Fast twitch vs slow twitch muscle fibers

The three fiber types

A fiber's type is set by the myosin it makes, the motor protein that pulls the muscle shorter. Adult human limb muscle makes three main versions, and each fiber is named after the one it carries:

• Type I (slow twitch). Slow to contract, low power, very fatigue resistant. These carry posture, walking and long efforts.
• Type IIa (fast twitch, fatigue resistant). Much faster and more powerful than type I, and better at repeating efforts than IIx.
• Type IIx (fast twitch, fastest). The fastest and most powerful human fiber, and the quickest to fatigue.

The clearest numbers come from Scott Trappe's lab, which tested single fibers from the thigh (vastus lateralis) of a former world champion hurdler, the 60m hurdles indoor world record holder (7.30s) and former 110m hurdles world record holder (12.91s). Each fiber was tested on its own in the lab:

Peak power here is per liter of muscle fiber, so the gap between types is not about size. Per volume, his IIx fibers made about twice the power of his IIa fibers and about 14 times the power of his type I fibers. They were also exceptional: the lab measured about 15–17 W/L in IIx fibers from healthy untrained people, so this sprinter's IIa fibers matched an untrained person's IIx.

What about type IIb?

Type IIb is the main fast fiber in mice and rats, where its myosin makes up about 70% of skeletal muscle myosin. Adult humans carry the gene, but the IIb protein is essentially absent from human limb muscle. Its RNA turns up in a small set of specialized muscles such as some jaw (masseter) fibers, and in limb muscle that is badly damaged and regenerating.

Older human studies called the fastest fibers "IIb" because staining methods couldn't tell them apart from the rodent type. Those fibers express IIx myosin. So when you read about type IIb fibers in human research or training articles, read it as IIx.

Hybrid fibers

Not every fiber is pure. Some make two kinds of myosin at once, usually I/IIa or IIa/IIx, and sit between the pure types in speed. In the thighs of 15 young men, hybrids were about 14% of fibers. In a group of older adults they were almost 5%. The champion hurdler had 4.5% I/IIa and 8.5% IIa/IIx hybrids.

How many hybrids a study finds depends on how it looks. The older staining method (myofibrillar ATPase) clearly undercounts them, and only testing fibers one at a time identifies them reliably. Hybrid counts also change with training, which is one reason researchers think fibers move through a hybrid stage when they change type.

A champion sprinter's fibers

The hurdler's thigh fibers broke down like this:

That is 71% fast twitch, and 24% pure IIx, the most the researchers had seen in an elite sprinter. Even so, nearly a third of his fibers were slow twitch.

Keep the limits in mind. This is one athlete, sampled near the end of his career, and the authors note it may not reflect his muscle at peak form. People also vary a lot. In one large North American sample, about a quarter of people had either under 35% or over 65% type I fibers in the same thigh muscle.

How much is genetic?

A good share. Pooling family and twin data, Simoneau and Bouchard estimated that about 45% of the variation in type I fiber proportion between people is inherited, about 40% comes from environment and activity, and about 15% is measurement error from sampling a small piece of muscle. Environment and training alone could account for differences of about 30 percentage points in type I fibers between people.

One gene has a clear link to sprinting. ACTN3 makes alpha-actinin-3, a protein found only in fast twitch fibers. About 18% of the white controls in the first big study had two copies of a common variant (the XX genotype) and make none of it. Among 107 elite sprint and power athletes, only 6% were XX, and none of the 35 women were.

Genes shift the odds without deciding the outcome. Some male elite sprint and power athletes in that study were XX, and inherited factors explain only part of fiber makeup. Training can still change a lot about the fibers you have.

Can fiber types change?

IIx to IIa: yes, with hard training

This is the best established change. In sedentary men who lifted heavy for three months, IIx myosin fell from about 9.3% to 2.0% of the muscle's myosin, IIa rose from 42% to 50%, the type II fibers grew, and strength went up. A 2021 review found heavy resistance training, sprint, power and plyometric work all tend to move IIx and IIa/IIx hybrid fibers toward pure IIa.

That can sound like losing speed, but IIa fibers are still fast twitch, and the training that causes the shift also makes the fibers bigger and stronger. The champion hurdler shows how powerful well-trained IIa fibers can be.

Slow to fast: contested

In six male sprinters, three months of intensive strength and interval training cut pure type I fibers from 52% to 41% and raised pure IIa fibers from 35% to 52%, with IIa/IIx hybrids also falling. The authors read it as fibers moving toward IIa from both directions. A later review notes the sprinters had just come off three weeks of rest, which may have exaggerated the change.

Reviews disagree on how far this goes. One describes type IIa and IIx interconversion as well recognized but the evidence on type I and type II interconverting as conflicting. Another concludes fibers can shift between slow and fast types but that long-term data in athletes have not settled the pattern.

Fast to slow: endurance volume

Longer, higher-volume endurance training pushes the other way. In novice marathon runners, 13 weeks of training raised type I fibers in the thigh from about 43% to 49% while IIa fell. In older adults, 16 weeks of endurance exercise moved fibers toward type I, with I/IIa hybrids increasing along the way.

The IIx overshoot

When the men in the lifting study stopped for three months, their IIx myosin climbed to about 17%, well above where they started. Their strength returned to its starting level. Extra IIx doesn't make you faster once the fibers have shrunk and the strength is gone, so the overshoot has no use as a training method.

Size changes more than type

The largest, most reliable effect of training is on fiber size. In the lifting study, type II fibers hypertrophied. With age, muscle loss comes mainly from type II fibers getting smaller, and six months of resistance training in older men grew type II fibers by 24%, which accounted for all of the gain in thigh muscle size.

What it means for training a sprinter

• Sprint and jump fast and often enough. Short, maximal efforts are the work that demands fast fibers. Sprint workouts and plyometrics for sprinters lay out sessions.
• Lift heavy enough to grow type II fibers. Strength training is how type II fibers get bigger, at any age. See does muscle make you slower for the kind of lifting that transfers.
• Keep endurance volume purposeful. High-volume endurance training is what moves fibers toward type I. Sprinters still need some conditioning, but long slow mileage isn't speed work.
• Give fast muscles time to recover. In a study of 20 men, those estimated to have mostly fast twitch fibers lost more power across three all-out 30-second sprints (61% vs 41%) and still hadn't recovered their strength five hours later, while the slow group recovered in 20 minutes. Fast athletes often need more recovery between hard sessions.
• Masters athletes: keep lifting. Type II fibers shrink with age, and resistance training grows them back.

Common mistakes

Writing yourself off as slow twitch

Genes explain only part of fiber makeup, and training changes fiber size and the IIx–IIa balance. Your times tell you more than a guess about your fibers.

Fearing that lifting kills fast fibers

Heavy lifting shifts IIx toward IIa, both fast twitch, while growing type II fibers and raising strength.

Chasing the IIx overshoot

Long layoffs raise IIx while strength drops. Use a planned taper, not months off.

Reading rodent IIb results as human

Humans run on IIx fibers. Mouse and rat data on IIb don't transfer directly.