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Light weights build muscle too
Closeness to failure
Closeness to failure is what counts. Sets don't have to reach failure, only come within a few reps of it. Anywhere from one rep to 30, getting close to failure seems to produce similar results, which lets people pick whichever form of discomfort they prefer.
Men and women
Women add lean mass at a similar rate to men: as a percentage of starting lean mass, there is no statistically significant difference between the sexes. Men add more in absolute terms because they start with more.
Female fatigue
Female muscle fibers adapt a little differently, and women tend to be somewhat less fatiguable and may recover a bit better. One theory, with no data behind it, is that this reflects lighter absolute loads. The heaviest powerlifters don't squat several times a week at 800 or 900 pounds, while lighter lifters of either sex often manage many hard sessions.
Why do light loads work near failure?
As a set nears failure, the muscle tires and the body recruits more motor units to keep producing force. Under Henneman's size principle, small motor units are recruited first and large ones last. A light load taken close to failure ends up recruiting the same large units a heavy load does at the start. The fibers are loaded under tension in both cases.
Which approach builds more strength?
In a 2017 meta-analysis in the Journal of Strength and Conditioning Research, Brad Schoenfeld and colleagues found muscle growth was similar across low and high loads. Strength gains, measured by one-rep max, were greater with heavier loads. Strength is a skill practiced at specific loads, so training heavy improves lifting heavy.
Why might women fatigue less than men?
Sandra Hunter at Marquette University has reviewed sex differences in fatigability. In many tasks, women fatigue less than men at the same relative intensity. Proposed reasons include a greater share of slow-twitch fibers in the muscles of women and lower absolute force, which leaves more blood flow to the muscle during contraction.
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