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Limitation of exercise capacity plays a central role in the life of the patient with chronic obstructive pulmonary disease (COPD), both as a marker of well-being1 and as an indicator of a poor prognosis.2 Our ability to characterise this crucial aspect of disease has grown rapidly in the last decade and with this so has our understanding of the many complex reasons for exercise impairment. It has long been recognised that the maximum ventilation during exercise is related to the initial FEV1 (forced expiratory volume in 1 s), with several formulae being developed to predict this. It was accepted that an inability to sustain a high level of ventilation would limit exercise performance in COPD, although exactly why this happened was uncertain. In the last decade there has been compelling evidence that changes in the operating lung volumes during exercise lead to mechanical limitation of inspiration and hence of tidal volume, which is associated with the sensation of breathlessness.3 4 Dynamic hyperinflation is a very consistent finding in COPD and can even occur early in the natural history of COPD, at least in symptomatic people.5 However, not all patients are limited exclusively by breathlessness on exertion, and data from the McMaster group in the 1990s pointed out that many patients were limited by a feeling of heaviness or fatigue in their legs, either along with breathlessness or dominating this sensation.6 As a result, attention began to turn to other factors, such as co-morbid cardiac disease and the possibility that skeletal muscle itself was not normal in COPD.7
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