Properties of slow fibres
Slow skeletal muscle fibres develop contractions more gradually and obtain most of their ATP by aerobic respiration. Their lower rate of lactate formation means that fatigue develops more slowly.
These fibres have a darker appearance and several features associated with maintaining an oxygen-dependent ATP supply.
- They contain numerous, relatively large mitochondria.
- They are supplied by a dense network of capillaries.
- They contain comparatively more myoglobin.
- Their reserves of glycogen and phosphocreatine are smaller than those in fast fibres.
- The calcium-ion reserve in the sarcoplasmic reticulum is smaller.
- Their ATP hydrolase activity is lower.
Where slow fibres are useful
Slow fibres are well suited to activity that continues for a long time at a relatively modest intensity. Examples include muscles used by animals during migration or prolonged stalking, together with human back and leg muscles.
The high mitochondrial content and rich capillary supply are consistent with the reliance of these fibres on aerobic ATP production.