Types of skeletal muscle fibres

Types of skeletal muscle fibres

Beyond the isotonic/isometric distinction, individual skeletal muscle fibres also differ physiologically along two independent axes: how fast they shorten, and how they generate ATP.

• Speed axis — fibres with myosin of high ATPase activity are fast fibres; fibres with lower ATPase activity are slow fibres.
• Metabolism axis — fibres rich in mitochondria and with a high capacity for oxidative phosphorylation are oxidative fibres (dependent on a steady blood supply of oxygen, and coloured red by their myoglobin content, hence "red muscle"); fibres with few mitochondria but abundant glycolytic enzymes and glycogen stores are glycolytic fibres (paler due to low myoglobin, hence "white muscle").

Combining these two axes gives the three physiological fibre types actually found in skeletal muscle:

1. Slow-oxidative fibres — low rate of ATP hydrolysis but capable of making large, sustained amounts of ATP; suited to prolonged, regular activity such as long-distance swimming. Long-distance runners have a high proportion of these fibres in their leg muscles.
2. Fast-oxidative fibres — high myosin ATPase activity combined with a strong ATP-making capacity, suiting them to rapid actions that must also be sustained for a while. …