Cluster sets aren’t bad, but CNS fatigue can be a threat… what’s your goal?

Howdy,

Cluster sets are smaller sets built-in a larger set with rest increments that typically range from 10-30 seconds (see below). Cluster sets involve taking around 90-95% of your 1RM and completing 5 sets of 3-5 repetitions, with 10-15 seconds of inter set rest between each rep. For example, you would complete one rep of the back squat, rest for 15 seconds, and repeat resting 15 seconds between each rep for a total of 5 reps. We want to use this technique mainly with large compound movements such as squats or bench presses, but it is possible to implement clusters with single joint movements such as the prone leg curl (if targeting hypertrophy). As cluster sets can be heavily taxing on the central nervous system (especially if very short rest), I would recommend programming them for no longer than 3 weeks during a strength phase (especially if you are an athlete).

What’s most important when figuring out rest for your cluster sets is gauging your capabilities for moving weight efficiently and safely. For example, if you’re missing reps due to the limited rest, then scale back the weight, or slightly increase your rest interval. When it comes to the rest between sets, it will be similar to what traditional sets look like. Ideally, take the rest you need to get in the work without missing reps, or having to drop intensities. Otherwise, short rest periods between sets mean that we train while CNS fatigue is still present from the preceding set – which is bad in every sense.

High-threshold motor units – the ones best suited for hypertrophy – are best recruited at high training intensities, accompanied by greater motor unit synchronization and greater muscular force. The requisite muscular tension needed for successfully completing a higher-intensity lift sends the message to the brain to bring out the big guns. (As always, we return to the nervous system.) This happens, too, during straight sets, but only as fatigue is reached. Cluster sets allow for motor unit synchronization, high levels of muscular force, and the recruitment of high-threshold motor units without fatigue. Heavier weights for more reps, gives us bigger muscles.

Whether we do a cluster set of 30 seconds or 10s pause, short rest periods increase the risk of training while experiencing CNS fatigue that could affect your next upcoming training at least (that’s why it should be closer to 30s if we do power/strength clusters). You are potentially not allowing enough time for the higher threshold motor units to recover (after a few sets), and therefore less of a hypertrophy effect at least. But this is individual, some athletes will recover faster. 

Traditionally, strength coaches have associated CNS fatigue with heavy strength training using certain multi-joint exercises (such as deadlifts). However, the research is very clear that CNS fatigue is actually greater after long-duration, low-intensity exercises than after short-duration, high-intensity exercise. One study found that CNS fatigue is greater after isometric contractions with 20% of maximum force than after similar isometric contractions with 80% of maximum force. With cluster sets, if very short rest between cluster reps (especially after doing few reps in a row, in the beginning of the set), we are doing pretty much strength-endurance (high load though), which is still CNS taxing. This method is not bad from the buliding specific endurance standpoint, but that’s for another topic.

BUT, DON’T GET ME WRONG HERE, THOSE SHORT REST PERIODS ARE THERE FOR THE PUROPSE OF QUICK “ENERGY RECHARGE” (REST) IN ORDER TO SOMEHOW KEEP WORKING – THEREFORE THIS TRICK WILL PROBABLY PRODUCE A BIT LESS CNS FATIGUE AND ALLOW FOR GREATER STRENGTH DEVELOPMENT AT LEAST. So, now we will see why one huge group of coaches claim that this is the best method for building strength (and pretty good for hypertrophy as well).

Most coaches believe that the 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. When you do 15 reps of dumbbell curls or bench presses, but your form begins breaking down, it’s not worthwhile. And if you don’t want your form to break down, you have to use lighter weights. Take a little break every few reps, though, and you get to reset and regain focus, hammering the form of an exercise while still using a heavier load. Quality volume (total load lifted multiplied by total reps, if you’re into the math of it) has always been the critical factor in building strength, power, and mass over time. Cluster sets have shown lower lactic acid accumulation and less neuromuscular fatigue. This means you can use them to pile up much higher volume than you’d get in normal workouts. This all is very true, especially if we are talking about high-rep stuff (if we still want to express high load under significant fatigue).

