There are several factors that can influence strength gains:
- You recruit and fatigue the fast-twitch muscle fibers.
- You develop the capacity to make the fast-twitch fibers "twitch" as fast as possible. This is called a high firing rate.
- You build muscle mass.
- You desensitize the Golgi tendon organs (GTO).
- You become more psychologically comfortable with the lift.
Clusters improve all of those!
You achieve maximum fast-twitch fiber recruitment when the load on the bar is around 80-82 percent of your maximum at that moment. Sure, you can get there by using lighter weights and using fatigue to increase the relative load of the bar.
But by using clusters with 88-90 percent of your max, you're recruiting all those fast-twitch fibers from the get-go. As a result, you won't have any reps that simply drain energy.
But it's not enough to recruit the fast-twitch fibers. The real strength gains will come from improving your capacity to use a high firing rate. This is a motor skill. And motor skill acquisition depends not only on the number of reps done with the skill emphasized, but on the ratio of "good" and "bad " reps.
The closer you are to your maximum strength, the higher the firing rate. Firing rate increases the most when you need even more force and you can no longer recruit more fibers. At 90 percent you have a very high firing rate from the start. If you do five cluster reps with 90 percent you'll get five reps with a very high firing rate and no reps with a low firing rate. From a motor learning standpoint, that's golden.
Now compare that to doing 10 reps with 70 percent. Because of fatigue you'll still end up with five to six reps where the fast-twitch fibers are maximally recruited and probably three reps with a high firing rate.
But you also get five reps with a lower firing rate. From a motor-learning perspective, this is vastly inferior. It's like trying to play golf and doing 30 great swings, 20 suboptimal ones, and 50 sh*tty ones. Chances are you won't improve rapidly.
Clusters are also very good at building muscle. Hypertrophy has a lot to do with the number of maximally-effective reps. A maximally-effective rep is a rep where you're recruiting as many fast-twitch fibers as you can. Since these have the greatest growth potential, it's all about stimulating them as much as possible.
As we just saw, when the load represents 80 percent of the max weight you can lift at that moment, you'll be recruiting the max number of fast-twitch fibers you can recruit.
You can get there by using less weight because each rep fatigues you. As you're fatiguing, your strength will go down (two to four percent per rep) so the weight on the bar is relatively heavier compared to what you can lift.
Here's an example:
| Rep | Weight on Bar | Fatigue Level | Relative Weight |
|---|---|---|---|
| 1 | 70% | 0% | 70% |
| 2 | 70% | 3% | 73% |
| 3 | 70% | 6% | 76% |
| 4 | 70% | 9% | 79% |
| 5 | 70% | 12% | 82% |
| 6 | 70% | 15% | 85% |
| 7 | 70% | 18% | 88% |
| 8 | 70% | 21% | 91% |
| 9 | 70% | 24% | 94% |
| 10 | 70% | 27% | 97% |
As you can see, by rep five you'd have maximally-effective reps. That gives you six of them in the set.
Now let's look at a cluster set. Because of the rest period you'll have some recovery, so fatigue is a bit slower.
| Rep | Weight on Bar | Fatigue Level | Relative Weight |
|---|---|---|---|
| 1 | 90% | 0% | 90% |
| 2 | 90% | 1.5% | 91.5% |
| 3 | 90% | 3% | 93% |
| 4 | 90% | 4.5% | 94.5% |
| 5 | 90% | 6% | 96% |
| 6 | 90% | 7.5% | 97.5% |
Clusters allow you to get as many growth-producing reps as you normally would in a higher-rep set, without having to waste energy doing reps that don't contribute to growth.
And because all the reps in a cluster will be above 85 percent of what you can lift at that moment (it'll range from 90 to 100 percent at the beginning of the rep) it means that not only are you recruiting all your recruitable fast-twitch fibers from the start, but each of those reps has a high firing rate.
The better you are at having your fibers twitch fast, the higher the firing rate. This means you'll be able to produce more force.
Developing the capacity to have the fibers fire at a high firing rate is a motor skill. Not only is it about the number of reps with a high firing rate that counts, but the ratio of reps with a high firing rate and reps with a normal one.
In a cluster of six reps, all six reps have a high firing rate. That's awesome for motor learning. In our 70 percent set above, you'll have three reps with a firing rate comparable to what it is during a cluster, and five reps with a low firing rate and two with a moderate one. From a motor learning standpoint, this is vastly inferior to clusters because of the inferior ratio.
But what if we compare a set of five cluster reps and a regular set of five reps?
In the regular set, you also have no wasted reps and you do all the reps with a pretty high firing rate. This is true, and sets of five are awesome for strength and size. But clusters are just a little bit better.
First because of the higher average load. In a cluster you use around 90 percent of your max and with regular sets of five between 80 and 85 percent. While fatigue evens out the relative load at the end of the set, the heavier weight still has a greater mechanical load than the lighter one, which will cause more muscle damage.
In a cluster with 90 percent versus a set of five at 82 percent, you still have three more reps with a very high firing rate (the closer you are to a 100 percent effort, the higher the firing rate). With 82 percent, it'll take you two to three reps to reach a relative load of 90 percent like you have in the cluster.
Finally, during a cluster set, peak power, force, and velocity are better maintained from rep to rep which makes for more quality reps and better motor learning (1).
For all of these reasons, clusters are my favorite strength-building method. It builds as much muscle mass as regular sets of 8 to 12 reps because of the number of maximally effective reps. It trains the nervous system as well as the Max Effort method, maybe even better because of the higher number of reps.
When doing clusters I suggest doing two work sets as each set has the same neurological impact as doing two sets of three reps at 90 percent.
