Cluster sets are about quality reps
A key advantage of the cluster set over your regular zounds-of-reps sets is all that rest. It keeps you from completely fatiguing your muscles.
It also flies in the face of old-school training. Classic muscle-building tells us it’s all about time-under-tension, the amount of time your muscles actually have to be working instead of relaxing. Time-under-tension is important too, but here’s what’s more important: Quality of movement.