What is the minimum number of stimulating reps in a workout that will cause hypertrophy?

If you enjoy this article, you will like my second book (see on Amazon).

In a previous article, I addressed the question of how many stimulating reps in a workout would maximize the dosage of mechanical tension leading to muscle growth. Even so, researchers have identified that strength-trained males can achieve gains in maximum strength when training with only a single set to failure in each workout.

While gains in maximum strength are produced by a number of mechanisms (and muscle growth is only one of them), this article addresses the question of how many stimulating reps in a workout are necessary for measurable amounts of hypertrophy to occur after long-term strength training.

What data are available? (part one)

From a practical perspective, the easiest way to identify the minimum number of stimulating reps in a workout is to look at early studies that compared different training volumes. Many of these early studies used very low workout volumes in the low volume group, and also used sensible training frequencies (recent studies have assessed much higher volumes).

Although there are many studies that have compared the effects of low and moderate training volumes, not all of these studies are strictly relevant for our understanding of the dose-response of muscle growth. Some studies did not require subjects to train to failure (or to a set proximity from failure) on each set, which makes it impossible to identify the number of stimulating reps that were done. Other studies were done in elderly people or in medical contexts. And some studies used less-than-ideal measurements for muscle growth.

Consequently, the studies listed below all involved training to failure, were done in untrained, young males and/or females, and used either ultrasound or MRI measurements of muscle thickness or cross-sectional area. This gives us confidence that we are using data that are largely relevant to the question of identifying the minimum number of stimulating reps in a workout. The only limitation is that they were all performed in untrained subjects. As we will see, this is an important consideration.

• Starkey (1996) — assessed the effects of 3 workouts per week for 14 weeks in untrained young males and females. Each workout comprised the knee extension and leg curl for 8–12RM to failure, with 1–3 minutes of rest between sets as required. One group did 3 sets of each leg exercise in each workout, while the other group did 1 set of each leg exercise. Vastus medialis muscle thickness increased significantly only in the high volume group, but hamstrings muscle thickness increased significantly and similarly in both groups.
• Rønnestad (2007) — assessed the effects of 3 workouts per week for 11 weeks in untrained young males. Each workout comprised the leg press, knee extension, leg curl, chest press, seated row, lat pull-down, biceps curl, and shoulder press exercises. One group did 3 sets of each leg exercise and 1 set of each upper body exercise in each workout, while the other group did 1 set of each leg exercise and 3 sets of each upper body exercise. Each set was done with an 7–10RM load to failure. Both groups achieved significant increases in quadriceps and hamstrings/adductors cross-sectional area, as measured by MRI scans, although the increases were significantly greater in the higher volume group.
• Bottaro (2010) — assessed the effects of 2 workouts per week for 12 weeks in untrained young males. One group did 3 sets of knee extensions and 1 set of elbow flexions in each workout, while the other group did 1 set of knee extensions and 3 sets of elbow flexions. Each set was done with an 8–12RM load to failure, with 3 minutes of rest between sets. Both groups achieved significant (and similar) increases in biceps muscle thickness. However, neither group significantly increased rectus femoris muscle thickness.
• Sooneste (2013) — assessed the effects of 2 workouts per week for 12 weeks in untrained young males. One group did 3 sets of seated dumbbell preacher curls in each workout, while the other group did 1 set. Each set was done with an 80% of 1RM load to failure, with 3 minutes of rest between sets. Both groups achieved significant increases in combined biceps brachii and brachialis cross-sectional area, as measured by MRI scans. The increases were significantly greater in the high volume group.

These studies show us that even 1 set of 1 exercise per workout (when the workouts are done 2 or 3 times per week) can cause measurable growth of the trained muscle, at least in untrained lifters. At face value, this suggests that we may only need 5 stimulating reps in each workout to cause hypertrophy (although doing more sets will likely increase the rate of muscle growth). However, given that untrained lifters can likely achieve more stimulating reps per set than trained lifters (which is what causes novice lifter gains), this finding may not be transferable. Nevertheless, since the untrained subjects did approximately 10 total reps per set in these studies, it seems that trained lifters would only need to perform two sets to failure (10 stimulating reps) at the very most to match the maximum possible number of stimulating reps done in these programs.

