Muscle fiber types: Type I, IIa, IIx

The 3 fiber types

Type I (slow twitch, oxidative):

Red color (high myoglobin)
High mitochondrial density
Fatigue-resistant
Low force, low speed
Postural muscles, long-distance running, cycling

Type IIa (fast-twitch oxidative-glycolytic):

Intermediate color
Moderate mitochondrial density
Reasonably fatigue-resistant
Moderate-high force, moderate-high speed
400m to 5K running, most resistance training

Type IIx (fast-twitch glycolytic):

White color
Low mitochondrial density
Fatigue rapidly
Highest force, highest speed
Sprinting, jumping, max-effort lifting

What determines someone's ratio

Genetics — about 50% of fiber-type ratio. The other 50% can shift with training, but you cannot convert Type I to Type IIx (or vice versa). You can shift IIx ↔ IIa with training, and you can change the proportions slightly through hypertrophy of one type vs another.

An elite sprinter is born with 70-80% Type II fibers. An elite marathoner is born with 70-80% Type I. Most people sit near 50/50.

How to train each type

Type I (slow twitch):

High reps, light loads (12-20+ reps)
Long-duration cardio
Time under tension techniques (slow tempos)
Rest 30-90s between sets

Type IIa (intermediate):

Moderate reps, moderate loads (6-12 reps at 70-85% 1RM)
Most "hypertrophy" training
Rest 90s-2min between sets

Type IIx (fast twitch):

Low reps, high loads (1-5 reps at 85-100% 1RM)
Plyometrics, sprints
Olympic lifts
Rest 3-5min between sets

Implications for programming

A typical hypertrophy program (6-12 reps) primarily targets Type IIa. To grow the fast-twitch fibers maximally, you also need heavier work. To grow the slow-twitch fibers maximally, you also need higher-rep work.

This is why advanced programs include multiple rep ranges across the week — they're targeting all three fiber types instead of one.

Real-world fiber type evidence

You can guess someone's fiber-type ratio without a biopsy:

Can they jump explosively? Lean toward Type II.
Do they run forever without fatigue? Lean toward Type I.
Do they max-lift well but gas quickly? Type II dominant.

This isn't precise, but it helps inform programming. A Type-II-dominant client will respond fast to heavy strength work. A Type-I-dominant client will respond better to volume.

TL;DR

3 fiber types: I = slow, oxidative, fatigue-resistant; IIa = intermediate; IIx = fast, glycolytic, powerful. Ratios are 50% genetic. Different rep ranges target different fibers. Most programs need multiple rep ranges to develop all three.