A cluster set is a group of single reps performed in a row with a short rest period between reps. For example, a bench press cluster set of 5 reps would involve benching 300 for 1 rep, resting briefly, benching 300 again for 1 rep, resting, etc., until 5 total reps have been completed.
The rest period varies; generally it's between 10 seconds to about a minute in length, with 15-30 seconds being the most common. Any rest of over 1 minute qualifies as individual sets of singles with limited rest, not a cluster set.
Most cluster sets are used with reasonably heavy weight – 85-95% of the 1RM – and for 6-20 total reps per set depending on the goal. During the break the lifter completely rests – with bench press and squat clusters you re-rack the bar; on pull-ups you just stand on the ground. The lifter isn't supporting the bar or weight during the rest period.
If a lifter can bench 300 pounds for a few reps as a standard set, that same lifter might well be able to lift 300 pounds for 10 reps (or more) cluster-style. This allows us to "cheat" and get more reps in at a scheduled weight.
Strong lifters know that the core of strength training revolves around getting in good quality reps at 85% or more of the 1RM. Clusters allow you to do so very effectively, so they're a great tool when training for maximal strength. Clusters can also help build work capacity at a specific intensity range.
Sample Cluster Sets
- Bench Press: 335 x 10 reps with 30 seconds rest
- Squat: 255 pounds x 10 reps with 30 seconds rest
- Bench Press: 330 pounds x 10 reps with 10 seconds rest
- Pull-Ups: 45 pounds attached x 10 reps with 20 seconds rest
- Deadlift: 405 pounds x 10 reps with 20 seconds rest
Most of the drawbacks with clusters come from either not understanding their purpose or improper programming. If you use too light of a weight, it'll feel very easy and the cluster set could continue almost indefinitely. On the other hand, go too heavy and you may not be able to get enough reps to be beneficial.
Because there's a break after every rep and no constant tension on the muscle, it's mainly a strength technique and not as suited for size. The set itself can also take a long time (a set of 10 reps with 30 seconds break is at least 5 minutes long) and if you work out in a group it can make warming up and completing the workout in a timely manner a challenge.