You can take a look at cluster sets as one kind of an “acute overtraining”… When we lift few sets (within one big set) to failure (and close to, usually 90% of 1RM and above), it places a great deal of stress and demand on the sympathetic nervous system and when done with more volume and intensity than the athlete can adapt to, sympathetic overtraining results (chronic part, which is where all the well-known dangerous symptoms start occuring). This places a great deal of stress on the CNS itself as well (especially if more than 5 reps per set included, which is pretty common when doing cluster sets). If you show noticeable decrease in reaction rate, poor motor coordination, lowered strength and power levels, inability to mentally focus… back off a little bit and take longer rest because those things will improve nothing (accumulation of fatigue during cluster sets is huge if number of reps is higher). Also (an extra, interesting, info), remember that the rate of strength recovery from a workout varies between muscles and between lifters. This is likely due in large part to inter-muscle and inter-individual differences in muscle fiber type proportion and voluntary activation. Fast twitch fibers are more easily damaged than slow twitch fibers (likely due to their less oxidative nature). A greater ability to recruit high-threshold motor units makes fast twitch fibers easier to activate (and damage). Thus, muscles (or lifters) with a higher proportion of fast twitch fibers and/or ability to recruit more motor units, will display slower recovery rates than muscles (or lifters) with a lower proportion of fast twitch fibers and/or an ability to access fewer motor units. We need to pay attention to this when doing clusters.

More fatiguing strength training workouts involve much greater cardiovascular demand (which is linked to a larger amount of CNS fatigue) and cause more muscle damage leading to excitation-contraction coupling failure (m. damage in larger % occurs especially during eccentric phase/training). Some strength training experts suggest that we can largely ignore CNS fatigue during strength training. However, as some studies showed, CNS fatigue is a completely normal part of strength training exercise (less compared to endurance training though). This is important, because CNS fatigue decreases our ability to recruit high-threshold motor units (it’s potentially detrimental phenomenon during exercise, because it can prevent the highly responsive muscle fibers of high-threshold  motor units from being trained). Therefore, reducing the impact of CNS fatigue on a workout is a viable strategy for enhancing hypertrophy and max strength, at least.

So, we all know that CNS fatigue occurs extensively during aerobic exercise, and only to a much lesser extent during strength training. Moreover, the amount of CNS fatigue seems to be greater when the duration of the set is longer (which definitely happens while doing clusters, but those small rest periods are helping a lot as well). It seems it’s determined by the duration of a set more than the amount of load we lift. This principle applies even when the load is the same but the nature of the muscle being trained allows for a longer duration of time spent before reaching failure. 

Among other things, cluster sets can be an useful tool for increasing one’s total work and volume during a workout. They allow for the potential to perform more reps with certain percentages while attempting to avoid form breakdown. 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.

According to some authors, cluster-style training does nine things better than traditional straight sets, or sets that have reps continuously strung together:

  1. Greater total power output during bench press training
  2. Greater force, velocity, and vertical displacement during Olympic lifts
  3. Greater power output during ballistic jump squats
  4. Better maintenance of technique throughout the unit of training
  5. Better maintenance of jump height and distance
  6. Lower ratings of perceived exertion after training
  7. Lower reliance on glycolytic pathways during training
  8. Similar increases of muscular strength after a training cycle
  9. Similar androgen and hormone responses to training

Both Charles Poliquin and Christian Thibaudeau think that clusters is the best strength training method of all time. Charles used to say: “You do one rep, rest for around 15-20 seconds, do another rep, rest 15-20 seconds, then do another rep. You do that until you get to five with a weight you’d normally only lift three times.” He added, “You do that for your squats, front squats, and deadlifts. That will rapidly increase your strength.” With this method, Christian went from 455 to 525 pounds in about six weeks. He knows, it sounds too good to be true, and it’s likely not a typical result. But he was hooked (as he said). In the months to follow he got his squat even heavier, eventually reaching 595 pounds. This technique also got his front squat from 395 pounds to 485. His push press also went up from 275 to 325… Years later he’d apply it to the bench press. It’s one of the main methods that allowed him to eventually reach a max of 445 at Dave Tate’s EliteFTS compound. He became a lifetime champion of clusters and have used it with a lot of pro athletes, international level amateur athletes, and strength athletes. The results are always impressive (he said). The only problem? The “traditional” Poliquin approach didn’t always work well. Some athletes (the stronger, more advanced lifters) often couldn’t get five cluster reps with their three-rep max. Some would burn-out. In fact, some actually had to use less weight than their 3RM.

