HYPERTROPHY CLUSTERS
BIG HYPE OR BIG HYPERTROPHY?
A little while back I posted on twitter (now X) how I went all in on cluster set formats within my training prescription for a number of different goals. I mentioned this is one of my favorite approaches for hypertrophy training presenting a simple format beyond that of traditional sets and reps schemes. Now let’s fast forward a few years, and it seems a number of people have begun to jump on the wave of this type of cluster set training calling this the “HYPERTROPHY CLUSTER”.
When considering sets and reps schemes I find myself going all in on clusters sets for most purposes –
Example: 💪 Hypertrophy: 2+2+2+2+2 (15-30secs intra rep rest) 🏋️♂️ Max Strength 1+1+1 or 2+2 (30-45 sec intra rep rest) 🤾♂️ Plyos 3+3+3+3+3 (10-30secs intra two rest) — Dr. Craig Turner (@CTsportsci) July 3, 2020
At the time of my original post I was questioned… “ WHY FOR HYPERTROPHY ? ” … and rightly so !
Given this set x repetition scheme is typically researched on increasing strength and power performance in comparison to traditional straight set schemes it was of course more theoretical than foundational as to why I used this for hypertrophy. At that point in time and even to the current date there is still a scarcity of research looking at clusters in relation to hypertrophy. Though after a few years of experimenting with protocols, and some further research, here’s a run down on “hypertrophy clusters”, what they are, how they might work and if this is all just big hype…
WHAT IS A HYPERTROPHY CLUSTER?
In truth there is no such thing as a “HYPERTROPHY CLUSTER”, at least by a purist definition.
However, this has seemingly become the name amongst some strength and conditioning coaches (and myself) given to the cluster format (sets x reps with intra-set rest) with the primary aim to build lean muscle mass (hypertrophy being the term referring to this of course).
The cluster format of training refers to the use of a smaller number of reps “clustered” by providing rest between a number of repetitions to split up a set. These short intra-set rest periods are performed before your main inter-set rest between sets (and subsequent next cluster block).
There are other format names such as the rest-pause method, which is generally also like a cluster set in principle (i.e. it uses rest to split up a set and create a cluster of reps). There are also protocols which use inter-repetition rest between single efforts (which is a different spin on a cluster format).
So as an operational definition for cluster sets we will go with is: a group of reps separated with intra-set rest before the set is terminated and the main inter-set rest period is performed.
A QUICK EXAMPLE OF A CLUSTER SET FORMAT:
For example, if you were performing a set of 12 reps in your program a cluster format may look like one of the following options:
You perform a total of 12 repetitions interspersed with rest periods (known as inter-set rest). Here we have two examples. One with 4 rep clusters and another with 2 rep clusters. Both total 12 repetitions, but get you there in different ways. This the typically raises the question:
But why would you do this instead of just performing 12 repetitions straight?
AN OVERVIEW OF THE CURRENT EVIDENCE FOR CLUSTER TRAINING
A number of research articles have explored cluster set training approaches with this typically being investigated in relation to strength and power. The advantages suggest that participants can perform greater sustained outputs, which then in turn can lead to greater adaptations in strength, power, a lower fatigue response and POTENTIALLY HYPERTROPHY…
When it comes to specifically measuring hypertrophy responses there is currently a lot less research than the other areas of S&C. This is likely due to the difficulty in measuring hypertrophy responses in a standardized way, and the need for longitudinal study designs that last a number of weeks so that hypertrophy can occur. Nevertheless there is still some evidence to date surrounding this concept.
Below is a brief review of some of the studies pertinent to this area:
SO WHY MIGHT THE CLUSTER SET METHOD BE IMPORTANT FOR HYPERTROPHY?
Essentially we can think of the cluster format providing a little bit of rest, which is just enough to mitigate some of the metabolic fatigue that you would more commonly experience within a traditional straight set format. This allows you to maximize your velocity capabilities across each of the reps within a set, maintaining higher outputs and the ability to perform more volume than that of the more traditional set schemes as detailed in some of the studies above.
