonable to expect that an improvement in airway function may lead to a similar gain in physical activity. In fact, neither of these phenomena is necessarily observed. The relationship between static lung function and exercise capacity is not predictive in COPD and, in addition, the forced expiratory volume in 1 s (FEV₁) alone cannot be used to judge disability. Furthermore, the effects of short-term improvements in airway function by bronchodilation do not reliably lead to improved exercise capacity. This is clearly a source of frustration—both for patients whose performance may not improve and for those who seek to demonstrate the benefits of a therapeutic intervention. There are several possible explanations for the apparent disconnection between changes in lung function and exercise performance. One explanation lies in understanding the nature of exercise capacity limitation in COPD, which may vary with the stage of the disease. Another factor may be the method chosen to assess activity limitation. We usually use formal, standardised whole body exercise tests as a …