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.