This could also mean that you’re improving your effective force production, and the time component of impulse. Therefore, the emphasis of cluster sets is seemingly to increase the mechanical strain / stress imposed by decreasing the metabolic stress and fatigue that may occur in other formats of training, which may be important for hypertrophy responses to occur (Krzysztofik et al, 2019).
From the table above we see that cluster sets are as effective for potential hypertrophy responses as traditional straight set training formats. But there are also some other positive aspects of cluster set training that warrant further discussion.
YOU CAN PERFORM MORE WITH CLUSTER SETS AT THE SAME RELATIVE INTENSITY AS TRADITIONAL SETS...
Here is an example of the same weight performed in the barbell back squat exercise, in which reps are performed in a straight set format to failure and clusters of 4 (with roughly 15 seconds rest between clusters).
TRADITIONAL SET TO FAILURE:
CLUSTER SET OF 4 REPS:
We can see that in this example the cluster format increases the total volume of repetitions within one set, in comparison to the repetitions to failure (RTF = 12, Cluster = 16) – one could argue that there was probably another few reps that could be performed before true failure in both conditions.
Nevertheless this shows that with clusters at the same relative weight we can push more volume. This is also demonstrated in the literature in which greater volume loads are noted with cluster set formats in comparison to traditional rep schemes (Iglesias-Soler et al, 2014; Oliver et al, 2015; Tufano et al, 2017).
It should be noted that I used a 10% velocity loss threshold based on the best rep for these sets. Mainly this was done for visual purposes. We see in the straight sets format at rep 5 the concentric velocity begins to decline below this threshold. Barring the last rep of the first cluster we see that velocity stays within 10% of the best rep until reps 15 and 16 (to which the set was terminated). Its likely we could’ve got one more set of clusters or more before true exhaustion kicked in based on the final velocities that we see by the end of the traditional set AMAP in comparison to the velocity seen in the cluster sets.
The other important aspect here is that velocity is maintained across the set beyond that of the straight set format. This means the individual is applying force more effectively per rep and as a result a faster impulse is created to perform the full repetition. This is favorable particularly for athletic populations who are looking to optimize strength and power qualities and is likely a reason why Oliver & Colleagues (2013) suggested clusters are great for hypertrophy in strength/power athletes.
The one potential drawback with this method is that session time may increase in comparison to straight sets due to the allotted rest times. However if volume was identically matched in the straight set format (most likely requiring an extra set) it’s likely these two protocols would equate. It could also be argued that the longer inter-set rests between clusters aren’t needed which could actually condense session time.
In addition to maintaining velocity across a set, cluster sets also offer another unique opportunity in comparison to traditional straight set formats…
CLUSTER SETS ALLOW FOR A HIGHER INTENSITY TO BE LIFTED FOR THE SAME RELATIVE VOLUME AS TRADITIONAL SETS...
Previously we demonstrated that at ~60% 1-RM that we can perform more reps before failure in comparison to the straight set format. The other option that clusters gives us is using a higher intensity and completing the same volume that you would complete in a traditional straight set of a lower intensity.
Below is an example of a cluster format doing 6×2 reps interspersed with 15-20 second rest….
This makes cluster sets a lucrative option, because now with the higher load we need to create more force and overall impulse to lift the load successfully. This means according to the size principle higher threshold motor units will be needed to create and sustain the given output to complete the work. We also see from the video that our average mean concentric velocity is similar to that displayed in the straight set format (Clusters = 0.57 m/s, RTF = 0.60 m/s) making both protocols comparable in terms of performance output, with the clusters accumulating the same volume at higher intensities.
As such with this higher velocity for the given external load, the activation of larger muscle units are needed before reaching fatigue. We are likely to see a greater relative contribution of type II and IIx muscle fibers, which means we may see greater adaptations in these fibers (e.g. potential adaptations such as targeted increases in cross sectional area), particularly as we are performing higher volumes of work at higher absolute intensities.
Oliver & Colleagues (2013) showed both cluster training and traditional training had positive responses on myosin heavy chain I, II, IIx and lean mass gains, with no differences between protocols. Though power in both bench press, vertical jump and back squat was improved to a greater extent in the cluster format group. A lot of coaches may call this “functional hypertrophy” in that with the changes in lean mass, there is a similar change in performance measures.
