The Science Behind Rep Ranges for Hypertrophy
We live in a world where rep ranges are deeply misunderstood. Fitness influencers will swear by some magical rep range their favorite bodybuilder taught them. Yeah, don’t listen to that guy.
Oh and anyone who talks about some outdated model of the strength/endurance continuum, yeah, don’t listen to that guy either. He probably doesn’t even lift.
But as somebody who simply wants answers on rep ranges, you’re left wondering, how many reps should I be doing?
Well, first let’s look at the strength endurance continuum to clear the air.
Many people still follow this outdated model. People assume that because lower reps are better for strength and higher reps are better for endurance, the middle rep range should be the hypertrophy rep range.
Indeed, strength is enhanced with lower reps and endurance is enhanced with higher reps because this is more specific towards those adaptations. When you do lower reps which is accompanied by heavier weights, you become better at lifting heavy things.
Accordingly, doing higher reps matches the demands of prolonged muscular endurance. Crossfit competitions are a good example of this.
However, hypertrophy is not specific towards a certain performance. Hypertrophy is specific towards the chemical response of bigger muscle tissue being constructed. This glorious process is called muscle protein synthesis and it gets triggered based on mechanical tension (1).
What is mechanical tension though? Let me explain as it will help you understand rep ranges better.
Resistance act as force against your muscles while the muscle contracts and stretches. As the muscle resists or controls this force, tons of individual fibers get recruited. Once recruited, the individual muscle fibers produce force to combat the resistance. This entire process is mechanical tension. More mechanical tension means more muscle protein synthesis.
So What Rep Range is Best?
“So If I do more reps, I’ll get more mechanical tension right?”
No, not quite. More reps or more time under tension doesn’t mean more mechanical tension. When you do more reps, you’re forced to use a lighter load which reduces the resistance of each individual rep, so even though you’re experiencing a longer duration of tension, the magnitude of the tension is smaller per rep.
Increasing the weight and hopping to lower rep ranges reverses these effects. By increasing the load, each rep is more difficult and stimulates a greater magnitude of tension. However, the total time accumulating that tension is reduced because heavier loads will reduce the amount of reps you can do.
So if you can put 2 and 2 together, it doesn’t really matter what rep range you use. The differences in the net amount of mechanical tension stimulated is the same.
Your 6-rep max will garner as much muscle growth as your 12-rep max (2). One requires less lifting of a heavier weight while the other requires more lifting of a lighter weight.
If you train both sets to the same proximity to failure, your biggest muscle fibers get recruited and stimulated to the same degree by the end of the set (3).
So while hypertrophy training isn’t as sensitive to rep ranges as other goals, there are 2 exceptions you should avoid.
- Very low rep (1-3) sets: These sets even though they’re extremely heavy, seem to be far too short to match the hypertrophy of other sets (4).
- Extremely high rep (40+) sets: These sets are too long where excess central nervous system fatigue reduces the fibers recruited, so when you reach failure, some fibers remain under stimulated or not stimulated at all (5).
To be clear, it’s not that these exceptions can’t produce hypertrophy. They can, but due to their drawbacks, they’re sub-optimal.
The Drawbacks of Different Rep Ranges
Despite some impractical situations, you can now see that the hypertrophy rep range is wider than most people think. Reps as low as 4-5 reps can still produce robust muscle gain along with reps as high as 30-35.
However, while nearly all rep ranges build similar amounts of muscle per set, how they fatigue you is quite different.
Higher rep sets produce more neuromuscular fatigue (6). This is the fatigue from the sum of your nervous system and your local muscle. Ever try to do one of those 20 rep max squat challenges? Yeah, you know what I mean. If you don’t, try it and you won’t be able to lift much after that. The neuromuscular fatigue is quite high.
Higher rep sets are reported to be more painful as well (7). That pump gets disgusting by the end of a 20-25 rep set compared to a 10-12 rep set.
However, lower reps have their own set of drawbacks. Lower rep sets apply more force per rep due to the heavy load. They consequently stress your joints and connective tissues more (8). Anyone who’s done extremely heavy sets often knows how crushing they can feel on your bones.
