Evidence indicates that low (<60% of 1-repetition maximum) and high (≥60% of 1-repetition maximum) loads produce similar muscle gain.[1] However, this notion starts to fall apart at the extremes.
Very light loads (15%–20% of 1-repetition maximum) seem to be inferior to higher loads for muscle gain,[2][3] and in order for loads at the lower end of the “hypertrophy repetition continuum” (i.e., 30% of 1-repetition maximum) to be as effective as higher loads, each set must be performed to muscular failure (i.e., the point at which another concentric repetition cannot be performed with proper form).[4][5][6]
This is much easier said than done, however. Training with low loads (30%–50% of 1-repetition maximum) to muscular failure tends to produce more discomfort, displeasure, and a higher rating of perceived exertion than training with higher loads.[7][8] As such, not everyone possesses the capacity to reach the point of muscular failure with low loads, making them a suboptimal option for muscle gain for many people.
On the other hand, very high loads (i.e., a 2- to 4-repetition maximum) may be inferior to moderate-to-high loads for muscle gain. One study found greater increases in lateral thigh muscle thickness when resistance-trained men performed 3 sets of an 8- to 12-repetition maximum compared to 3 sets of a 2- to 4-repetition maximum.[9] In another study that equated volume load (i.e., load multiplied by repetitions multiplied by sets) between conditions, 7 sets of a 2- to 4-repetition maximum were as effective as 3 sets of an 8- to 12-repetition maximum at increasing biceps muscle thickness. However, the very-high-load group reported high levels of fatigue, both physically and psychologically, and joint-related issues, which was not the case in the moderate-load group.[10]
Therefore, in order for very high loads to produce similar muscle gain to moderate-to-high loads, more sets need to be performed, which takes more time to complete and risks inducing a greater strain on connective tissue. From a practical standpoint, training with moderate-to-high loads (i.e., a 5- to 10-repetition maximum) may be the best approach for muscle gain for most people.