SURELY THERE ARE SOME CONCERNS WITH THIS METHOD?
Naturally when a method sounds too good to be true there is likely going to be some skepticism that accompanies it.
Mainly when it comes to high volume cluster sets, at high intensities, the primary concern that arises is central fatigue (i.e. with high neuromuscular demands how does the central nervous system [CNS] respond) and peripheral fatigue (i.e. muscular fiber damage and soreness).
From the current research it seems that the hormonal responses are either similar or reduced in comparison to traditional sets schemes (Oliver et al 2015; Oliver et al 2016). For example cortisol (an indicator of stress) responses were lower post training for cluster sets format in comparison to traditional 4×10 training (Oliver et al 2015). Given that there are also consistent reports of cluster sets improving power output and strength, it is likely that CNS responses are manageable as positive adaptations are reported with several weeks of cluster training exposure (Tufano et al, 2018; Davies et al, 2021; Davies et al 2022).
The other concern is typically based around the current research that suggests we see similar responses with clusters and traditional sets and reps schemes (Davies et al, 2021)…
" WHY WOULDNT WE JUST USE TRADITIONAL SETS x REPS SCHEMES INSTEAD? "
There are a couple of things to unpack with this one:
Given you get similar responses in hypertrophy, it’s the potential additional benefits that make clusters lucrative. If we can get improved strength / power, and as such functional hypertrophy (i.e. the changes in size are also accompanied by positive changes in performance so you won’t slow down etc.…), then this method seems like a good option.
One of the primary reasons we likely see no differences in hypertrophy between traditional reps schemes and cluster sets is because research protocols typically volume / volume + load match the conditions. Which potentially takes away one of the primary factors of clusters (the ability to do more with more). Therefore it is likely the relationship to volume load & subsequent stress why we see no differences in markers of hypertrophy.
Clusters generally are accompanied with lower fatigue responses in relation to traditional sets. This actually suggests that clusters may be more appropriate for populations who perform concurrent training like sports athletes, or those that need to train at higher frequencies. This means the residual fatigue is likely less than traditional methods and so the carry over to the next session may be mitigated. However, we need more evidence to substantiate these claims, but in principle based on the current evidence this would make sense.
A lot of the current literature has a number of limitations. Most studies volume match work in an attempt to standardize whether the rest format is important. As we know hypertrophy responses are dependent on volume load, mechanical and metabolic stress, it’s unsurprising that more roads lead to Rome (hypertrophy responses with different training programs).
Most studies also use the same or similar cluster set protocols (30 sec inter-set rest), which may not sufficiently cause enough metabolic stress to be even more favorable for hypertrophy (as shown above we can use shorter rest times than a lot of the reported literature). Lastly the studied populations are generally ranging from untrained to powerlifters, though very few if any replication of these studies exist.
The last point is if it shows similar responses in hypertrophy then it offers an alternative option to the pre-existing methods of training and could be important for overload as an “advanced alternative training method”. Its not that a cluster needs to demonstrate superiority, its that it needs to demonstrate the targeted response. Based on the current literature it seems to do so.
SO SHOULD CLUSTER SETS REPLACE OTHER METHODS FOR HYPERTROPHY?
At face value I am reserved to say that HYPERTROPHY CLUSTERS are the best method to increase your muscle gains (but they might be in certain cases).
We know that hypertrophy adaptations can happen with a number of different training stimuli:
Traditional straight sets of high reps with lighter load training
Drop sets
Traditional straight sets of low reps with high set number
Pyramid sets
Training to failure
Tempo training
Accentuated eccentric load
Pre-exhaustion methods
Super sets
Tri sets
Use of alternative methodologies such as blood flow restriction with lower loads
Some even suggest stretching
Cluster set formats are just another programming methodology at the end of the day. To say this method is superior would be purely speculative at this point. Though we can potentially argue the versatility of cluster sets in comparison to traditional sets, with the ability to establish a infinite number of training structures, rep:rest intervals etc… based on the person’s needs for the given day.
THE ADDITIONAL CASE ARGUMENT FOR "HYPERTROPHY CLUSTERS"...
