FORM ENDURANCEAcademyHOW TRAINING WORKS · 10 MIN
Understand fibre recruitment and the two main things aerobic training builds: capillaries and mitochondria.
Inside the muscle fibre
Cross-section of a thigh-muscle fascicle, about 80 fibres
Fibre types in a typical endurance athlete’s thigh muscle: about 55% Type I, 35% Type IIa and 10% Type IIx. Type I fibres run a little smaller.
Approximate, typical untrained → trained change in the thigh muscle; individual responses vary widely. Recruitment is an illustration of the size principle, not a measurement.
STEP 1 OF 5
A muscle is bundles of fibres, and not all fibres are alike. Type I fibres are slow, fatigue-resistant and packed with mitochondria. Type IIa are faster and still fairly enduring. Type IIx are the fastest and tire quickly.
Endurance athletes usually have more Type I in their leg muscles, but every muscle is a mix.
Every muscle is a mix of fibre types.
A muscle is bundles of fibres, and not all fibres are alike. Type I fibres are slow, fatigue-resistant and packed with mitochondria. Type IIa are faster and still fairly enduring. Type IIx are the fastest and tire quickly.
Endurance athletes usually have more Type I in their leg muscles, but every muscle is a mix.
The takeaway: Every muscle is a mix of fibre types.
Your nervous system recruits fibres in order of size: the small Type I fibres first, the bigger fast ones only as the effort climbs. This is the size principle.
At easy intensity only a fraction of the fibres are working, and nearly all of them are Type I. That is why easy sessions can go on for hours.
The takeaway: Easy effort recruits mostly slow, fatigue-resistant fibres.
Drag the intensity up. Past threshold the IIa fibres join in, and near a sprint even the IIx fibres are working.
Fast fibres make more force but tire sooner and lean on carbohydrate. That is one reason hard efforts cannot last.
The takeaway: Harder efforts recruit fibres that fatigue faster.
Capillaries are the smallest blood vessels, delivering oxygen and fuel to each fibre. With consistent aerobic training, the number around each fibre rises, roughly 20–30% over a few months.
More capillaries mean oxygen has less far to travel, and the fibre can work aerobically at a higher intensity.
The takeaway: Weeks of easy volume build the delivery network.
Mitochondria are where fat and carbohydrate are burned with oxygen. Endurance training can raise their volume in the muscle by around 40%.
More capillaries and mitochondria are much of what "aerobic base" means in practice. Most endurance methods keep the bulk of training easy to build exactly this. The training philosophies pages explain how each method does it.
The takeaway: Aerobic base is something real that you can build.
Teaching models of typical patterns in healthy, trained adults. Individual responses vary. Not medical advice.
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