Skeletal Muscles Factor

Skeletal muscles are made up of muscle fibres, which fall into two broad categories: slow-twitch fibres (Type I) and fast-twitch fibres (Type II). Most muscles contain a mixture of both, and the exact proportion of each depends on a person's genetics, hormones and training habits. This composition plays a dominant role in how much strength, endurance and speed a person can develop.

Every skeletal muscle also shows four basic properties:

• Contractility — the ability to shorten and produce force.
• Excitability — the ability to respond to a nerve stimulus.
• Extensibility — the ability to stretch or lengthen.
• Elasticity — the ability to return to its original length after being stretched.

These properties, together with the mix of fibre types, determine which components of fitness a muscle can support.

Slow-twitch fibres (Type I / slow oxidative) contain large numbers of oxidative enzymes, have more capillaries and hold a higher concentration of myoglobin and mitochondrial enzymes than fast-twitch fibres. This makes them well suited to aerobic activity and highly resistant to fatigue. Their rich blood supply gives them a red colour. They contract slowly but keep contracting for long periods without tiring, releasing energy steadily. Slow-twitch fibres are important in long-distance running, swimming and cycling.

Fast-twitch fibres (Type II / fast glycolytic) hold a good volume of glycolytic enzymes that promote anaerobic activity, but because they contain fewer mitochondria their aerobic capacity is limited and they tire quickly. They do not depend on a blood supply to produce energy, so their colour is lighter than that of slow-twitch fibres. They contract rapidly, use a lot of energy and produce small amounts of energy very quickly. Fast-twitch fibres are important in explosive, anaerobic actions such as jumps, throws and sprints. …