Normal individuals have a large reserve in lung function that allows breathing capacity to increase at least twenty to thirty times during periods of vigorous physical activity. It is not the ability to breathe that normally limits maximum exercise capability, nor does physical activity normally change lung function, either in the short-term or over time with training. With lung disease, however, lung function declines and can impose breathing limitations on physical activity. This may result in excessive shortness of breath with daily activities that previously could be carried out without great difficulty.
In some susceptible individuals, exercise may cause the airways ("breathing tubes") to narrow, reducing lung function and causing abnormal shortness of breath with physical activity. Such persons are thought to have "reactive" or "twitchy" airways.
ANDREW L. RIES
Mahler, D. A. (1993). "Exercise-Induced Asthma." Medical Science of Sports and Exercise 25:554–561.
Ries, A. L.; Bullock, P. J.; Larsen, C. A.; Limberg, T. M.; Myers, R.; Pfister, T.; Sassi-Dambron, D. E.; and Sheldon, J. B. (2001). Shortness of Breath: A Guide to Better Living and Breathing, 6th edition. St. Louis, MO: Mosby.