I think the image says everything, but there is an entire study that goes along with these cross-sections of the thighs of athletes and a non-athlete. The dark matter that looks like the meat in a steak is muscle. (Just like the meat in a steak.) Look at how little muscle the non-athlete has.
Here's their synopsis: This study contradicts the common observation that muscle mass and strength decline as a function of aging alone. Instead, these declines may signal the effect of chronic disuse rather than muscle aging. This maintenance of muscle mass and strength may decrease or eliminate the falls, functional decline, and loss of independence that are commonly seen in aging adults.