IF YOU WERE to pit an old-time runner against an athlete 30-40 years his junior, something interesting would happen. Sure, the kid might outrun Gramps in the short term, but the seasoned runner would have one invisible advantage over a long distance: more efficient muscles.

That’s because a lifelong regimen of aerobic exercise—like cycling, rowing, running, or swimming—trains muscles to wring every last bit of energy from the body’s natural fuel sources, according to a new study published in Medicine & Science in Sports & Exercise.

Researchers set out to see if exercise changes how muscles store fuel (glycogen and fatty acids), the composition of muscle fiber type (whether slow- or fast-twitch), and the muscles’ ability to produce energy (oxidative capacity) in seasoned endurance athletes.

In the study, researchers examined muscle biopsy samples from 14 “young” recreational athletes (age 18–39), and 13 “older” master endurance athletes (age 60–75). The researchers then compared the biopsies according to how often the athletes exercised (five times per week) and mode of training. They also measured the athletes’ fat and glycogen oxidation before and after they underwent a graded cycle ergometer test at maximum effort.

Their findings: Muscles do get better with age—for the most part. The “older trained muscle” in the masters athletes tended to have more fatty acids (which serve as fuel) and a higher percentage of oxidative muscle fibers to produce more energy. The older athletes’ muscles also burned fat more consistently during moderate exercise. Their only disadvantage? Lower levels of glycogen, the body’s secondary long-term source of energy.

So if you want to stave off the negative effects of aging and a sedentary lifestyle, and make exercise metabolically more efficient for your body, you’ve got to get off the couch and get moving for as long as you’re still kickin’.