Effective reps and fatigue
There are a quite a few competing theories about what method of training causes the most hypertrophy. The most popular of these is the 'effective reps' model. Below I will outline what the effective reps model is and why regulating rest times helps optimize the number of effective reps you can have in a set.
Effective reps is the idea that only some of the reps in a set of resistance exercise cause hypertrophy. These reps are the 'effective reps'. For instance, in a set of 10 on the barbell bench press, only 4 of those reps might actually cause hypertrophy. So if you stopped that set of 10 reps at 6 the rep mark, you wouldn't get that hypertrophy. Notice that this doesn't render the first 6 reps useless as they still need to be executed to reach those last few effective reps.
The proposed mechanism for why these reps are effective in causing hypertrophy is roughly as follows. In order to stimulate hypertrophy, high threshold motor units (HMU) need to be recruited, so they can be worked and grow. This is achieved by having to use more effort in a rep when the rep is getting harder as more and more motor units fatigue. On this logic, there will be two factors that increase fatigue during a set.
1) number of reps
The more reps that are completed, or the longer the set goes on, the more your lower threshold motor units will fatigue so the high threshold motor units have to be activated to complete the reps.
2) the load
Heavier loads will also fatigue the muscle to a greater extent because the higher tensions involved with heavier weights will fatigue the lower threshold units faster.
Obviously, the heavier the load is, the less reps you are going to able to complete. If the weight is too heavy then not as many of these effective reps can take place. Studies suggest that the most optimal number of reps for hypertrophy lies somewhere in the 5-8 rep range This is the rep range where the weight, reps and therefore effort is highest.
In a future article, I will look at why the number of effective reps might be different for different movements and why you don't actually need to worry about what reps are effective or not but for now that is outside the scope of this article.
So from this we know that fatigued muscles are good for hypertrophy because this is how the large high threshold motor units are activated. Therefore it might seem beneficial to have really short rests between sets, so the muscle can stay in that fatigued state. The only problem here, is if that rest is too short, then the number of effective reps might drop dramatically because the lifter has not recovered enough. The lifter needs to be able to performs as many effective reps as possible in a session for maximum hypertrophic outcomes.
In the next section I want to discuss what actually helps a muscle recover and just how long that might take.
ATP - the fuel of the body
When you complete a set of resistance exercise that has lasted more than ten seconds, you have already used Adenosine Triphosphate (ATP), the energy fuel of the body, in a few different ways. After the first rep the muscles cells that cause the biomechanical movement use some of the free ATP around the muscle cell. You do another rep and muscles need to find more ATP so a compound called creatine phosphate, which is already in the muscle cell (CP) is used to make even more ATP quite quickly. This lasts for about 10 seconds.
As you continue your sets and reps, you start using the large reservoir of the carbohydrate glycogen which is also stored around the muscle. This is a slower process than the creatine phosphate system. Glycogen further helps replenish the depleting ATP and creatine phosphate stores even more.
This is why we need to rest between sets. If you do not rest then you just cannot produce the ATP required to move the weight any longer. So rest times should be used in this respect to optimize recovery so we replenish as much of that glycogen and creatine phosphate as possible.
Now you could just do your one set and leave your workout there. You could go home and eat all the bread, pasta and cakes and voila, your glycogen and CP levels will be nice and restored. But there is one issue, you haven’t done enough effective/stimulating reps!
In order to grow and get stronger, your muscles need to recruit those high threshold motor units which happens when your muscles experience fatigue, as explained in the above section.
When you are trying to build muscle and get stronger, the two ideas expressed above are in conflict. On one hand you are trying to let all your ATP producing compounds be fully restored, so you can perform the next set as well as possible, and on the other you are trying to fatigue the muscles with reps and heavy weights to stimulate growth of individual muscle fibres.
You need to rest long enough such that you can complete the programmed set with as much weight as possible without resting so long that your muscles recover entirely and so do not activate as many high threshold motor units in the ensuing set.
You can think of this conflict between accumulating fatigue and energy recuperation with the following analogy. Imagine a video game character who needs to build up a powerful sword strike. The power bar builds and builds and then at the perfect moment you execute the strike. The better timed the strike (starting your set), the more powerful the effect (the hypertrophic outcome).
With the mechanism out of the way, we can now look at the studies that have been done on rest times and resistance training.
What does the science say?
There are two notable studies that have found that a longer rather than a shorter rest are more optimal for muscle hypertrophy. Obviously it matters what we define as short and long. A 2016 study (Schoenfeld, 2016) looked at the differences over an eight week period between a group that trained with 3 minute rest intervals, normally associated with strength, and a 1 minute rest interval which is normally associated with hypertrophy. The 21 young resistance trained men completed 3 sets of 8-12 reps on seven different exercises three times a week. One rep max on squats and bench press increased for the group training with a longer rest period, which was expected but more interestingly, there was also significantly more hypertrophy in the upper leg muscles and a trend for more hypertrophy in the triceps and biceps. This is in line with another study (Buresh, 2009)that found that arm cross sectional area grew significantly more with a longer rest of 2.5 minutes rather than a shorter rest of 1 minute in 12 untrained males after a ten week training period.
