Chronic obstructive pulmonary disease (COPD) is a disease of the lungs that is usually caused by smoking. In COPD, lung damage gradually worsens over time. People with COPD are short of breath and sometimes cough and wheeze. Symptoms slowly get worse over time, but most patients also have intermittent bouts of worsening of symptoms. Lung infections, cold weather, and exertion may bring on these bouts. In addition to medications, treatment guidelines recommend high-intensity, continuous exercise to improve lung function. However, this type of exercise can be difficult for people with COPD to tolerate. Some believe that short intervals of exercise may also benefit patients with COPD and may be easier for them to accomplish, but high-quality studies comparing the 2 types of exercise have been unavailable.