I think it’s also important to highlight that this method has some other further utilities, which isn’t often discussed, that may provide support as a preference method within hypertrophy training:
In populations that fatigue easier (I.e. older/elderly populations, novice athletes or a person returning from injury) this method may make the session more manageable than traditional set methods, reducing the perceived effort and mitigating fatigue that straight sets may accumulate. We again would likely see the ability of these populations to enhance their sessions with either higher volume loads or being able to perform a higher absolute intensity
Well trained athletes/lifters who need new forms of overload. A well trained athlete/lifter may have made it this far and probably not used cluster sets (or at least the infinite combinations of cluster set formats). Therefore this provides a means to establish overload through novel training format that the athlete hasn’t used before.
We know a key to adaptation is creating overload and functional overreaching, as such the ability of the athlete to do more with more, would allow further volume load to be performed per session, particularly pushing more repetitions for a given intensity than what they typically may be used to.
We can condense session time with Clusters. The cluster set option doesn’t only offer the ability to do more volume with more load. But they also offer the option to perform more reps in a shortened rest period, which may save a chunk of time in comparison to straight set formats. We are often looking for the minimal effective dose for the maximal adaptations so this should also be the case when doing lighter warm up sets.
LETS SUMMARIZE THIS DISCUSSION...
WHY WOULD CLUSTER SETS WORK FOR HYPERTROPHY ?
The potential answer to why clusters work for hypertrophy is likely due to the ability to increase the training volume load, increase mechanical stress and reduce fatigue responses during a session, while providing similar hormonal responses to that of traditional straight sets. Specifically the ability to do more volume with more intensity relative to other traditional training approaches also means that higher threshold motor units are preferentially recruited.
From our current understanding of muscle hypertrophy there is a dose dependent response (i.e there needs to be enough training volume performed to stimulate growth and cross sectional area changes in the muscle). Seemingly this points us in the direction of the amount of volume muscle contraction / strain stimulus that’s being provided, as well as metabolic factors such as fatigue to be accumulated. Though currently there isn’t an exact volume, intensity and rest configuration that suggests optimal hypertrophy (Krzysztofik et al, 2019), cluster sets could certainly be a pathway to hypertrophy adaptations.
WHY ARE CLUSTERS A LUCRATIVE METHOD ?
Recent evidence suggests that training to failure isn’t necessary for hypertrophy adaptations (Grgic et al 2022). As the previous example (above) showed, we can actually avoid training to failure at the same relative intensity with a cluster set format, while maintaining higher velocities in the movement.
Therefore cluster set training offers us a method that allows high training volumes without the increased metabolic fatigue that one might experience when training to failure. Which suggests we can skirt between higher mechanical strain without accumulated metabolic fatigue. The question here though remains is that metabolic response needed to further enhance hypertrophy responses?
The other benefits also likely outweigh the negligible differences in hypertrophy between different exercise protocols, meaning you likely get further adaptations beyond that of just hypertrophy (e.g. functional hypertrophy, increased strength and power and the ability to manage fatigue responses).
ARE CLUSTER IMPORTANT FOR ATHLETES ?
We also mentioned previously that a higher relative load for the same absolute volume as training to failure is achievable with cluster sets. The study by Azari & Colleagues (2021), in powerlifters also demonstrated increased lower body and upper body power in conjunction with similar strength gains when compared to traditional sets. This potentially means the preferential recruitment of larger motor units faster, which may suggest greater adaptations to type II and IIx fibers.
Because of the exposure to more intensity, it’s likely greater strength adaptations are also going to occur in conjunction with muscle cross sectional area changes, which is likely a desired combination for sport performance athletes. However we need more studies to investigate this response further particularly in well trained populations in relation to typically training processes that would be used.
WHAT ARE SOME OTHER THOUGHTS ON THE CURRENT EVIDENCE FOR HYPERTROPHY CLUSTERS ?
From the current research there is clearly still a lot of gaps when it comes to cluster sets and hypertrophy. There are also likely a few more studies that exist that are not covered in this article.
However, the sparsity of effects on hypertrophy measured directly suggests that future research should focus on this area to establish the effectiveness of cluster sets. Particularly those sets that use shorter rest periods similar to those demonstrated with the 3/7 method by Stragier & Colleagues (2019).