A true 5-rep max done often can be brutal if you have weak joints.
In addition, lower rep sets have a higher risk of injury because any deviation in form shifts more unintended stress on certain structures of your body. For example, your right knee and hip will be pretty achy if you accidentally deviate your body to the right during a 1-rep max attempt.
So What Should I do?
In general, I would suggest doing a variety of rep ranges. This way you balance out the drawbacks by not doing too much of one rep range. The middle rep ranges are also a good general rule of thumb to stick to.
I still like the variety recommendation better though as some preliminary research finds people who vary their rep ranges grew a bit more muscle (9).
For example, if you do 2 quad exercises, you can do a lower rep barbell back squat and a higher rep leg extension during the same workout or within the week.
Generally speaking, compound exercises work better with lower reps and isolation exercises work better with higher reps.
Ultimately, don’t overthink it. If you’re training hard and making performance increases, it doesn’t matter much which rep range you stay in.
- Wackerhage, Henning, et al. “Stimuli and Sensors That Initiate Skeletal Muscle Hypertrophy Following Resistance Exercise.” Journal of Applied Physiology, 9 Jan. 2019, journals.physiology.org/doi/full/10.1152/japplphysiol.00685.2018.
- Schoenfeld, Brad J, et al. “Muscular Adaptations in Low- versus High-Load Resistance Training: A Meta-Analysis.” European Journal of Sport Science, U.S. National Library of Medicine, 2016, www.ncbi.nlm.nih.gov/pubmed/25530577.
- Morton, Robert W., et al. “Muscle Fibre Activation Is Unaffected by Load and Repetition Duration When Resistance Exercise Is Performed to Task Failure.” The Physiological Society, John Wiley & Sons, Ltd, 27 July 2019, physoc.onlinelibrary.wiley.com/doi/abs/10.1113/JP278056.
- Schoenfeld, Brad J, et al. “Differential Effects of Heavy Versus Moderate Loads on Measures of Strength and Hypertrophy in Resistance-Trained Men.” Journal of Sports Science & Medicine, Uludag University, 1 Dec. 2016, www.ncbi.nlm.nih.gov/pubmed/27928218.
- Lasevicius, Thiago, et al. “Effects of Different Intensities of Resistance Training with Equated Volume Load on Muscle Strength and Hypertrophy.” European Journal of Sport Science, U.S. National Library of Medicine, July 2018, www.ncbi.nlm.nih.gov/pubmed/29564973.
- Haun, Cody T, et al. “Molecular, Neuromuscular, and Recovery Responses to Light versus Heavy Resistance Exercise in Young Men.” Physiological Reports, John Wiley and Sons Inc., Sept. 2017, www.ncbi.nlm.nih.gov/pmc/articles/PMC5617935/.
- Ribeiro AS;Dos Santos ED;Nunes JP;Schoenfeld BJ; “Acute Effects of Different Training Loads on Affective Responses in Resistance-Trained Men.” International Journal of Sports Medicine, U.S. National Library of Medicine, pubmed.ncbi.nlm.nih.gov/31499564-acute-effects-of-different-training-loads-on-affective-responses-in-resistance-trained-men/.
- Bohm, Sebastian, et al. “Human Tendon Adaptation in Response to Mechanical Loading: a Systematic Review and Meta-Analysis of Exercise Intervention Studies on Healthy Adults.” Sports Medicine – Open, Springer International Publishing, Dec. 2015, www.ncbi.nlm.nih.gov/pmc/articles/PMC4532714/.
- Schoenfeld BJ;Contreras B;Ogborn D;Galpin A;Krieger J;Sonmez GT; “Effects of Varied Versus Constant Loading Zones on Muscular Adaptations in Trained Men.” International Journal of Sports Medicine, U.S. National Library of Medicine, pubmed.ncbi.nlm.nih.gov/27042999-effects-of-varied-versus-constant-loading-zones-on-muscular-adaptations-in-trained-men/.