The aim of pulmonary rehabilitation is to improve exercise capacity and reduce dyspnoea. As well as improving the patient's overall condition, pulmonary rehabilitation provides a means to monitor the patient regularly. During physical exercise, the increased activity of the muscle system increases cardiac and respiratory output. If the patient's respiratory status deteriorates, as occurs, for example, before the onset of an exacerbation, exercise capacity could be reasonably expected to reduce. The investigators hypothesised, therefore, that changes in the physical capacity of patients with COPD during a rehabilitation session could provide a predictive indication regarding the risk of occurrence of exacerbation.