Chronic diseases are major killers in the modern era. Physical inactivity is a primary cause of most chronic diseases. (Source) Physical activity and inactivity mechanisms differ; gene-environment interaction [including aerobic training adaptations, personalized medicine, and co-twin physical activity]; and specificity of adaptations to type of training.
The body rapidly maladapts to insufficient physical activity, and if continued, results decrease both total and quality years of life. Taken together, conclusive evidence exists that physical inactivity is one important cause of most chronic diseases. In addition, physical activity primarily prevents, or delays, chronic diseases, implying that chronic disease need not be an inevitable outcome during life.