Muscle grows when you challenge it enough to cause fatigue — signaling the body to rebuild the muscle stronger. “The most important factor is muscle fatigue, the point where you can’t do another rep with good form. Without that, 1 lb weights won’t create much change,” says Jacqueline Gavino, DrPH, MPH, the director of fitness at Pritikin Longevity Center in Miami.
One way to do this is through slow, controlled movements, which keep muscles under tension for longer and are especially effective for building muscular endurance — the ability of muscles to exert force repeatedly over an extended period without fatiguing — with light weights, Fiutem says.
Increasing your rep count works similarly. “Generally, performing higher repetitions — like 15 or more — helps develop muscular endurance rather than maximal strength,” Fiutem says. That’s because lighter weights combined with higher rep counts mainly target type 1, or slow-twitch muscle fibers, which are built for sustained effort over time.
Push those reps far enough to reach fatigue, though, and you’ll also recruit type 2, or fast-twitch, fibers — the ones responsible for strength gains and muscle growth, Dr. Gavino adds.
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