From time immemorial—or at least since the 1950s—the mantra for people looking to boost muscular size and strength has been to lift heavy weights (usually defined as those that can be lifted only from three to 10 times with proper form).
However, a study published in the Journal of Applied Physiology says that half-century of research is, well, heavy-handed (Mitchell et al., 2012).
Researchers at McMaster University in Canada recruited 18 healthy men to participate in the study, all of whom were in their early 20s and recreationally active, but who hadn’t lifted weights before. Over a period of 10 weeks, subjects were assigned a combination of three different programs that required them to complete as many repetitions as possible on the leg extension machine, depending upon their assigned programs—eight to 12 reps for the heaviest weights and 25 to 30 reps for the lightest. The three programs used in the combinations were:
The researchers concluded that doing more repetitions using lighter weights is just as effective at muscle-building as the heavy-weight protocol—as long as the subjects trained to momentary muscular failure (a.k.a. fatigue, or the inability to finish your last rep in a set with proper form).
Stuart Phillips, Ph.D., one of the researchers and a professor in McMaster’s department of kinesiology, explains that these findings are consistent with the size principle, in which the body, when needed to perform a task, recruits all its motor units (nerves that activate muscle fibers), from the smallest to the largest.
The smallest motor units activate slow-twitch fibers, and the largest units activate fast-twitch fibers—the ones that generate greater force. “People in the strength-conditioning world have said, correctly, that if you lift a heavier weight, you activate both the small and large motor units. But the size principle says that if the smaller motor units (slow-twitch fibers) get fatigued, the muscle dips into the fast-twitch fibers.”