Reps at 80% of 1RM Might Predict Your Muscle Fiber Type

Key points:

  • A new study tested if the number of reps participants could complete at 80% of 1RM in the squat correlated with their muscle fiber type ratio.
  • There was a small, inverse correlation (r=-0.38) between the number of reps participants could do and their percentage of fast-twitch fibers.
  • The mean percentage of fast-twitch fibers was 58% in participants completing 5–8 reps at 80% of 1RM and 44% in participants completing 11–15 reps. In participants completing 9–10 reps, the mean muscle fiber type ratio was 50/50.

Are you “rep strong” or “max strong”? Whether you’re leaning towards one or the other, part of the explanation might lie in your muscle fiber type ratio.

Fast- and Slow-Twitch Muscle Fiber Types

You have two major muscle fiber types:

  • Slow-twitch, or type I, are suited to aerobic activities or high numbers of submaximal contractions. They tire slowly but at the cost of speed: they contract slower and are not as explosive as their counterpart.
  • Fast-twitch, or type II, are your fastest muscle fibers. The price of this speed is stamina, and they tire faster than the slow-twitch fibers.

In the population as a whole, the mean muscle fiber type ratio is about 50/50 for most muscles in our bodies. In an individual, however, they can differ vastly.

Typically, people with a high percentage of slow-twitch muscle fibers do well in endurance sports like running and cycling. People with a high ratio of fast-twitch muscle fibers, on the other hand, do well in power sports, such as sprinting, throwing, and jumping.

The good news is: for pure strength, your muscle fiber type likely doesn’t matter all too much.

That is because both slow-twitch and fast-twitch muscle fiber types create almost the same amount of force relative to their size.1 And, in strength sports like powerlifting and most of the popular lifts we do in the gym, the contractions happen slow enough that it doesn’t really matter if your muscle fibers are mostly slow-twitch or fast-twitch. What matters most is that they are big enough to create a lot of force.

Training Based On Muscle Fiber Types

One idea that has been around for ages is that you might get better training results (i.e., gain muscle and strength faster) if you train according to your muscle fiber type distribution:

  • Predominantly slow-twitch? Do more high-rep training.
  • Predominantly fast-twitch? Do more low-rep training with fast contractions.

At least so the saying goes.

I won’t dive into the sparse research available on this topic; Greg already did a great job on that. Suffice to say that the picture is murky at best. Even if you did know your fiber type distribution in your major muscles, we still don’t know exactly how you should train to benefit the most, even if the recommendations in the bullet list above might be a reasonable starting point.

Even if we don’t fully know what to do with the information if we had it (if your goal is strength and size), wouldn’t it be interesting to know your muscle fiber type distribution just out of curiosity?

The gold standard to determine muscle fiber type distribution is to do a muscle biopsy, which is invasive and not available to the public. Luckily, a study testing a far more practical method to determine your muscle fiber type was just published.

Can Squat Reps at 80% of 1RM Predict Your Muscle Fiber Type?

In a new study,2 a group of researchers wanted to try a 20 year old theory: that the number of reps you can do in the squat at 80% of your 1RM can predict your quadriceps muscle fiber type ratio.3

To test this, they recruited 30 participants: 10 female and 20 male. All participants were between the age of 18 and 40, were regularly performing resistance training, and had at least two years of training experience in the squat.

The researchers had the participants warm-up, and then gradually work up to a 1RM squat. That is, the heaviest weight they could successfully lift one time.

Then, the participants rested for 15 minutes before doing as many reps as possible in a single set at 80% of that 1RM.

At another visit to the lab, the participants’ vastus lateralis (the largest quadriceps muscle) was biopsied, and its muscle fibers were analyzed.

Correlation Between Muscle Fibers and The Number of Reps

After analyzing the results, the researchers found a small but significant inverse correlation (r=-0.38) between the number of reps performed and the percentage of fast-twitch muscle fibers.

All participants completed between 5–15 reps at 80% of their freshly tested 1RM. Here is the mean percentage of fast-twitch muscle fibers, grouped by the number of reps they could do:

  • 5–8 reps: 58% fast-twitch
  • 9–10 reps: 51% fast-twitch
  • 11–15 reps: 44% fast-twitch
Repetitions at 80% of 1RM and muscle fiber type

A correlation of r=0.38 can be interpreted that 14% (0.38^2) of the variation in repetition performance is explained by variations in fast-twitch muscle fiber percentage, which is frankly not a lot.

Another similar study from 2008, albeit with far fewer participants, didn’t see a correlation between reps in the leg press at 70 or 85% of 1RM and muscle fiber type.4 They did, however, see a correlation between reps at 70% of 1RM and capillarization. Capillarization is the number of capillaries in your muscles, supplying blood flow to your muscle fibers. Generally, endurance-style training increases capillarization more than resistance training.

Interpretation and Take-Away

So, can you use a rep test at 80% of a completely fresh 1RM in the squat to predict your muscle fiber type distribution in your quads? Well, it might give you an inkling, but take another look at the scatter plot above, and you’ll see that the variation is big. Your fiber type distribution isn’t the only factor determining how rep strong you are; factors like capillarization and what you are accustomed to also play in:

  • If you train primarily low reps, your 1RM will pull away relative to your rep strength.
  • If you train primarily high reps, you will get better at repping at a high percentage of your 1RM.

Even if you don’t want to do a 1RM test followed by a max rep test, you might still have an idea of whether or not some of your muscles lean more towards fast- or slow-twitch. The problem is that we still don’t really know how you should change up your training even if you knew your muscle fiber type distribution. Low or high reps? Fast or slow? We don’t know.

What I’m taking away from this study, apart from that I found it really interesting, is a reminder that our bodies are different in many small ways. Therefore, I find it plausible that we respond best to different types of training, even if the difference is only that we enjoy one way of training more than the other.

  • Do you prefer to build your strength and muscle using mostly sets of 8–12 reps in the squat? Then do so in much of your training, and practice a little at lifting heavier when you are planning to max out.
  • Do you prefer doing lots and lots of singles, doubles, and triples? Do it, but make sure to get your hypertrophy training in somehow. Perhaps by finishing with one or two higher rep sets or by complementing with other exercises.

I hope we get to return to this topic in the future. In the meantime, if you are looking for training inspiration, we have a ton of proven training programs right here: Training Programs.

Thank you for reading, and good luck with your training.

More reading:

References

  1. Exp Physiol. 2011 May;96(5):539-47. Relationship between force and size in human single muscle fibres.
  2. Biol Sport. 2021 Jun; 38(2): 277–283. Prediction of muscle fiber composition using multiple repetition testing.
  3. Karp JR. Muscle fiber types and training. Strength Cond J. 2001;23(5):21.
  4. J Strength Cond Res. 2008 May;22(3):845-50. Fiber type composition and capillary density in relation to submaximal number of repetitions in resistance exercise.