Heavy sets.
A meta-analysis of 21 studies found that gains in one-rep-max strength were greater with high-load than with low-load training, while muscle size grew similarly either way.2 The ACSM’s 2026 position stand agrees: strength was enhanced by lifting heavier loads, 80% of your one-rep max or more, for two to three sets.1 So if the goal is the number on the bar, some of your sets have to be heavy. Muscle size is more forgiving; strength asks for the weight. (The build-muscle page covers the other goal.)