What data are available? (part two)

Although most studies in trained lifters have assessed the effects of much higher workout volumes, there is one study that may well have assessed the effects of a single set performed to failure in this population.

• 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 leg workout involved knee extensions, squats, and leg presses. Unlike the other quadriceps muscles, the rectus femoris is a two-joint muscle (it functions as a hip flexor as well as a knee extensor) and therefore it is logical that it would contribute minimally during combined hip extension and knee extension exercises. Indeed, research has identified that the rectus femoris has virtually no leverage (internal moment arm) during the squat exercise, and its activation is substantially reduced when performing hip extension in combination with knee extension during dynamometry exercise with the hip and knee each at 90 degrees of flexion. Consequently, it is unsurprising that while the knee extension exercise trains the rectus femoris very effectively, the squat does not train this muscle. Whether the leg press can train the rectus femoris is unclear, since it involves hip extension, but the amount of hip extension is less than during the squat exercise. If neither the squat or leg press can train the rectus femoris, then this study essentially assessed the effects of 1, 2, and 4 sets of knee extensions per workout (and per week) on rectus femoris development (and all training groups increased rectus femoris cross-sectional area, with no significant differences between groups). If the leg press can train the rectus femoris, then the study compared the effects of 2, 4, and 8 sets of exercise per workout (and per week).

This study suggests that trained lifters might be able to achieve a meaningful and measurable amount of muscle growth after strength training with either one or two sets to failure in each workout (when doing this workout just once a week). This would indicate that a minimum of either 5 or 10 stimulating reps are necessary in a workout to bring about hypertrophy.

What data are available? (part three)

Given that the literature indicate that untrained individuals can attain meaningful muscle growth with a single set performed to failure, it would be valuable to look at the muscle growth (or lack thereof) caused by strength training with a smaller hypertrophic stimulus. This could be tested either by performing a single set with a moderate load with a certain number of reps in reserve (e.g. 8 reps with a 10RM, which might be expected to provide 3 stimulating reps) or by performing a single set to failure with a heavy load (e.g. a 3RM, which should also provide 3 stimulating reps).

At the time of writing this article, I am not aware that any such studies exist. However, in one study, a group of strength-trained subjects lifted daily for 21 days, with different training programs of elbow flexion exercises for each arm. One arm was allocated to do a 1RM test (3–5 attempts per session) in addition to a maximal voluntary isometric contraction test (2 attempts of 3 seconds per session). The other arm was allocated to do the same tests in addition to a workout of 3 sets of 10 reps of single-arm dumbbell curls with 70% of 1RM (probably equal to 12RM, which equals 2 reps in reserve assuming no fatigue from one set to the next).

The short duration and the daily training frequency of this study make it difficult to interpret. If the study had been carried out over a longer period, with workouts done 3 times a week, it would have been extremely valuable. As it was performed, it is likely that inflammatory signaling arising from muscle damage caused by the earlier workouts triggered central nervous system (CNS) fatigue. CNS fatigue would be expected to make subsequent daily workouts less effective at stimulating hypertrophy. Indeed, similar short durations of muscle-damaging exercise have been shown to reach a plateau in muscle size soon after starting the program.

Even so, the researchers found that after 21 workouts, the arm that was trained with the tests (5–7 stimulating reps per day) achieved no muscle growth, while the arm that was trained with both the tests and the workout (18 stimulating reps per day) did. Taken at face value, this suggests that we probably need more than 5 stimulating reps per workout to trigger muscle growth (but it does not preclude the possibility that only 10 stimulating reps are necessary). Nevertheless, as noted above, the study duration was short, thereby allowing little time for muscle growth, and there was a likelihood that muscle damage caused CNS fatigue in this study, leading to a reduced hypertrophic stimulus of each workout.

What is the takeaway?

Untrained lifters doing a single set to failure in each workout will experience meaningful muscle growth (whether this occurs due to only 5 stimulating reps per workout is doubtful). Trained lifters can almost certainly achieve measurable muscle growth by using two sets to failure in each workout (which is equal to 10 stimulating reps).

If you enjoyed this article, you will like my second book (see on Amazon).