Most runners and HYROX athletes talk about heart rate zones.
But what many people don't know is how your muscle fibres tell the real story.
And understanding this is how you go from training hard to training smart.
The Muscle Fibre Hierarchy
Your body doesn't use every fibre at once.
It follows what's known as Henneman's Size Principle — recruiting from smallest to largest motor units as demand increases:
- Type I fibres – slow-twitch, highly oxidative, built for endurance.
- Type IIa fibres – fast oxidative-glycolytic, hybrids that adapt to both aerobic and anaerobic stress.
- Type IIx fibres – fast glycolytic, explosive but fatigue quickly.
Each training zone recruits and develops a different set of fibres.
That's why different intensities create different adaptations — and why you need both easy and hard sessions to build a complete engine.
How Fibre Recruitment Maps to Training Zones
| Zone | Fibre Recruitment | What You're Training |
|---|---|---|
| Zone 1–2 (Below LT1) | Type I fully active | Aerobic base, mitochondrial growth, fat oxidation |
| Zone 3 (Around LT1 → Below LT2) | Type I + IIa begin joining | Hybrid fibres adapting to endurance |
| Zone 4 (Around LT2) | Type I + IIa heavily recruited | Maximal aerobic power, lactate tolerance |
| Zone 5–6 (Above LT2) | Type I + IIa + IIx | High-intensity power, neuromuscular drive |
Training around LT1 maximally challenges your Type I fibres.
Pushing toward LT2 recruits more Type IIa fibres, teaching them to use oxygen more efficiently.
Go beyond LT2 and you bring in Type IIx fibres — powerful, but less efficient, and suited for short bursts of effort.
Why It Matters
If most of your training sits in the "moderate" middle, you're missing both ends of the spectrum:
- You're not giving Type I fibres enough low-intensity time to improve efficiency
- You're not challenging Type IIa fibres hard enough to build power and raise your ceiling.
To build a complete engine, you need both:
- Long Zone 2 sessions to develop endurance and efficiency.
- Threshold and tempo work to build capacity and speed.
This balance is the difference between getting fit and becoming truly efficient.
How Run Pace Pal Makes It Simple
The Tempo + Zone 2 Builder in Run Pace Pal takes the guesswork out of this process:
- Designs progressive aerobic base programmes (Zone 2) using HR drift analysis.
- Builds tempo workouts matched to your actual threshold pace.
- Tracks efficiency over time so you know you're getting fitter, not just working harder.
Try it today: Run Pace Pal Tempo + Zone 2 Builder
When to Consider Lab Testing
Field tests and app-based tools are great for structure and progression.
But if you want to see exactly where Zone 2 ends and Tempo begins, lab testing provides the full picture.
At Box Nutrition, our lactate and VO₂max tests give you:
- LT1 (Aerobic Threshold) — your Zone 2 anchor.
- LT2 (Anaerobic Threshold) — your Tempo anchor.
- Full curve analysis — showing how much room you have to improve.
📧 Book your test here: boxnutrition.co.uk/book-online or email info@boxnutrition.co.uk
The bottom line: Train your muscle fibres, not just your heart rate. Zone 2 builds your base. Threshold builds your ceiling. Together, they build a complete runner.
