Best Rep Range (Weight Load) for Muscle Growth

October 12th, 2007 by Paul Johnson

How many reps and weight to use for a set, is a controversial topic among bodybuilders. Many bodybuilders look for that “magic rep number” in their training routine that stimulates the muscle best for growth. Most bodybuilders believe that very heavy weight (below 5 reps to failure), mostly causes strength gains and very little muscle growth. On the flipside, high reps light weight, causes mostly muscular endurance and no strength or muscle gains. Part of the reason of these beliefs, is based on experience and on the science of muscle fiber types.

We know from basic physiology that there are two major muscle fiber types. They are anatomically, metabolically, and functionally different. There is no disagreement here as this is well documented in science. The type 2 muscle fibers, which are responsible for power and most muscle growth (hypertrophy), respond best to heavy weights. The fast twitch type 1 muscle fibers, do better with light weight done repeatedly multiple times.

So why the controversy? It seems like the perfect bodybuilder workout, would be to stimulate the type 2 muscle fibers the most by doing one rep max sets to failure. This is where it gets complicated. First you are neglecting your other muscle fiber types, which may make up the majority of your muscle. The type 1 fibers don’t grow as well, but they do have the ability to get bigger. They also can increase glycogen stores, which helps draw in water, making your muscles bigger. Another factor is called time under tension. Short sets with a few reps, may not be enough time to adequately cause a cascade of hormonal reactions needed to stimulate significant muscle growth.

Bodybuilders through experience have found that very low reps are good for strength, but not helpful for building up muscle size. Powerlifters traditionally do low reps, usually around rep 5. Very high reps(like 15+) also seem to not be helpful for muscle growth either. The key seems to be somewhere in the middle between these extremes, where you are stimulating both muscle fibers simultaneously to a significant degree.

Rep Range (weight) and hypertrophy studies:

I decided to see if I could find some research studies to help me zero in on the “magic rep range” between the 5 and 15 rep marks.

A study was published J Physiol. 2004 Aug 1;558(Pt 3):1005-12 done on 15 men for 3 months. They did excercises for 4-5 sets starting out at a lighter 12 RM. Every few weeks the weight increased to eventually 6 RM. Satellite cell activity was significant in both groups, especially at the one and 3 month mark. Muscle hypertrophy happened significantly in both groups too, 6.7% at 30 days and 30% at 90 days. Rep range didn’t seem to have much of a factor according to this study.

Another study was published in J Physiol 547.P, P16 measuring the quadriceps between 60% and 90% of the one rep max. They found that anything above 65% had no changes in protein synthesis. This concludes that using a heavier weight than 65% of your, probably won’t increase muscle growth. A study published in Eur J Appl Physiol. 2002 Nov;88(1-2):50-60. Epub 2002 Aug 15 done on 32 untrained men for 8 weeks. One group did 3-5 reps for 4 sets with 3 minutes rest. The second group did 9-11 reps to failure with 2 minute rest. The 3rd group did high reps of 20-28 with one minute rest between sets. Another group was the control. Before and after the 8 weeks they measured fiber-type composition, cross-sectional area, myosin heavy chain (MHC) content, and capillarization cardiovascularity, muscle endurance, and one rep max. The results showed all 3 fiber types hypertrophied with all the groups, the low and intermediate group had more muscle hypertrophy. The changes in muscle fibers were the same in all resistance groups. Researchers concluded in untrained men, low and medium reps (weight intensity) gave the same training adaptions.

These studies imply, that weights anywhere between 6-12 reps in a set to failure, are going to give all similiar muscle growth. One problem with these studies, is that they were done on untrained or inexperienced weightlifters. High reps wouldn’t do much for a experienced lifter.

They also don’t take into account, that a bodybuilder will have to occasionally add weight and decrease his reps in order to get his muscles to continually adapt. If you keep doing reps at 10-12 for the same weight every week, you will plateau very quickly. Periodization also teaches using different rep ranges(among other things) to keep the muscles guessing.

Another thing to consider is muscle fiber makeup. The amount of each type 2 fibers we have in our muscles, varies widely by a large percentage between each muscle and between people. Therefore it will take different weight loads and repetitions to stimulate the muscle optimally. You can read a previous article, where I discuss more about muscle fiber type and training Weight training based on muscle fiber type

In summary, there is no exact magic rep number/weight load because you will have to constantly change it, to keep your muscles adapting to new stimulus. Even if you could do the same workout everytime, everyone has such different muscle fiber makeup, which optimally stimulates muscles at different weights and repetitions, depending on percentage of type 2 fibers in a muscle.