Do Different Muscle Fibers Need Different Rep Ranges?

Muscle fiber types and rep ranges: what did the new review find?

Varovic, Larsen and Grgic combined eight studies involving 195 participants. They modeled training load against changes in type I and type II fiber size measured through muscle biopsies. This was a synthesis of existing experiments.

The model suggested relatively greater type I growth at lighter loads and type II growth at heavier loads. However, every pairwise contrast between fiber types had a 95% confidence interval that included zero. The apparent pattern could not establish a clear difference.

Six studies involved untrained participants, all biopsies came from one thigh muscle—the vastus lateralis—and the data contained no observed loads between 53% and 70% of one-repetition maximum. Those gaps limit muscle-specific prescriptions. [1]

A fiber result is not a whole-muscle rep prescription

Type I fibers are often called slow-twitch; type II, fast-twitch. A muscle contains a mixture. The programming question is whether choosing a load based on its expected fiber composition actually improves growth of that muscle.

An earlier experiment tested that idea in the calves. Twenty-six untrained men trained for eight weeks, using one leg for lighter-load work at 20–30 reps and the other for heavier-load work at 6–10 reps. Researchers measured the soleus, typically rich in type I fibers, and the gastrocnemius, which has a more mixed composition.

Muscle-thickness gains were similar between loading conditions. This small trial does not settle the question for every muscle or experienced lifter, and ultrasound did not measure individual fiber growth. It does challenge the simple rule that a supposedly “slow-twitch muscle” must be trained with high reps. [2]

My interpretation: a proposed mechanism becomes useful for programming when it helps us choose an approach that improves the outcome we care about.

What the broader evidence says about heavy and light weights

A 2017 meta-analysis of 21 studies found similar hypertrophy across lower and higher loads when sets were performed to momentary muscular failure. Heavier loading produced larger improvements in one-repetition-maximum strength. Growing muscle and maximizing a heavy single are related goals, but the loading choices can differ. [3]

In one 12-week trial, 49 trained men used either 20–25 or 8–12 reps per set, with sets taken to volitional failure. Both groups increased lean mass and muscle-fiber size without significant between-group differences in those outcomes. That supports flexibility within the conditions tested; it does not show that effort or exercise execution are irrelevant. [4]

These comparisons also matter when reading a program: 15 reps is a count, not an effort target. If you could perform many more repetitions, the set is different from a 15-rep set near your limit. Our training intensity guide explains load and effort, while the RIR guide explains how to describe the repetitions you have left.

Taking every set to failure is a separate decision. An eight-week study in trained adults found similar quadriceps growth with failure training and stopping around 1–2 reps short, with more acute fatigue when training to failure. This finding applies to the exercises and program studied; it is not proof that every distance from failure works equally well. [5]

Choose your range, then judge how it works

The following is a THG application example, built around the coaching ranges above. Use it as a starting framework to adapt to your own training.

A simple comparison is more useful than constantly changing rep targets. Keep the exercise setup and your intended effort reasonably consistent, record what you do, and use those notes when deciding whether the range suits you.

For your next session: pick one exercise and write down why you chose its rep range. Include the load, reps and estimated RIR in your log. That gives you something concrete to review before changing the program.