Strength training volume for hypertrophy always seems to be a contentious topic, whether people are arguing about one set versus multiple sets, or whether they are arguing about the exact number of sets at which the dose-response relationship plateaus (or even starts to have a negative impact).
Even so, in many of these arguments, there is rarely a clear understanding of exactly what volume is, how it is defined, and how the research has linked different amounts of volume to hypertrophy.
In addition, we need to understand which reps actually stimulate hypertrophy (not all reps do, otherwise endurance exercise would produce a lot of muscle growth, as it usually involves a huge number of reps). Only when a rep stimulates hypertrophy should it really be counted towards volume. We also need to appreciate the substantial difference between “workout volume” and “weekly training volume,” and how each of those things have been studied by researchers.
What is volume?
Volume is the way in which we measure the size of the dose of a strength training program. In this respect, it is quite similar to time under tension, which is another very commonly misunderstood concept. Greater volumes provide a larger dose of training, and produce a greater stimulating effect on the muscle fibers to increase in size.
Volume is measured in a number of ways by researchers, although none are really that satisfactory. The most common ways are (1) the total number of sets to failure, (2) the total number of reps (sets x reps), and (3) the volume load (sets x reps x weight).
Studies have only linked the number of sets to failure to a dose-response on muscle growth. Measured in this way, greater volumes (number of sets to failure) lead to more hypertrophy. Measured in either of the other two ways (total number of reps or volume load), there is no relationship whatsoever between volume and the amount of hypertrophy that occurs after training.
For example, when two groups of strength-trained individuals perform similar training programs using 3 sets of the same 7 exercises to muscular failure but with either light loads (25–35RM) or moderate loads (8–12RM), they achieve the same amount of hypertrophy, and yet training with lighter loads involves far more volume (sets x reps) and volume load (sets x reps x weight) than training with moderate loads.
Similarly, the relationship between training volume and hypertrophy also disappears when we do not take sets to failure. This can be seen in studies of German Volume training, where moderate and high volume training programs cause similar muscle growth. It can also be seen when comparing training programs of light or moderate loads with the same volume load, where failure is not reached, in which strength training with a moderate load leads to greater muscle growth.
Failing to realize that research has only really linked increases in this one, single measurement of training volume to hypertrophy can easily lead even quite experienced commentators astray.
Even so, there is a very good reason why this must be the case.
What is volume really?
Biologically, volume is really just the number of stimulating reps that are performed for a muscle group in each set.
A stimulating rep is one that involves (1) the recruitment of high-threshold motor units (and therefore the activation of their associated muscle fibers), and (2) a slow shortening velocity.
We need to recruit high-threshold motor units for two reasons. Firstly, despite being quite few in number, they actually control the large majority of muscle fibers. Secondly, the very slow twitch muscle fibers that are controlled by low-threshold motor units are not very responsive to the workout stimulus, and tend not to grow after training.
The recruitment of high-threshold motor units is determined largely by the effort involved in performing a movement. When we lift a heavy load, or move a light load explosively, or lift a light load to muscular failure, we are using a high level of effort, and motor unit recruitment is high.
However, simply recruiting the motor units (and activating the associated muscle fibers) is not enough. We also need to expose the activated muscle fibers to high levels of mechanical loading. This is accomplished through the force-velocity relationship. When the activated muscle fibers shorten slowly, they produce a lot of force, and therefore experience high levels of mechanical loading. When the activated muscle fibers shorten quickly (as in very fast movements), they produce little force, and experience little mechanical loading. This is why lifting light loads explosively involves maximal motor unit recruitment but does not stimulate muscle growth.
Importantly, the last five or so reps of a set taken to failure involve similarly high levels of motor unit recruitment and similarly slow bar speeds as a result of increased local muscular fatigue. Therefore, regardless of the weight on the bar, the muscle fibers controlled by high-threshold motor units experience approximately the same number of stimulating reps, when training to failure (training with a certain number of reps in reserve produces the same effect, albeit with 1 or 2 fewer stimulating reps, as appropriate).
