Training to muscular failure, otherwise known as momentary failure (i.e., the point at which another concentric repetition cannot be completed with proper form), is not necessary to increase muscle strength.[1] In fact, ending each set a few reps shy of failure appears to be superior to training to failure for maximizing gains in 1RM strength.[2][3]
In contrast, training closer to failure appears to be better for increasing muscle size in a dose-response manner,[4] meaning that on a per set basis, a set performed to failure is the most conducive to muscle gain. However, this relationship appears to be influenced by load.
When using light loads (30%–40% of 1-repetition maximum), sets should be performed to failure to maximize muscle gain,[5] but as the load gets heavier, training to failure becomes less important.[4] Further research on the relationship between load, proximity to failure, and muscle gain is needed, but evidence suggests that in the context of heavy loads (≥80% of 1-repetition maximum), training to failure and stopping a set several reps shy of failure produce similar muscle gain.[6]