Skeletal muscle fibres

Classification depends on: Maximal velocities of shortening: fast and slow. Fast fibres readily fatigue, while slow fibres can maintain smooth tetanic contractions for long periods of time. Major pathway used to form ATP: oxidative and glycolytic. Based on the above properties, the following types can be identified:

Slow-oxidative (type I): low myosin ATPase activity; high oxidative capacity

Table 9.4 Skeletal muscle cell types (based on the time between the occurrence of the muscle action fibre potential and the peak of the resulting tension)

Fast twitch

Slow twitch

Vmax

High

Low

Myosin

High

Low

ATPase activity

Glycolytic metabolism

High

Low

Oxidative metabolism

Low

High

Mitochondrial content

Low

High

Myoglobin content

Low

High

Fast-oxidative (type Ila): high myosin ATPase activity; high oxidative capacity Fast-glycolytic (type lib): high myosin ATPase activity; high glycolytic capacity Type I fibres predominate in muscles depending on sustained tone, e.g. postural muscles, and are fatigue-resistant. On the other hand, type II fibres predominate in muscles used for rapid movement and fatigue rapidly from lactic acid accumulation.

Muscle fatigue is related to a reduction in glycogen content of muscle fibres, oxygen lack associated with increased blood and muscle lactic acid levels and fatigue at the neuromuscular junction.

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