This is why the number of sets to failure is often a good way of measuring training volume, regardless of the weight on the bar, because every set to failure usually contains the same number of stimulating reps. When we do not perform sets to failure, it is not clear how many stimulating reps are done in each set. In fact, the number of stimulating reps is often smaller when some so-called higher volume routines are performed!
When is the number of sets to failure a poor measurement of volume?
Although the number of sets to failure is often a good measure of the number of stimulating reps in a set, sometimes it is not. It fails to be a good measure if the number of stimulating reps is cut short for some reason. While this always happens when we stop a set short of muscular failure, it can also occur during a set despite reaching failure.
During a set of strength training exercise, our ability to exert force can be negatively affected by either (1) reducing the ability of the muscle itself to exert force (called “peripheral fatigue”), or (2) reducing the ability of the central nervous system to activate the muscle so that it produces force (called “central nervous system (CNS) fatigue”). The activation of the muscle by the CNS is accomplished by the recruitment of motor units, in size order, from low-threshold to high-threshold.
Consequently, both peripheral and CNS fatigue contribute to us reaching muscular failure. Importantly, CNS fatigue reduces the level of motor unit recruitment, while peripheral fatigue increases it. However, CNS fatigue takes precedence by virtue of being the ultimate controller of muscle function. So when there is more CNS fatigue (either because of a shorter recovery time, a greater aerobic demand, or increased lactate accumulation), we can reach failure in the exercise before actually achieving full motor unit recruitment.
For example, when short rest periods are used, muscular failure is reached on each set but the stimulating effect of the set is less than expected. This can be attributed to greater CNS fatigue with short rests (either due to the reduced recovery time for this type of fatigue, the greater aerobic demand, or the increased lactate accumulation). This same logic can be applied to increasing training volume, because CNS fatigue increases over the course of a workout. This means that later sets likely contain fewer stimulating reps than earlier sets. This can help explain the effects of exercise order and also suggests that adding extra sets to a workout will have progressively smaller and smaller incremental benefits.
In addition to affecting our ability to accomplish stimulating reps in workouts with short rest periods, CNS fatigue can reduce the number of stimulating reps achieved in a workout, if we have not yet recovered from a previous workout. If we train a muscle too soon after the last workout, there will be muscle damage present. When this muscle damage is present, it can lead to accompanying CNS fatigue. This is why training too frequently each week can fail to produce greater muscle growth, even when the volume is greater with the higher frequency.
Essentially, CNS fatigue experienced during workouts (either because it accumulates within the workout, or because it is still present from a previous workout) means that we cannot simply add up the number of sets to failure performed in all of our workouts in a week and expect to be able to calculate the total number of stimulating reps per week. That is not a valid calculation. Some of the later sets in a workout, and some of the later workouts in a week will not involve as many stimulating reps, because of CNS fatigue.
When does an exercise stimulate a muscle?
One last factor that must be taken into account when measuring training volume is the effect of an exercise on a muscle group.
Stimulating reps can only be counted in full when an exercise produces full motor unit recruitment for the worked muscle group, which usually means that the muscle group has to be the limiting factor for the exercise. Yet, many exercises involve multiple muscle groups, and not all of them are worked equally hard. For example, the squat probably stimulates the quadriceps maximally, as they are the limiting factor, but likely leaves some stimulating reps in the tank for the working hip extensors (adductor magnus and gluteus maximus), especially when a traditional high bar squat variation is used.
Moreover, the impact of an exercise may not be quite as obvious as we like to think. Although the squat is a good exercise for the quadriceps, it only really develops the single-joint quadriceps. The rectus femoris is actually very poorly trained by this exercise, which in reality makes sense, because it is a hip flexor and the production of a hip flexion turning force would oppose the important work being done by the hip extensors. (This causes problems both in practice and also in research. For example, when studies measure the rectus femoris after squat and leg press training, we are not really going to get a good insight into the effects of volume on that muscle group).
