The fibres differing characteristics make them particularly useful to athletes in specific sporting events. For example, a marathon runner would need a high percentage of slow twitch fibres in his/her legs and the 100m sprinter would need a higher percentage of fast twitch fibres, to ensure success in their event.

Huxley's Sliding Filament Theory

The sliding filament theory is used to explain how muscle contracts and shortens when stimulated by a nerve impulse. The simplified sequence of events is as follows:

1. The impulse (electrical signal/message) travels down the nerve until it reaches the muscle at the neuromuscular junction (also known as motor endplate).

2. Synaptic vesicles in the neuromuscular junction release the chemical that they contain (acetylecholine) into the gap between the nerve and the muscle (known as the synaptic cleft) this chemical transmits the impulse across the gap to the surfce of the muscle (sarcolemma).

3. The impulse spreads along the sarcolemma until it goes deep into the muscle down specialised inlets (T-tubules).

4. Whilst going down the T-tubules the impulse can spread throughout a network of tubes called the sarcoplasmic reticulum (SR). The impulse passing through the SR causes it to release calcium ions throughout the muscle.

5. Within the muscle the contractile machinery is made up two protein filaments, the myosin (thick) filament and the actin (thin) filament. Released calcium binds to part of the actin strand (troponin) and causes specialised 'binding' sites to become uncovered.

6. Once the binding sites are exposed the thicker myosin filament is strongly attracted to them and reaches up and takes hold of the actin filament, forming a 'crossbridge'. (Animation point 1).

7. Using ATP energy the myosin heads perform a pull or powerstroke on the actin filaments causing them to move and shorten the section of the muscle fibre (Animation point 2). This happens all the way along the muscle fibres length. The broken down ATP molecule is released as ADP + P.