I’ve never seen anything that better depicts how we lose muscle as we age if we don’t use it. I’ve seen other studies that correlate a better quality of life as we age with increased fitness, but a picture really is worth a thousand words.
The image below is from MRI cross sections of the quadricep muscles from a research study published The Physician and Sportsmedicine (link down at time of posting). The study took detailed measurements of 40 masters athletes between the ages of 40 and 81, and found a surprising lack of age-related muscle loss. The top pic is a cross section of the thigh of a 40 year old triathlete. The bottom is a triathlete at 70. The middle is a sedentary 74 year old man.
I think this summary from the study summarizes it best.
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. Evaluation of masters athletes removes disuse as a confounding variable in the study of lower-extremity function and loss of lean muscle mass. 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.