There are three types of muscle fibre: 1, 2a and 2b. Their function is determined by their size and metabolism. Type 1 fibres oxidise glucose to produce adenosine triphosphate (ATP). They are of small diameter, contract slowly, produce modest force and are resistant to fatigue. Type 2b fibres are large in diameter, produce ATP anaerobically, have fast twitch tensions, generate large force but fatigue rapidly. Type 2a fibres are intermediate. A table showing the details of the classification of muscle fibre types is provided. The chapter concludes with a brief description of cross-bridge cycling.