Introduction The Effects of Physical Activity on Health and Disease T his chapter examines the relationship of physical activity and cardiorespiratory fitness to a variety of health problems. The primary focus is on diseases and conditions for which sufficient data exist to evaluate an association with physical activity, the strength of such relationships, and their potential biologic mechanisms. Unless otherwise specified, the term physical activity should be understood to refer to endurance-type physical activity.
Physical activity is difficult to measure directly.
Three types of physical activity measures have been used in observational studies over the last 40 years. Most studies have relied on self-reported level of physical activity, as recalled by people prompted by a questionnaire or interview. Some studies have relied on occupation to classify people according to how likely they were to be physically active at work. Epidemiologic studies of physical activity and health have compared the activity levels of people who have or develop diseases and those who do not. Cohort studies follow populations forward in time to observe how physical activity habits affect disease occurrence or death.