More studies are likely needed to look at un-matched conditions with real world examples of cluster sets performed in trained populations. I understand the need for controlling variables in the laboratory setting. However from experience this method has been really useful in developing athletes in both strength and size.
FINAL REMARKS:
Cluster set formats can likely offer an alternative set x rep x rest scheme in comparison to other training methods.
It’s likely that this method isn’t superior for hypertrophy to other training methods. However it does tick a lot of boxes in terms of training load, adaptations and mitigating fatigue and creates hypertrophy responses
We need more applied studies in more populations to assess the efficacy of this method with regards to hypertrophy.
I will continue to use the term “HYPERTROPHY CLUSTERS” when referring to higher session volumes with higher intensities utilizing shorter rest periods (<15 secs inter-set rest), which currently isn’t as well studied as some other areas of cluster set training. My theory here is this may increase metabolic stress in conjunction with mechanical stress with the cluster format and in turn improve hypertrophy responses beyond that which we typically see through the more “strain” based protocols that have been published to date. Nevertheless from our current knowledge clusters are a viable method for hypertrophy training, but are not the BEST method.
Arazi, H., Khoshnoud, A., Asadi, A., & Tufano, J. J. (2021). The effect of resistance training set configuration on strength and muscular performance adaptations in male powerlifters. Scientific Reports, 11(1), 7844.
Davies, T. B., Tran, D. L., Hogan, C. M., Haff, G. G., & Latella, C. (2021). Chronic effects of altering resistance training set configurations using cluster sets: a systematic review and meta-analysis. Sports Medicine, 51, 707-736.
Davies, T. B., Halaki, M., Orr, R., Mitchell, L., Helms, E. R., Clarke, J., & Hackett, D. A. (2022). Effect of set-structure on upper-body muscular hypertrophy and performance in recreationally-trained male and female. The Journal of Strength & Conditioning Research.
Grgic, J., Schoenfeld, B. J., Orazem, J., & Sabol, F. (2022). Effects of resistance training performed to repetition failure or non-failure on muscular strength and hypertrophy: a systematic review and meta-analysis. Journal of sport and health science, 11(2), 202-211.
Iglesias-Soler, E., Carballeira, E., Sanchez-Otero, T., Mayo, X., & Fernandez-del-Olmo, M. (2014). Performance of maximum number of repetitions with cluster-set configuration. International journal of sports physiology and performance, 9(4), 637-642.
Krzysztofik, M., Wilk, M., Wojdała, G., & Gołaś, A. (2019). Maximizing muscle hypertrophy: a systematic review of advanced resistance training techniques and methods. International journal of environmental research and public health, 16(24), 4897.
Oliver, J. M., Jagim, A. R., Sanchez, A. C., Mardock, M. A., Kelly, K. A., Meredith, H. J., … & Kreider, R. B. (2013). Greater gains in strength and power with intraset rest intervals in hypertrophic training. The Journal of Strength & Conditioning Research, 27(11), 3116-3131.
Oliver, J. M., Kreutzer, A., Jenke, S., Phillips, M. D., Mitchell, J. B., & Jones, M. T. (2015). Acute response to cluster sets in trained and untrained men. European journal of applied physiology, 115, 2383-2393.
Oliver, J. M., Jenke, S. C., Mata, J. D., Kreutzer, A., & Jones, M. T. (2016). Acute effect of cluster and traditional set configurations on myokines associated with hypertrophy. International journal of sports medicine, 37(13), 1019-1024.
Stragier, S., Baudry, S., Carpentier, A., & Duchateau, J. (2019). Efficacy of a new strength training design: the 3/7 method. European journal of applied physiology, 119, 1093-1104.
Tufano, J. J., Conlon, J. A., Nimphius, S., Brown, L. E., Banyard, H. G., Williamson, B. D., … & Haff, G. G. (2017). Cluster sets: permitting greater mechanical stress without decreasing relative velocity. International journal of sports physiology and performance, 12(4), 463-469.
Tufano, J. J., Halaj, M., Kampmiller, T., Novosad, A., & Buzgo, G. (2018). Cluster sets vs. traditional sets: Levelling out the playing field using a power-based threshold. PLoS One, 13(11), e0208035.
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