Conversely, while we may program bench presses for the pectoralis major and anterior deltoids, they are actually similarly effective for the triceps brachii. So if we are following a body part split routine, and include bench presses for the chest and shoulder muscles, and then carry out a separate day involving single-joint exercises for our arms, then we are actually training the triceps twice as often as the chest and shoulders.
Ultimately, whether an exercise stimulates a muscle must be determined by an understanding of the biomechanics of the exercise, and not by whether it has traditionally been included in workouts for that body part. Including multi-joint exercises will always lead to some confusion, but that is a problem we may need to work with, if we are to benefit from using them.
What does this mean in practice?
In practice, this means that we can produce the following working definitions of workout and weekly volume.
Workout volume — volume is the number of stimulating reps achieved for a muscle group in a workout. This is likely to be approximately 5 reps per set to failure with an exercise that involves that muscle as the limiting factor, although if short rest periods are used then this number will be smaller. Also, later sets in a workout may not contain quite as many stimulating reps as earlier sets, due to CNS fatigue.
Weekly volume — volume is the number of stimulating reps achieved in each workout. This is likely to be approximately 5 reps per set to failure with an exercise that involves that muscle as the limiting factor in each workout, although if workouts performed earlier in the week lead to substantial muscle damage, then CNS fatigue will cause a reduction in the number of stimulating reps achieved in each set to failure in later workouts.
Consequently, while many strength training experts measure weekly volume as the sum of the sets to failure done each week, this is *not* valid, because of CNS fatigue caused by muscle damage. Similarly, while many research groups try to identify the weekly volume that produces the most hypertrophy by altering the number of sets in a workout while maintaining the number of workouts per week the same, this is a not a valid calculation, because of the interaction between workout frequency and workout volume.
So how does workout volume affect muscle growth?
What is the dose-response effect of workout volume on hypertrophy?
Workout volume clearly produces a (non-linear) dose-response effect on post-workout increases in signaling in the mTOR pathway, in increases in muscle protein synthesis (MPS) rates, and in myogenic signaling responses. Studying each of these dose-responses carefully can help us understand better how workout volume affects muscle growth.
mTOR pathway — The dose-response of workout volume on signaling in the mTOR pathway seems to increase fairly linearly with increasing workout volume.
MPS rates — The dose-response of workout volume on post-workout increases in MPS rates is a gradual increase up to a plateau, after which there is no change in the response (either up or down). This makes sense, because MPS rates are determined by the rate of protein synthesis per ribosome (translational efficiency) and by the amount of translational machinery in a given volume of muscle (translational capacity). Temporary increases in MPS, such as after single workouts, are likely caused by an increase in translational efficiency, while long-term strength training is believed to increase translational capacity, which requires the production of new translational machinery through ribosome biogenesis. Since there is a limit to how much efficiency can be increased, this strongly indicates the presence of a plateau in the post-workout increases in MPS rates as a result of training volume.
(Note that it is not completely unusual for post-workout increases in muscle protein synthesis to differ from post-workout mTOR signaling, although the two responses normally move in tandem. Indeed, mTOR signaling has been observed to diverge markedly from MPS responses on occasion).
Myogenic signaling responses — The dose-response of workout volume on myogenic signaling responses seems to to be delayed and even slightly reduced at high volumes. The delayed response has been attributed to the negative effects of muscle damage, since greater workout volumes produce more muscle damage, and since the post-workout MPS response occurs both in order to repair muscle damage and also in order to produce muscle growth. Therefore, when a (high volume) workout causes a lot of muscle damage, this could lead to a need to devote more of the post-workout increase in MPS rates to muscle damage repair, and less to hypertrophy. This could then imply that the mTOR signaling response is also partly related to the need to repair muscle damage.
The muscle damage response to a workout is also important to consider.
