The fiber you stopped asking for

What does fast-twitch actually mean?

Your muscles are built from two broad kinds of fiber. Slow-twitch, type I, is the endurance fiber. It fires at low force, resists fatigue, and carries you through a long run or a long day on your feet. Fast-twitch, type II, is the other one. It fires hard, produces far more force, and quits fast.

Type II is what a sprint is made of. A jump. A heavy single. The rep that moves slowly because everything else in you is already done. It also carries most of the size and shape people are actually training for. The glutes, hamstrings, shoulders and calves that read as athletic on a body lean heavily fast-twitch.

You do not get to pick your ratio. That part is genetics. You do get to pick whether the fast fibers you have are being used or sitting there dormant, and that is the whole argument.

Why does fast-twitch go first?

Muscle loss with age is not even. The body does not shrink uniformly from 30 onward. It lets go of type II first, and it lets go of it hardest.

The size of that difference is worth sitting with. A 2013 study in Experimental Gerontology took muscle biopsies from 25 healthy men averaging 23 and 26 healthy men averaging 71. Type II fibers were 29% smaller in the older group. Type I fibers showed no significant difference at all. The muscle that goes with age is, specifically, the fast kind.

It shows up as function before it shows up in the mirror. A 2012 review in Frontiers in Physiology reports strength being lost two to five times faster than muscle mass, and power faster than strength. The cohort study it cites for that, following 1,678 older adults for five years, found men losing about 16% of their knee extensor strength while losing only 5% of the muscle. People find out they got slow long before they find out they got smaller.

The fibers do not disappear on a birthday. They go quiet, because nothing in a normal adult week asks them a question. Ask, and a good deal of what looked like aging turns out to have been disuse.

Why does normal training miss it?

There are two popular ways to train and neither one gets there on its own.

Steady cardio is a slow-twitch activity by definition. That is what it is for. An hour at a conversational pace is an hour of telling your fast fibers they are not needed.

Lifting is closer, but it depends how you lift. Fibers are recruited in order, smallest first. Type II only comes in when the demand is high enough that the slow fibers cannot cover it: a heavy load, or a light load moved with real intent to move it fast, or a set taken close enough to failure that everything has to help. Sets of ten at a comfortable weight moved at a comfortable speed stop short of that line.

Which is how a man trains four days a week for a decade and still cannot sprint.

What actually trains fast-twitch?

1. 01

   Speed

   Sprinting is the cleanest stimulus there is, because there is no way to sprint slowly. Hills and sled pushes are the same signal with less that can go wrong.

2. 02

   Force

   Heavy compound lifts, loaded into the range where the bar moves slowly whether you want it to or not.

3. 03

   Intent

   Jumps, throws, and lighter lifts moved as fast as you are able to move them. The load is not the point here. The speed of the attempt is.

None of it takes long, and that is the part people get wrong. Fast-twitch work is short by nature, because the fiber that makes the most force is the fiber that runs out first. Ten good sprints is a session. Add volume past that and you are training something else, in a tired body.

What does a week of it look like?

Two or three exposures. Not more.

One day where the point is speed: sprints or hills, full recovery between efforts, and you stop the moment the efforts start falling off. One or two days where the point is force: heavy compounds, real loads, reps low enough to keep every one of them clean.

Explosive work goes at the front of a session on fresh legs, or it is not explosive work. Everything else, the accessory volume and the conditioning and the parts that build shape, goes after it.

That ordering is most of what a program is for. The Ripped Athlete runs it across twelve weeks so you are not deciding it standing in a gym on a Monday.

What should you expect, honestly?

The first thing that changes is not the mirror. It is having a second gear again. Stairs, a sprint to catch something, the ability to move fast without your body filing a complaint.

The look follows, because the fibers that produce force are the ones with the most room to grow, and because sprint and power work spends energy in a way that shows.

How fast and how much depends on your training history, your age, your sleep and what you eat. Anyone who hands you a number without knowing those four things is guessing.

What if you are over 40, or starting from nothing?

It matters more, not less. The fiber declining fastest is the one with the most to gain from being trained, and it does respond. In that same 2013 study, the older men, averaging 71, went on to train for six months: their type II fiber size increased by 24%, and that growth accounted for essentially all of the muscle they gained. If it works at 71 it is not closing at 43.

It also means the on-ramp is real. Tendon and connective tissue adapt slower than muscle does, so nearly all of the injury risk in sprint and power work sits in the first few weeks, and nearly all of it comes from going too hard too early. Build up. Run at around three-quarters of your best for a couple of weeks before you go and find out what all-out feels like.

This is training guidance, not medical advice. If you have an injury history or a heart condition, get cleared before you sprint.

Questions on this

Where the numbers come from

• Experimental Gerontology, 2013 (PMID 23425621)

  Muscle biopsies from 25 healthy young men, average age 23, and 26 healthy older men, average age 71. Type II fibers were 29% smaller in the older group; the difference in type I fibers was not statistically significant. The same older men then trained for six months, after which their type II fiber size had increased by 24%.

• Mitchell et al., Frontiers in Physiology, 2012

  A quantitative review of age-related decline in muscle size and strength. It reports strength lost two to five times faster than muscle mass, and power faster than strength. The five-year strength figures quoted on these pages come from a 1,678-person cohort study (Hughes et al., 2002) that this review summarizes, not from the review's own measurements.

Read next

• The exercises that actually train fast-twitch

  Which movements actually recruit type-II fiber, which ones only look like they do, and the one test that tells you which is which.

• What your athletic age is measuring

  The Athletic Age Test returns a number and a dormancy figure. Here is what goes into it, what each result means, and what it is not.

• Bodybuilding vs athletic training

  Two ways to train, two different bodies. What each one builds, what each one costs you, and how to tell which one you have actually been doing.