For advanced trainees who may be hitting a plateau, cluster sets are a technique that allows us to work with heavier weights or more volume than we normally could. This helps us place new demands on our muscles and has the potential to get us past our plateau. During a cluster set you break up your reps into clusters and take short breaks in between them. So instead of doing a set of 6 reps, your could do three clusters of 2 reps with 30 seconds between them. At the end of the cluster set, you end up doing the same amount of reps: 6.
The biggest advantage to cluster sets is that they allow you to move heavier weights, which results in more strength gain and/or more muscle mass added. Let's think about this rationally:
Imagine I ask you to squat a certain weight. You get under the bar and manage to get 6 reps. By the 6th rep you are completely done, you could not do another rep. We then add a little bit more weight on the bar. You know for sure you wouldn't be able to do 6 reps at that weight, so we do a cluster set. You get under the bar, do 2 reps and then put the bar down. After taking 30 seconds, you do another 2 reps. Take another 30 seconds break, and then do your final 2 reps. And voila! That's 6 reps at a heavier weight.
Cluster sets can be used for strength or for hypertrophy depending on your rep scheme. You could do clusters of 3-3-3-3 or 4-4-4 for hypertrophy, as long as the total reps are in the 8-15 range. For strength you can do clusters of 1-1-1-1 or 2-2-2 or any combinations as long as the rep range stays in the 2-6 rep range.
There are many, many combinations of clusters out there and I will outline a few popular ones at the end of this article.
The only limitation (in my eyes) for cluster sets is how long they take. Doing a normal set of 6 reps take no longer than 30 seconds. A 2-2-2 cluster is going to take at least 90 seconds, three times as long. When you plan on doing them, just plan for a little bit more time in the gym.
Who should do them
Cluster sets are an advanced technique to put on strength and size. If you are new to the gym I recommend you find your 5 rep max on a lift with a linear progression first and then go through at least two volume-strength cycle. That should take a least a year.
If you've worked out for 2+ years, are confident with your form and have already experienced with volume and strength phases, then go ahead and give the clusters a try.
When should you do them?
You can program your cluster sets according to their goal, so strength cluster go in your strength phases and hypertrophy clusters go in your volume phases.
Since they allow you to lift heavier weights, putting them at the end of a strength phase and beginning of a volume phase makes the most sense in my eyes.
Warning note: Cluster sets will fatigue your central nervous system a LOT. You need to be careful not to over do them and for that reason only do them for 2-4 weeks at a time. I recommend taking a deload week after your cluster set program.
What exercises should you use?
All of them.
You will see below that you can arrange your cluster sets many different ways and you can find something that work for every exercise.
How many cluster sets per workout?
Your programming and workout split will be the first determinant of this. You won't be doing the same cluster set workouts if you are working out 5 days per week vs 3 days per week. At the beginning, start by only doing two exercises as cluster sets and then see how you react. Remember that these are pretty taxing on the central nervous system so take your time implementing them in your routine.
Back squats: 4 cluster sets
Pull ups: 4 cluster sets
Incline bench press: 4 normal sets
Hamstring curls: 4 normal sets