1. Normal respiratory muscles have a large functional reserve. 2. Muscles have a tremendous capacity for adaptation. Adaptation is task-specific (eg, muscles use similar motor units for the same tasks). 3. Muscular fatigue results when motor units are required to perform an unaccustomed task. Continued effort in the face of fatigue, especially high-tension low-repetition effort, may produce a reversible "use atrophy" (eg, fiber damage, splitting, and regeneration). 4. Exercising damaged or regenerating muscle may produce irreversible damage. 5. Exercising the respiratory muscles of some patients may make them more susceptible to fatigue or, at least, produce no further improvement in function. Alternating rest and exercise improves pulmonary function tests in some patients. 6. Retraining a weak or damaged muscle requires that it first be "shut down" and rested before attempting retraining. 7. Training a rested muscle to different tasks--before these tasks are needed--may be the critical step in successful rehabilitation. 8. Resistance breathing probably improves both respiratory muscle strength and respiratory muscle endurance. 9. There are at least three immediate tasks for clinicians to define: Where in the present natural history of COPD should respiratory muscles be rested? How long should they be rested? How best can they be retrained?