Currently, the maximum volume that strength-trained lifters should use for optimal gains is unclear. As a result, there is vigorous debate ongoing between proponents of very high volumes and those who advocate moderate volumes.
Nevertheless, we can make sense of the literature quite easily if we are prepared to dig into the details of each study, and look at what was done in each case. These details shed a lot of light on the volume debate, and reveal that it may not be that contentious after all.
What data are available? (Part one)
Few studies have assessed the effects of training volume in strength-trained subjects (when comparing the number of sets to failure), although there are lots in untrained people. A recent meta-analysis included only 3 studies in strength-trained lifters (the first two on the list below, in addition to another one that did not measure muscle-specific changes in size). Since then, another 4 studies have been published, which gives 6 in total.
Before we begin, a couple of key points.
Firstly, we need to compare groups that trained with the same proximity to failure (and not just the same number of reps with the same percentage of 1RM), because the proximity to failure is what determines the mechanical tension that is experienced by each working muscle fiber, due to the force-velocity relationship. This equalizes the dose of training for each set between groups, at least when ignoring central nervous system (CNS) fatigue. Thus, we need to exclude two studies that have assessed the effects of training volume without controlling for proximity to failure, even though they were carried out in strength-trained lifters.
Secondly, we need to be aware that the researchers who conducted these studies did not always accurately identify the exercises that stimulate muscle growth in a body part. Consequently, their counts for the number of sets to failure for each muscle may need to be modified to calculate the correct number. After all, no student of biomechanics believes that the overhead press (which involves shoulder abduction) is a particularly effective exercise for the pectoralis major (which is a shoulder adductor), and it is now clear that the squat is not an effective exercise for the rectus femoris. Sometimes, these adjustments make no difference to the overall results. Other times, they make a big difference (as we will see in the very first study on the list below).
What data are available? (Part two)
The following six studies have assessed the effects of training volume in strength-trained lifters. Here they are in order of appearance.
Ostrowski (1997) ? assessed the effects of body part split training on changes in rectus femoris and triceps brachii muscle size. The intent of the researchers was to use 1, 2, and 4 sets per exercise, and 3 exercises per muscle group, to compare the effects of 3, 6, and 12 sets per workout (and per week). Yet, the use of only one exercise for the rectus femoris in the leg workout (squats and leg presses probably don?t work this muscle) resulted in that comparison being between 1, 2, and 4 sets per workout (and per week). The use of 3 triceps exercises in the arm workout and 4 in the chest/shoulders workout resulted in that comparison being between 7, 14, and 28 sets per week, albeit across 2 workouts. While there were no significant effects of training volume, rectus femoris size seemed to increase more after 4 sets per week than after 1 or 2 sets per week. Triceps muscle size increases were similar after 14 and 28 sets per week, indicating that a plateau was reached somewhere between those volumes.
Radaelli (2015) ? assessed the effects of full body training, 3 times a week, on changes in biceps brachii and triceps brachii muscle size. Either 1, 3, or 5 sets were done for each exercise. Two exercises were done in each workout for the biceps, and three for the triceps. Thus, the number of sets per muscle group was 2, 6, or 10 per workout (6, 18, or 30 per week) for the biceps, and 3, 9, or 15 per workout (9, 27, or 45 per week) for the triceps. Increases in biceps and triceps muscle sizes were progressively greater with increasing volumes, with no obvious plateau.
Schoenfeld (2018) ? assessed the effects of full body training, 3 times a week, on changes in biceps brachii, triceps brachii and leg muscle size. Either 1, 3, or 5 sets were done for each exercise. Two exercises were done in each workout for each arm muscle, and 3 for the leg muscles. Thus, the number of sets per muscle group was 2, 6, or 10 per workout (6, 18, or 30 per week) for each arm muscle, and 3, 9, or 15 per workout (9, 27, or 45 per week) for the legs. Increases in biceps and leg muscle sizes were progressively greater with increasing volumes, with no obvious plateau. Although not significant, the increases in triceps size showed the same pattern.
Heaselgrave (2018) ? assessed the effects of training with either one or two workouts per week on increases in biceps brachii muscle size. One group did 1 workout per week with 3 sets of 3 exercises (9 sets per workout and per week). Another group did 2 of the same workout (9 sets per workout and 18 sets per week). A third group did 2 workouts with 4?5 sets per exercise, for a (13?14 sets per workout and 27 sets per week). While not taken to failure, each set was done with 2 repetitions in reserve (RIR). While not significant, the increases in biceps muscle size were greatest after 18 sets a week.
