There are different training methods in endurance training. Above all, the continuous method and the interval method are significant to the physical training with COPD.

Continuous method: The physical exertion is continuous. E.g. "walking fast" at a steady pace or a certain work rate (as illustrated 70 % Wmax on a cycle ergometer. Depending on the intensity, the response of e.g. respiration or heart rate varies: - Steady values = without significant fatigue - Increase of values due to "sustaining" the   high pace/ intensity = fatigue

Interval method: strenuous and light exertion take turns = exercise and a rewarding break take turns = sequence exercise: break. Exercising in intervals is the method that enables to perform strenuous daily activities which otherwise lead to interruptions due to overstraining/breathlessness. It is strain that is more higher for a short period of time than what you would be able to perform non-stop for a longer period of time (more than continuous performance) followed by short breaks in which you strain less to recover (rewarding break).