The optimal rep range for muscle growth isn’t what you think

Chapters:

00:00 Intro

00:06 Study 1: High vs Low reps

01:08 Study 2: Type I vs Type II muscle fibers

01:45 MH Physique App

02:10 Endurance vs Explosiveness

02:54 The size principle

03:58 True momentary muscle failure

05:00 Time under tension

05:52 The data

06:56 Fatigue management and Progressive overload

08:07 Conclusion

Transcript:

Two new studies suggest that high reps might just be criminally underrated for muscle growth.

In the first study, group of participants trained one side of their body with sets of 8 to 12. The classic hypertrophy range. The other side of their body, specifically the biceps and the quads, was trained with sets of 20-25 reps. Both sides were trained to volitional failure. After 10 weeks there were no significant differences between the sides in measures of muscle growth, as measured by actual muscle size via ultrasound, or muscle protein synthesis as measured by myofibrillar muscle protein synthesis. These data provide strong evidence across multiple muscle groups in the same individual, using multiple measures of muscle growth, that resistance training load does not determine how much muscle you will build.

Most research at this point is quite clear that across a range of approximately 4-30 reps, and probably actually much higher reps than that, you will build equivalent muscle growth as long as you go close to failure on a set per set basis. So a set of four reps, a set of eight reps, a set of 20 reps, even a set of 30 or maybe more reps will build the same amount of muscle on average.

However, the second study finds that this growth may be concentrated in different muscle fibers. This was a meta-analysis of eight studies, still in preprint, which concluded: “Training with low loads (20 to 30% of 1RM)…”, so that’s very low loads, “…favored type I fiber growth. In contrast, increasing load was associated with relatively greater hypertrophy in type II muscle fibers.”

So high reps might be better for our slow twitch muscle fibers, which are more endurance oriented, and lower reps, higher intensities might be better for type II fibers, which are the more fast twitch muscle fibers geared for explosive force output, but they don’t have quite the same endurance as the type I fibers.

These findings sound very intuitive. If you train more endurance style, you will train more endurance style muscle fibers. If you train with high force outputs and heavy weights, you will train the muscle fibers that generate the most explosive force. However, it’s actually not that simple, and previous research has been very conflicted on whether this is true. A previous meta-analysis that compared lower versus higher reps in fact, found no significant difference in which muscle fibers were trained. And while some studies on blood flow restriction training and a bunch of Russian studies that used constant tension training did find more type I muscle fiber hypertrophy with very high reps, many other studies have failed to find significant differences between different muscle fibers in their growth rates with different repetition ranges.

Mechanistically, you would also not necessarily expect heavier weights to be better for type II muscle fibers due to the size principle. The size principle of motor unit recruitment states that the bigger motor units with the more type II muscle fibers kick in only later as needed to help the smaller motor units with more type I muscle fibers as the sets progresses and higher force outputs are needed due to fatigue.

So if you are lifting a weight, you’re going closer to failure, fatigue accumulates, at a certain point the type I fibers and the smaller motor units are no longer sufficient to be able to lift this amount of weight. So this amount of force generation is no longer possible due to fatigue that causes the force outputs that you can produce to decline. At this point, the bigger guys have to come in to help, which is mostly the bigger motor units with more type II muscle fibers.

So if you go close to failure in the end you will still recruit all of the type II muscle fibers even with a very low load. So a set of 30 repetitions at the end of the set, as you go closer to failure, you will still recruit all of the type II muscle fibers, just as if you had done the set with, say, your 8 rep max. One caveat here is that most people have a very hard time going to true momentary muscle failure with very low loads. If you tried to do something like squats to your 30 rep max, at some point it basically turns into cardio and almost nobody is actually able mentally to push themselves to true momentary muscle failure.

Even with an exercise that is a lot more forgiving, like biceps curls, it is very hard to find your true 30 rep max. You basically think that you’re close to failure by rep number 20 or so, and then you have like 10 more grinding repetitions in you. This is much more effortful than a set of 8, where you usually feel at after rep 6 or so that it gets very hard. And then even if you give up earlier, it’s 1 or 2 repetitions maybe in difference, whereas with the high rep set it might be 5 to 10 repetitions where you really feel every repetition: “I think this is the last one.” So most people, and this is established in research, have a very hard time accurately estimating how close to failure they are with very high rep sets.

If you’re not going close to failure it’s very possible that you do not get sufficient time under tension of the highest threshold motor units with high rep training. For the type I fibers, the slow twitch muscle fibers, it makes more mechanistic sense that high rep training would be beneficial for them, because they are exposed to more time under tension. See, that first part of the set is basically just accumulating fatigue for the type II muscle fibers and the higher threshold multi units to be precise, and they only kick in later, so the last 8 reps of the 30 rep max are basically when you’re actually targeting the type II fibers really well as opposed to a set of 8 with your 8 rep max where you target them from the beginning already.

But for the type one muscle fibers, you have the extra 20 repetitions or so of time under tension, whereas with a set of 8 rep max, yes, the type I muscle fibers are again active from the start, but they only get 8 reps of time under tension. With the sets of 30 you get basically those 20 reps of time under tension for the type I fibers, and then you get the type II fibers that kick in later. So you also still get their muscle growth.

The data, however, did not clearly support this idea either. In fact, the data in general were statistically quite weak. And that’s in part due to the inherent problem of grouping a lot of studies together in a meta analysis like this. But it’s also in part because the data here essentially show, based on the main models, very limited effect on the type I fibers, but more of an effect similarly on the type II fibers. I would also note that while the general trend was that higher reps were better for type I fibers, and lower reps were better for type II fibers, based on these models, if you take an at face value, they would be logically contradictory because type I and type II fibers should sum to the total.

Now there was no effect on total growth, but if you do find an effect on type II growth, then logically type I growth has to go down. So these graphs should be opposite to each other. And they weren’t. So overall the data were not statistically very strong, and the contrasts were most meaningful at very high repetitions, so like 20 to 30% of 1RM, which means that in practice these differences might just be marginal for most individuals, which would also explain why most studies have not been able to find a difference. Regardless of the limitations in the data there are significant potential benefits for fatigue management and muscle growth of incorporating a much wider variety of rep ranges than the traditional 6 to 12 reps, and there is really nothing to lose other than your sanity, ff course, when you try sets of 20 plus reps on many compound exercises. But regardless, your joints may thank you, and it might help you mitigate fatigue by emphasizing slightly different muscle fibers with the high and the low rep training.

Therefore, I teach most of my students in my online course, and the method that I also use in my app is to use a variety of rep ranges. And one method that works particularly well to achieve this is daily undulating periodization. I also generally like, for practical reasons and these types of research findings, to use much higher repetition ranges for isolation work and lower rep ranges for compound work. So if for chest, for example, you have flies and bench press, I recommend that you do your bench presses with a higher intensity and your flies with a lower intensity.

This generally also works well for practical reasons, because compound exercises turn into cardio when you try to do them for very high reps, and isolation exercises typically have an easier time progressing in reps, so progressive overload is easier if you try to add, say, 5 or 10 pounds or kilos to a cable stack for lateral raises or flies. It’s very problematic for many individuals, especially women, but progressing in reps is generally easier.

So the take home message for both theoretical and practical reasons is to use a wide variety of rep ranges to stimulate muscle growth and manage fatigue. I hope this research update helps you with your training program design. If you want to learn absolutely everything you need to take your physique to your maximum muscular potential as a serious lifter, then check out my online course. And if you want to put it into practice, check out my app. Links are in the description and the app now has a two week free trial which includes a free program. So check it out. See you next time.