When muscle damaging-exercise is extremely severe, it can lead to muscle fiber loss. Muscle damage can range from very small disruptions to the myofibrils and cytoskeleton of the muscle fiber, to tears in the cell membrane, to ruptures of the muscle fiber, and ultimately to its wholesale destruction. When a muscle fiber is slightly damaged, it can be repaired. In such cases, the damaged areas are cleared away and replaced with new structures. When a muscle fiber is very severely damaged, it becomes necrotic and is completely removed. Afterwards, a new muscle fiber is grown inside the cell membrane of the old one. This is called regeneration. It is possible that some muscle fibers are not regenerated after they have become necrotic. This is then observed as the loss of muscle fibers or muscle mass.
Overall, we can therefore describe the dose-response of workout volume as being formed of three elements: (1) a non-linear relationship between workout volume and MPS rates up to a plateau, and (2) an increasing (probably linear) relationship between training volume and the amount of MPS that must be directed to muscle damage repair instead of to hypertrophy, and (3) a (probably non-linear) increase in the number of fibers that become necrotic and yet are subsequently not regenerated. Together, these three relationships suggest that there might be a U-shaped curve of workout volume on hypertrophy, rather than simply an increase up to a plateau.
N.B. Effects of using different volume definitions
It is important to note that the above analysis refers to volume as defined by the number of stimulating reps. Clearly, CNS fatigue will also have an influence on the actual number of stimulating reps that are performed during a workout. CNS fatigue therefore also has an effect on the relationship between the more common measures of volume (such as the number of sets to muscular failure) and the resulting muscle growth after training.
What is the dose-response effect of weekly volume on hypertrophy?
To identify the real dose-response effect of weekly volume on hypertrophy requires us to find the right combination of workouts over the week that allows the maximum number of stimulating reps to be performed.
Importantly, this will depend on the optimal training frequency for (1) the individual, and (2) the muscle group. Optimal training frequency has been shown to vary between individuals (most likely because the susceptibility to muscle damage also varies between individuals), and muscles take different durations of time to repair the resulting muscle damage from the same volume of training, which implies that training them with the same frequency will lead to quite different results. Also, the level of psychological stress that an individual is currently experiencing affects the rate of workout recovery, most likely by altering the speed at which muscle damage is repaired.
In general, many strength training workouts allow full recovery of muscle damage (and associated CNS fatigue) within 24 hours, but high volume workouts typically require 48 hours (for the lower body) or 72 hours (for the upper body). Even so, there are some studies that have recorded a markedly suppressed ability to produce force (indicating muscle damage) for far longer, even up to 5 days post-workout. Therefore, the effect of recovery rate on the optimal weekly training volume is very important.
However, the majority of the literature exploring the effects of training volume has examined the effects of altering the number of sets to failure in single workouts over the course of a week (where some groups do 1 set per exercise in each workout, while other groups do 2 or more sets per exercise).
Such studies tell us about the maximum workout volume that can be used in the context of a given training frequency for the majority of the individuals in the cohort. For example, a recent study examined what happens if strength-trained individuals perform 1, 3, or 5 sets per exercise, within the context of a training program involving 3 workouts per week. These studies do not tell us the maximum weekly volume that could be used, if we also modified training frequency to best fit the stimulating reps into the week according to the optimal balance between workout volume and training frequency (and the answer is likely to be different for different individuals and muscle groups!)
Failing to spot this problem can easily lead even experienced commentators to give guidelines about maximum weekly volume based on a body of literature that doesn’t actually answer that question.
What is the takeaway?
There is probably a specific amount of volume that can be performed in a workout that maximizes the amount of hypertrophy that results for a given muscle, because there are good reasons why moderate volumes will produce superior growth than low volumes, and why excessively high volumes will produce inferior growth to more moderate volumes.
Similarly, there is probably a specific amount of volume that can be done in a week that maximizes the amount of hypertrophy that results for a given muscle. However, this may not involve performing individual workouts with the amount of volume that maximizes the amount of hypertrophy for a given muscle, because it is the optimal combination of frequency and volume that leads to the weekly volume that produces the most muscle growth.