Rest Periods for Size
When using the big compound movements for building size, you want to use rest intervals that aren't that far off from what you'd use in a strength protocol. When you're using compound movements you don't want to create excessive CNS fatigue, so you should rest long enough to allow for at least a near-maximal neural recovery between sets.
The goal of the compound movement when training for size is not to burn, destroy, or annihilate the muscle, but to progressively use more weight in the proper size-stimulating zone (6-8 and 8-12 rep ranges). So take around two minutes between sets of the same exercise.
For isolation exercises, fatigue (especially cumulative muscle fiber fatigue) is the main goal, so rest intervals should be shorter. Not so short that your strength drops off too much from set to set, but you should try to gradually take less rest over time.
A strength drop-off of 5% per set of isolation work is acceptable, and a total drop-off from the first to last set of 20% is a good target. In other words, shoot for a reduction in performance of 20% between your first and last set of an isolation movement. This reduction can either come from reps or load.
For example, if on the first set you perform 12 reps with 140 pounds, a 20% reduction could mean:
- Doing 9 reps with 140 pounds on the last set
- Doing 12 reps with 110 pounds on that last set
If you can't achieve a 20% drop-off in four sets of isolation work, it means that you're either not training hard enough or that you're taking too much time between sets.
For isolation work when training for size, the rest intervals should be anywhere from 30 to 75 seconds.