Barbalho (2019, March) ? assessed the effects of training using a body part split routine on changes in biceps brachii, triceps brachii, pectoralis major, quadriceps, and gluteus maximus muscle size. Each body part was trained once per week. Depending on their group, subjects did either 5, 10, 15, or 20 total sets per workout (either 2, 4, 5, or 7 sets each on the first 2 exercises, and either 1, 2, 5 or 6 sets for the third exercise in the workout). Exercise selection was such that some of the muscles were indeed trained with 5, 10, 15, or 20 sets per workout (and per week), but others were only trained with 4, 8, 10, or 14 sets per workout (and per week). Even so, there was a clear plateau in the gains achieved above 4?5 sets per workout (and per week), and in fact the gains in size were greater after training using 5 or 10 total sets per workout than after training with 15 or 20 total sets per workout.
Barbalho (2019, June) ? this study was the same in design as the one above, and there was again a clear plateau in the gains in muscle size that were achieved above 4?5 sets per workout (and per week). In this study, muscle size was measured at two time points. The two higher volume groups (15 and 20 sets per workout and per week) tended to display reductions in muscle size in the latter half of the study period. In contrast, the lower volume groups (5 and 10 sets per workout and per week) did not.
These results all seem very contradictory, so what does this all mean?
What does this mean in terms of stimulating reps?
These studies divide quite clearly into two categories (this observation was pointed out to me by my friend Boz, who also noted the implications of the differences in training variables between the categories).
One category suggests that increasing volume can cause increasing amounts of muscle growth even at very high numbers of stimulating reps, and the studies in this group do not identify any plateau in hypertrophy with increases in volume. The other category suggests that muscle growth is maximized with far more moderate numbers, and these studies identify obvious plateaus. Even so, looking at the characteristics of these studies makes it clearer what is going on beneath the surface.
The very high volume category ? Radaelli (2015) and Schoenfeld (2018) indicate that volume continues to increase muscle growth even up to 45 sets per week (225 stimulating reps). In both studies, subjects trained the tested muscles 3 times a week, while the rest periods between sets/exercises were fairly short, at just 90?120 seconds.
The moderate volume category ? Barbalho (2019, March) and Barbalho (2019, June) both found that there was a clear plateau above 5 sets per workout and per week (25 stimulating reps), and a trend towards reduced muscle growth when using 15 or more sets per workout and per week (75 stimulating reps). Rest periods ranged from 30 seconds to 4 minutes in both studies. Heaselgrave (2018) found muscle growth was maximum with 18 sets per week, albeit when using 2 reps in reserve on each set (54 stimulating reps), 3 minutes of rest, and training the tested muscle 2 times a week. Ostrowski (1997) also found a plateau above 14 sets per week (70 stimulating reps), with 3 minutes of rest, and training the tested muscle 2 times a week.
Why are the categories so different?
The main difference between the categories is the training frequency used. The very high volume category trained 3 times per week, while the moderate category trained each muscle just once or twice a week. Also, the two studies at the higher end of the moderate category both involved training the muscle twice a week, while the two studies at the lower end involved training the muscle once a week. This suggests that a slightly higher frequency may allow lifters to benefit from a higher weekly volume.
Critics of this observation will note that (1)training frequency has routinely been found to have little effect on muscle growth when volumes are matched, and (2) training frequency seems to be quite individual, with some people benefitting from a higher training frequency and others benefitting from a lower frequency. Yet, the observation made in this analysis is different. Here, we find that greater effective weekly volumes can be attained by using higher frequencies, most likely due to the reduced damage that is experienced in each workout when workout volumes are lower.
To be clear, in this analysis, we are not assessing the effect of frequency in isolation from weekly volume. Rather, we are assessing how frequency affects the maximum effective volume that can be achieved in a week. The results indicate that we can attain higher effective volumes when we use a higher frequency. As an aside, this means that the idea of a maximum weekly volume is not a valid construct, since it will vary substantially depending on what training frequency we choose.
What is the maximum number of stimulating reps per workout and per week?
