Do short rest periods help or hinder muscle growth?

Bodybuilders have traditionally used short rest periods during strength training, often to help them achieve a greater muscle pump, or to increase the burning sensations that were once attributed to lactic acid.

Such sensations are commonly perceived to be indicative of an effective workout. But whether this is true or not, do short rest periods help or hinder muscle growth?

How does rest period duration affect muscle growth?

Although short (1–2-minute) rest periods are popular among bodybuilders, they do not enhance muscle growth, when the total volume (as defined by the number of sets to failure) done in each workout is the same. In fact, longer (3-minute) rest periods produce more hypertrophy than short (1-minute) rest periods, in strength-trained males.

How might this happen?

Can lower volumes explain why rest period duration affects muscle growth? (part I)

Some researchers have suggested that short rest periods might cause less hypertrophy, because they lead to lower training volumes. While this initially sounds like a plausible explanation, we can quickly see that it is highly unlikely, and also fails to understand exactly how training volume has been linked to muscle growth.

It is true that short rest periods reduce the number of reps that can be achieved on each set, and this reduces the total volume (sets x reps) and volume load (sets x reps x weight) that is accomplished over a long-term training program.

However, volume (number of sets to failure) is not the same thing as volume (sets x reps) or volume load (sets x reps x weight). And, importantly, volume (number of sets to failure) is the only measurement of volume that has been linked to muscle growth.

Indeed, the literature shows that greater volumes (number of sets to failure) produce more hypertrophy, and this result is dose-responsive up to quite high volumes. Even so, the result is not affected by the number of reps that are done in each set, when using moderate or moderately light loads. When the weight is between 5RM and 30RM, the amount of hypertrophy that is achieved is the same, although training with lighter loads involves several times more volume (sets x reps) and volume load (sets x reps x weight) than training with moderate loads, and greater increases in both volume (sets x reps) and volume load (sets x reps x weight) over a training program.

In other words, volume (sets x reps) and volume load (sets x reps x weight) are completely disassociated from the amount of hypertrophy, but volume (number of sets to failure) is strongly linked to the amount of hypertrophy that happens after a long-term strength training program.

(This is because the only reps that produce hypertrophy during conventional strength training are those that involve a high level of motor unit recruitment at the same time as a slow muscle fiber shortening velocity, and these are probably the final five reps of any set performed to failure, when lifting any moderate or light weight).

Suggesting that short rest periods might cause less hypertrophy because they involve a smaller volume (sets x reps) or volume load (sets x reps x weight) is not even remotely a valid explanation, because these measures of volume are clearly not related to muscle growth.

So why do short rest periods cause less muscle growth?

Can lower volumes explain why rest period duration affects muscle growth? (part II)

Some researchers have suggested that reduced volume (sets x reps) and volume load (sets x reps x weight) might not be the reason why short rest periods might lead to reduced hypertrophy.

In one important study, researchers tested a moderately high volume workout for the quadriceps comprising 4 sets of leg presses and 4 sets of knee extensions with 75% of 1RM, with each set performed to failure. One group used 1-minute rest periods, and the other group used 5-minute rest periods. The group who used 5-minute rest periods achieved 14–15% more volume (total reps) and 13–17% more volume load (sets x reps x weight).

However, the group who used 5-minute rest periods increased muscle protein synthesis rate (of the myofibrillar fraction) by 139%, while the group who used 1-minute rest periods increased muscle protein synthesis rate by only 68%, when measured over the 4 hours post-workout.

The researchers pointed out that it would be extremely unlikely for the small difference in volume (total reps) or volume load (sets x reps x weight) to be able to explain this very large difference in post-workout muscle protein synthesis response (unfortunately, they failed to observe that neither volume (total reps) nor volume load (sets x reps x weight) can explain the amount of hypertrophy that results from a workout anyway).

Since they felt that such small differences in volume were unlikely to be able to explain the very large differences in muscle protein synthesis rates, the researchers proposed an alternative hypothesis, which is that short rest periods increase central nervous system fatigue, as a result of the greater accumulation of lactate.

To appreciate how this hypothesis works, we need to understand the various aspects of fatigue during strength training.

How does rest period duration affect fatigue?

Fatigue is a temporary reduction in the ability to produce force.

Our ability to exert force can be reduced by either (1) reducing the ability of the muscle itself to exert force (which is called “peripheral fatigue” and which is measured as involuntary, electrically-stimulated force), or (2) reducing the ability of the central nervous system to activate the muscle so that it produces force (which is called “central nervous system fatigue” and which is measured as the difference between involuntary and voluntary force). The activation of the muscle by the central nervous system is accomplished by the recruitment of motor units, in size order, from low-threshold to high-threshold.

Importantly, when we experience central nervous system fatigue, this reduces the level of motor unit recruitment, regardless of the degree of peripheral fatigue, since it takes precedence by virtue of being the ultimate controller of muscle function.

Let’s take a look at central fatigue first.

#1. Central fatigue

Central fatigue can occur either because of a reduction in the size of the signal sent from the brain or the spinal cord, or because of an increase in afferent feedback that subsequently reduces motor neuron excitability.