When you’re very strong (or have a high ratio of fast-twitch fibers) the 15-20 seconds of rest can be insufficient for taking advantage of the potentiation effect. The sub-optimal rest decreases performance capacity enough to mask the increase in performance that’d come from the neural activation from the heavy rep.

Some people might need at least 30 seconds of rest to get the full benefits of clusters for strength gains. However, if your goal is to get both size and strength as an advanced lifter, Poliquin clusters are super solid.

I also find the Poliquin cluster to be superior for women since they don’t seem to require the same amount of rest to recover.

Carl Miller used two main types of clusters. An extensive (more volume) method and an intensive (higher load) method.

The weight used depended on the type of movement selected. For example, on technically complex movements like the snatch or clean & jerk, a lower percentage would be used compared to the squat, press, or deadlift.

These types of clusters are more appropriate for very strong athletes or those with a high ratio of fast-twitch fibers. Very strong lifters will need the longer rest between reps to take advantage of the potentiation effect of clusters. Miller clusters are a pure strength method. They will result in less muscle growth than the Poliquin cluster.

All of these cluster approaches will be effective at increasing strength rapidly. However, the stronger you are, the more you should go with the Miller work-to-rest ratios. On the bigger lifts – like squats, front squats, deadlifts, and the Olympic lifts – the Miller ratios might also work better.

The Poliquin work-to-rest ratio will give you a bit more muscle growth stimulus. This could be an interesting tool for athletes who need to maintain a high level of force production with short rests. I’m thinking CrossFit athletes and MMA fighters.

The accentuated eccentric clusters provide a stronger stimulus than the normal sets, but they cost a lot more and will require a lot more recovery time. As such, these more demanding options should only be used by people with a background in high-level strength work. And the volume must be kept lower (clusters in general, in most cases).

In order to grow greatly and build massive strength, you need to recruit and fatigue fast-twitch muscle fibers. Recruitment is maximized when the load on the bar is at least 80-82 percent of your max strength. In a cluster set, all of your reps fall into that zone. That means every rep is maximally effective at stimulating hypertrophy and increasing strength. You’re also able to do this with no “garbage volume” since all reps are maximally effective at recruiting the fast-twitch fibers.

Stimulating growth is, in large part, a matter of putting mechanical stress on the highest number of muscle fibers possible. First you must recruit these fibers, then every time you lengthen those fibers while they’re under load (eccentric/negative portion of the rep) you trigger the mechanisms that lead to growth (that’s why slow eccentrics is super important for muscle growth). The heavier the weight, the greater the mechanical load on the fibers.

With maximal loading comes a lower number of reps per set, which means fewer occasions to impose the mechanical stress while the fibers are lengthening. But with a cluster set, you get more reps in a set while also maximizing load. That makes cluster training a powerful hypertrophy method.

As we said, many coaches claim that 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.

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.

You develop the capacity to make the fast-twitch fibers “twitch” as fast as possible. This is called a “high-firing rate” and it’s the real key to strength. The faster your fibers can fire (fire more frequently in a given unit of time) the more force you produce. This firing rate increases exponentially when the load is more than 85 percent of your max. With clusters, every rep is in that zone too. So the more of these reps you have relative to your total number of reps, the better you are at programing your nervous system to make your fibers fire with a high rate. As a result, you get stronger.

You get a higher quality of heavy work. Because of the intra-set recovery period, your capacity to produce force is better maintained from rep to rep. This leads to better technique maintenance.

If you want rapid strength gains, clusters are definitely worth considering. Christian T. started using them over 20 years ago and haven’t stopped. It’s something that has stood the test of time and been shown to work for pretty much everybody. Give it a shot if you’re ready to gain strength fast.

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