Those studies that have assessed the effects of training muscle groups once a week have identified that 4?5 sets to failure (20?25 stimulating reps) maximizes the growth of a muscle group after a single workout each week (Barbalho [2019, March], Barbalho [2019, June], and Ostrowski ). Studies that have assessed the effects of training a muscle twice a week have found that doing the equivalent of 11?14 sets to failure (55?70 stimulating reps) per week is sufficient to maximize muscle growth (Heaselgrave  and Ostrowski ).
There seems to be a fairly obvious relationship there!
In both groups of studies, the maximum effective volume per workout is between 5?7 sets to failure (25?35 stimulating reps), although the likelihood is that the number is closer to the lower end of that range (5 sets to failure or 25 stimulating reps). Yet, doing two workouts per week allows twice the maximum effective volume per week than only one workout per week.
Assuming that we can recover from similar volumes when training 3 times a week, the maximum effective volume per week would be approximately 15 sets to failure (75 stimulating reps) per muscle group.
Why is the very high volume category so different?
The very high volume category of studies indicates that volume continues to increase muscle growth even up to 45 sets per week (225 stimulating reps). Even so, these studies both used quite short rest periods (90?120 seconds).
Short rests lead to less muscle growth when training with the same number of sets to failure, caused by smaller increases in post-workout muscle protein synthesis rates, likely due to central nervous system (CNS) fatigue. Moreover, CNS fatigue builds up over a workout, making each incremental set produce a smaller and smaller effect.
It seems likely that the large number of sets that was necessary to maximize hypertrophy in the very high volume category of studies was due to the prevalence of CNS fatigue (both from short rest periods and from the large volumes), which necessitated additional sets to achieve the required stimulus. It is probable that the same effects would be achieved using fewer sets to failure with lower volumes and longer rest periods.
Even so, there was still a dose-response effect across the volumes used in these studies, which suggests that CNS recovery was occurring from one workout to the next. Therefore, we can reasonably assume that training three times a week with relatively high volumes is feasible.
What does this mean in practice?
When training a muscle 3 times a week, the maximum effective volume for the average strength-trained lifter is most likely to be approximately 5 sets per workout and 15 sets per week (25 stimulating reps per workout and therefore 75 stimulating reps per week). When training a muscle 2 times a week, the maximum volume is likely to be 10 sets per week (50 stimulating reps). When training a muscle once a week, the maximum volume is likely to be 5 sets per week (or 25 stimulating reps). In all cases, this assumes that rest periods are sufficiently long between sets.
Ultimately, the maximum limit for volume each week is probably partly determined by the stimulus achieved in a single workout, and partly by the number of workouts done in a week. This is because the maximum limit for volume in a workout seems to be only 5 sets, but it is possible to recover from such a workout with a few days, thereby allowing multiple such workouts to be done over the course of a week.
What else is important?
One key point that may not be immediately obvious is that in the three studies that identified some evidence for a plateau above 5 sets in a single workout (Barbalho [2019, March], Barbalho [2019, June], and Ostrowski ), doing additional volume in the same workout for the tested muscle group did not have any negative effects until 10 sets per workout were exceeded (Barbalho [2019, March] and Barbalho [2019, June]).
This is important, because it means that we can program exercises that work different torso muscles in the same workout. For example, in a workout comprising 5 sets each of the bench press and overhead press, the triceps receives 10 sets worth of stimulus, while the pectoralis major receives only 5 sets. The amount of stimulus experienced by the deltoids is arguable, depending on how you divide up its regions, but is probably 10 sets for the anterior deltoids and 5 sets for the middle deltoids. Even so, the effective stimulus from that workout is similar for the triceps, pectoralis major, anterior deltoids, and middle deltoids, since the plateau occurs above 5 sets, but none of the muscles experiences more than 10 sets.
What is the takeaway?
The workout volume that will cause the most hypertrophy seems to be approximately 5 sets per muscle group per workout, so long as rest periods are sufficiently long between sets. Doing up to 10 sets for a muscle group per workout seems to have no further beneficial effects under such conditions, but equally has no negative effects. Doing more than 10 sets for a single muscle group in one workout may have negative effects, likely due to the excessive amount of muscle damage that is caused.
It seems to be possible to do multiple such workouts in a week (at least three). Therefore, the weekly volume that will cause the most hypertrophy is likely to be approximately 15 sets per muscle group per week, but only if that volume is allocated evenly over 3 similar workouts. Indeed, there is no such thing as a maximum effective weekly volume when stated independently of the number of workouts being done per week, since the maximum effective weekly volume will vary substantially according to training frequency.