Not all studies seem to be in line with Schoenfeld’s however. In a group of 13 moderately resistance trained men, Ahtiainen et al found no significant differences in muscle mass or strength between one group training with a 2 minute rest interval and another group with a 5 minute rest period over a 6 month period (Ahtiainen, 2005). Additionally another study (Villaneuva, 2015) with a group of 22 older men, found that lean body mass increased with a shorter 1 minute rest rather than a longer 4 minute rest on after an 8 week resistance training period.
To demystify these conflicting results, I want to refer back to the original mechanisms I outlined above. First of all, rest times are not some relative value of short and long- a sufficient rest time should be some minimum bound of absolute time that it takes for a muscle takes to replenish its glycogen and creatine phosphate stores such that the trainee can complete another stimulating set of resistance exercise. The studies which found an increase with hypertrophy over the longer period (Schoenfeld and Buresh) were 2 minutes or more for the LONG period (3 minutes and 2.5 minutes respectively). The study which found no differences in hypertrophy (Ahtianen), but still an increase in hypertrophy in both the long and short period, had the shorter period set to 2 minutes. This suggests that the 2 minute rest period is enough time for ATP stores to recover sufficiently to maximize hypertrophic outcomes. It also suggests that having a longer rest period, in this case, didn’t affect hypertrophic outcomes negatively.
So why did the Villaneuva experiment show more lean muscle gained for the one minute group? The design of the Villanueva experiment probably actually highlights how more reps closer to failure, or more stimulating reps causes more hypertrophy. Because both volume and sets and reps were equated, this meant that those in the short rest (1 minute) group hit more stimulating reps. The longer rest group of four minutes were perhaps allowed too much time to recover so there weren’t as many stimulating reps with the same sets, reps and volume as the shorter rest group.
What would enhance the effect of hypertrophy for the group for the 4 minute interval would be to make sure they are training with the same number of stimulating reps. Both the Buresh and Schoenfeld ststudies designed the workouts so that the participants were instructed to finish a set when they did not think they could do another with good form. This is roughly similar to an rpe 9 or 9.5, to have one or possibly one more rep in the tank. This meant they could get closer to muscular failure with heavier loads, due to the longer rest time – which is the perfect recipe for hypertrophy.
For most exercises, a two minute rest period seems to be adequate for sufficient ATP recuperation to maximise hypertrophic outcomes. It is important to remember that the more muscle mass involved in a lift the more time you will need to recover. For example, performing a bicep curl is going to require a much shorter rest period than doing a deadlift. Additionally, the more effort a lift requires, the more ATP it will need to use because more high threshold motor units are being activated, so lifting heavier weights will also require more rest if the time under tension is the same.
Optimal rest time for hypertrophy
Hypertrophy will occur when enough effective reps have been completed. This is because effective reps stimulate high threshold motor units. You activate higher threshold motor units by using more effort, which means using heavier weights over several sets and reps.
If you are doing many sets, say 5, you might want to consider resting longer towards the end of the sets to maximise this effect. My suggestion is actually that rest times should get longer as you complete more sets because building fatigue will require more and more ATP replenishment to complete the next set with the same number of reps. This is not a suggestion you will come across in many standard programs, but you'll find that you naturally want to rest longer as the sets go on.
This rest time for most compound exercises for hypertrophy should be between 1 and 3 mins depending on how much muscle mass is involved and how many reps you are meant to be completing. If more muscle mass is involved, go more towards the 2-3 minute range (like deadlift, squat, push press etc) or less muscle mass go more into the 1 – 2 minute range.
Along the same line, there are some rest time schemes for certain exercises like rest pause sets and cluster sets which allow fatigue to remain high and rests short enough to maximise the number of effective reps. This is something I plan to write on in the future so keep on the look out.
Lastly, in this article, I have mainly looked at the optimal rest times for hypertrophy and not strength training. Whilst the two are similar, optimal strength outcomes will require longer rests so performance of the reps is optimal too. Strength training requires larger neurological adaptations as it’s all about specificity and perfecting motor patterns. Heavy sets of less than 5 reps also hit the central nervous system harder, contributing to overall central fatigue throughout the session (this is distinct from muscular fatigue). So when strength training make sure you are resting at least 3 mins and up to 10 mins between sets for heavy compound movements.
Chris Beardsley, https://medium.com/@SandCResearch/how-do-different-types-of-fatigue-affect-hypertrophy-and-recovery-4b5bd500bcb
Ahtiainen, JP, Pakarinen, A, Alen, M, Kraemer, WJ, and Hakkinen, K. Short vs. long rest period between the sets in hypertrophic resistance training: Influence on muscle strength, size, and hormonal adaptations in trained men. J Strength Cond Res 19: 572–582, 2005.
Buresh, R, Berg, K, and French, J. The effect of resistive exercise rest interval on hormonal response, strength, and hypertrophy with training. J Strength Cond Res 23: 62–71, 2009.
Schoenfeld BJ. Longer inteerset rest periods enhance muscle strength and hypertrophy in resistance trained men. J Strength Cond Res. Jul;30(7).1805-12. 2016.
Villanueva, MG, Lane, CJ, and Schroeder, ET. Short rest interval lengths between sets optimally enhance body composition and performance with 8 weeks of strength resistance training in older men. Eur J Appl Physiol 115: 295–308, 2015.