Muscle fibre types, and why most of what you have read is oversold

What people are told

That you have fast-twitch and slow-twitch fibres in a fixed ratio, that this ratio determines whether you are a sprinter or an endurance athlete, and that you should train for your type — high repetitions if you are slow-twitch, low if you are fast.

Often accompanied by a field test claiming to determine your ratio.

What is actually known

The fibre types are real. Human muscle contains fibres differing in contractile speed, fatigue resistance and metabolic profile, usually described as Type I (slower, fatigue-resistant) and Type II with subtypes (faster, more powerful, fatiguing sooner). This is established histology, not theory.

Proportions differ between muscles and between people. Soleus is predominantly slow — appropriate for a muscle holding you upright all day. Gastrocnemius has a higher proportion of faster fibres.

Recruitment is orderly. Motor units are generally recruited from smallest to largest as force demand rises, which means faster fibres come in at higher forces — or at lower forces once the slower ones fatigue. This has a consequence people find counterintuitive: a light set taken close to failure recruits the faster fibres too, because fatigue drives recruitment upward.

Now the part that is oversold. Determining your fibre composition requires a muscle biopsy. Field tests claiming to estimate it — reps at a percentage of maximum, jump tests — have not been shown to do so reliably, and the results people build programmes on are not measuring the thing they name.

And the training prescription does not follow even if the ratio were known. The idea that slow-twitch people should train high-rep and fast-twitch low-rep is a plausible-sounding inference that has not been demonstrated to produce better outcomes.

What follows from that

Do not train for a fibre type you have not measured and probably cannot.

Rep ranges are less rigid than the fibre-type story implies. A useful amount of research points to the total quantity of challenging work mattering more than the specific rep range, across a fairly wide band — which is the same conclusion progressive overload arrives at from a different direction.

Where a muscle's composition is genuinely known and useful, it is because of anatomy — the seated calf raise exists because of where soleus attaches, not because of a fibre-type test.

What we still cannot say

Your fibre composition, without a biopsy.

How much composition can change with training. Shifts between Type II subtypes are reported; wholesale conversion between Type I and Type II is much less clear and is argued about.

Whether individualising training by fibre type would help if we could measure it. It is a reasonable hypothesis that has not been established, and it is currently sold as though it had.