The dark side of the training response is degeneration. Over doing it or under doing it is the dark side of exercise. In the sweet spot of physical activity the training response regenerates muscles and joints. As we age we exercise less and get out of shape and we can become prone to using the same movement patterns over and over. The over reliance upon too few a variety of movements is one way a human body can degenerate or repetitively strain (Cinderella Hypothesis). 

Degeneration does not occur due to age otherwise we would degenerate all over our body because it is all the same age and the prevalence of MSK pain would rise with age and it does not. Informing us that age is not a cause of MSK conditions.