Type I slow oxidative, type IIa fast oxidative, type IIx fast glycolytic
Mnemonic
“One is slow, two is fast”:
| Type I | Type IIa | Type IIx | |
|---|---|---|---|
| Speed | Slow | Fast | Fast |
| Metabolism | Oxidative | Oxidative-glycolytic | Glycolytic |
| Mitochondria | Many | Many | Few |
| Myoglobin | High, so red | Moderate | Low, so white |
| Fatigue | Resistant | Intermediate | Rapid |
| Example | Postural muscles, marathon | Middle distance | Sprinting |
“Slow red oxidative, fast white glycolytic”, and the red colour comes from myoglobin and capillary density, not from haemoglobin.
Endurance training increases mitochondria and capillaries within existing fibres and can shift IIx towards IIa, but does not convert type II into type I; resistance training hypertrophies type II fibres preferentially.
Ageing and disuse preferentially lose type II fibres, which is why the elderly lose power before they lose endurance, and why falls prevention focuses on resistance work.
Expansion
| Feature | Type I | Type IIa | Type IIx |
|---|---|---|---|
| Contraction | Slow | Fast | Fastest |
| Metabolism | Oxidative | Oxidative and glycolytic | Glycolytic |
| Mitochondria | Many | Many | Few |
| Myoglobin | High (red) | Moderate | Low (white) |
| Capillaries | Dense | Dense | Sparse |
| Fatigue | Resistant | Intermediate | Rapid |
| Force | Low | High | Highest |
| Example | Postural muscles, soleus | Sprinting |
Recruitment follows the size principle: small motor neurones supplying type I fibres are recruited first, with larger type II units added as force demands increase. This gives fine control at low forces and reserves the fatigable fibres for maximal effort.
Training shifts properties: endurance training increases mitochondrial density, capillarity and oxidative enzymes, converting IIx towards IIa. Resistance training causes hypertrophy, predominantly of type II fibres.
Fibre type proportion is largely genetically determined, and elite endurance and sprint athletes show markedly different distributions.