When central nervous system fatigue is high, this reduces the number of motor units that are recruited. Since motor units are always recruited in size order, this mainly stops high-threshold motor units from being recruited, and these are the motor units that control the large numbers of highly responsive muscle fibers that grow after long-term strength training. By preventing these muscle fibers from being activated, central nervous system fatigue can inhibit the hypertrophic stimulus achieved by any given set.

There are at least three reasons why short rest periods might lead to greater central nervous system fatigue than longer rest periods.

Firstly, while most research has shown that central nervous system fatigue after strength training is fairly short-lived, it is certainly present in the 30 minutes after a workout. Moreover, it is also present within a workout, at least once a certain amount of volume has been accomplished, although it probably decays exponentially after each set. Therefore, using shorter rest periods makes it more likely that we will commence a subsequent set while central nervous system fatigue is still present. This will prevent us reaching full motor unit recruitment in that next set.

Secondly, exercise that involves a greater aerobic demand may lead to greater central nervous system fatigue, since aerobic exercise seems to produce central nervous system fatigue more readily than strength training. Short rest periods might therefore lead to greater central nervous system fatigue during a workout, because they increase this aerobic demand.

Thirdly, researchers have suggested that blood lactate accumulation might be implicated in the development of central nervous system fatigue, perhaps because metabolite accumulation often occurs in tandem with increasing discomforting sensations, which lead to greater afferent feedback. We already know that training to muscular failure with light loads probably does not lead to quite the same level of motor unit recruitment as training to failure with heavy loads, and this is most likely due to the greater afferent feedback associated with metabolite accumulation. In the case of light loads, this reduced level of motor unit recruitment seems too small to cause a smaller gain in muscle size after training (compared to heavy loads), but the effect might be large enough to produce a deficit in hypertrophy when using short rests (compared to longer rests).

Now, let’s look at peripheral fatigue.

#2. Peripheral fatigue

When the muscle experiences peripheral fatigue, this reduces the amount of force that each muscle fiber can produce, and so the central nervous system increases the level of motor unit recruitment to compensate. This increase in motor unit recruitment increases the number of active muscle fibers.

When peripheral fatigue is very high (as when strength training to failure with light loads), the central nervous system recruits all available motor units, which activates the majority of the muscle fibers inside the muscle. Therefore, by activating all of the muscle fibers (while they are shortening at slow speeds, due to fatigue), peripheral fatigue can contribute to the hypertrophic stimulus.

It is important to note that peripheral fatigue can lead to increased motor unit recruitment without the accumulation of lactate (and therefore without any metabolic stress). This occurs because the processes that lead to fatigue can vary, and probably do not involve the accumulation of metabolites anyway. Therefore, we cannot determine how much peripheral fatigue is present with different rest period durations based on blood lactate levels, although these are a good guide to the level of metabolite accumulation.

The amount of metabolic stress experienced during strength training is not always affected by the duration of the rest period. Some research shows that for rest periods between 30 seconds and 2 minutes in duration, the amount of blood lactate that accumulates is the same. Other research shows that when comparing rest periods of 1 minute and 5 minutes, the amount of blood lactate that accumulates is greater when using the shorter rest period duration. This suggests that there may be a key rest period duration between 2–3 minutes at which blood lactate accumulation begins to reduce, but levels are maintained similarly high with shorter rest periods.

In any event, metabolite accumulation was greater when using the shorter (1-minute) rest period than when using the longer rest (5-minute) rest period, even though myofibrillar protein synthesis rates were elevated more with the longer rest periods, which is yet another piece of evidence that casts doubt on the role of metabolites in hypertrophy.

What does this mean in practice?

Each set that is performed to failure involves a certain number of stimulating reps (probably five). These are those reps at the end of a set that involve high levels of motor unit recruitment at the same time as a slow muscle fiber shortening velocity.

When we perform strength training, this triggers a certain level of central nervous system fatigue. This central nervous system may be greater when using short rest periods, perhaps due to the exponential decay of the central nervous system fatigue after each set, or the aerobically-demanding nature of the exercise, or the greater afferent feedback related to an accumulation of metabolites.

Starting the next set of an exercise while we are still experiencing high levels of central nervous system fatigue will necessarily prevent us from reaching full motor unit recruitment, which will stop us from stimulating the large numbers of highly responsive muscle fibers that are governed by the high-threshold motor units. Essentially, the set will be terminated by the central nervous system before we accomplish all (five) of the stimulating reps. This will reduce the post-workout increase in muscle protein synthesis, and the long-term gains in muscle size.

Practically, we could compensate for the reduced number of stimulating reps that occurs when central nervous system fatigue is present by doing extra sets. Either way, the workout may well take approximately the same length of time.

What is the takeaway?

Short rest periods of 1–2 minutes between sets produce less muscle growth compared to longer rest periods of 3–5 minutes. This is probably caused by the higher levels of central nervous system fatigue that are present at the point of starting the next set, when using a shorter rest period duration, which reduces the level of motor unit recruitment we can achieve, and therefore decreases the number of stimulating reps in each set to failure. Practically, we can compensate for the reduced number of stimulating reps doing additional sets, although resting longer might be preferable for most people, given that the end result is the same.