You'll find an excellent long article at RedNova on the way in which exercise relates to metabolism, gene expression, chronic disease, mitochondrial function, biochemistry, health and longevity. Very good stuff.
To emphasize just how profound this effect is and how recently it has become critical, consider that a century ago type-2 diabetes was never seen in humans younger than 40 years. Even two decades ago it was routinely called "adult-onset diabetes." Today, physicians are seeing "adult" diabetes in 10-year-olds. But it's not just diabetes that's cropping up in youth. Sixty percent of overweight teenagers already have at least one risk factor for coronary artery disease. Whatever merits youth sports may have for social development, young people (and old) need physical activity outside of sports just to maintain metabolic homeostasis to prevent many chronic diseases that will shorten their lives.
Work to understand how physical inactivity changes the mixture of proteins made as old skeletal muscle becomes physically inactive remains ongoing. Booth and Scott Pattison of the University of Missouri used small glass plates containing thousands of copies of genes to measure mRNA made by muscle. We found that more than 700 genes (out of about 24,000 measured) changed when comparing old to young skeletal muscles. Since many patients with, for example, broken hips successfully survive the surgical repair but later die with weak skeletal muscles, we wanted to identify the genes responsible for the inability of the weak skeletal muscles to get strong again in old humans. Old rats experience a similar loss in ability to enlarge skeletal muscle after limb immobilization, so we looked at differences using the animal model. A total of 354 genes differed in their expression in skeletal muscle between young and old rats with immobilized limbs. As we write, our current work focuses on determining which of these genes are the culprits that prevent recovery of skeletal-muscle strength after limb immobilization.
By now we hope you'll agree that skeletal muscle is a fascinating and largely underrated tissue. But a richer scientific understanding of its function may also require some changes in the linguistic world. The profound effects of underutilizing skeletal muscle suggest that the old adage "use it or lose it" turns out to be a gross understatement.
Use it or lose it indeed - moderate exercise is continually demonstrated to be one of the four most important things you can do right now for your long term health and longevity. (The other three being a good calorie restriction diet, supplementation and supporting research into real anti-aging medicine).