Fast vs. Slow-Twitch: Why Training Both Matters

Ever wondered why some people are lightning-quick sprinters while others seem built for marathons? It comes down to your muscle fibre types: fast-twitch and slow-twitch.

The best news? No matter what you were “born with,” the right training can help you improve both.

Fast-Twitch (Type II) Muscle Fibres

These are your explosive, powerful fibres—the ones that fire quickly and help you move big loads or sprint fast.

• Found in sprinting, Olympic lifting, jumping, HIIT
• Contract rapidly for power and speed
• Fatigue quickly (not built for the long haul)

Slow-Twitch (Type I) Muscle Fibres

These are your endurance fibres—they keep you moving steadily without burning out.

• Used in Zone 2 cardio, long runs, hikes, bodyweight movements
• Slower to fatigue, great for sustained effort
• Packed with mitochondria, making them efficient at fat-burning and supporting overall metabolic health

Why You Should Train Both

Focusing on just one side of the spectrum leaves gaps in your fitness. Here’s why you need a balance:

• Fast-twitch: Crucial for preventing falls, maintaining speed and strength as you age
• Slow-twitch: Builds aerobic efficiency, boosts cardiovascular health, and helps with recovery between hard efforts

Practical Tips

Here’s how to make sure you’re hitting both types of muscle fibres in your training:

• Mix it up → Heavy lifting, short sprints, and plyometric movements for fast-twitch development
• Add Zone 2 work → A weekly session of longer, steady-state cardio for slow-twitch endurance
• Vary your rep ranges → 3–6 reps for strength and power, 10–20 reps for muscular endurance

This is exactly why CrossFit works so well—it blends different training styles so you’re constantly developing across the spectrum.

Final Thoughts

Training both fast and slow-twitch fibres doesn’t just make you “well-rounded.” It improves performance, longevity, and body composition all at once.

So next time you’re in class, remember: the variety isn’t random—it’s there to help you unlock the full potential of your body.

See you at the barbell (and on the rower),
— Tom, Kathryn and The EDC Team