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The Muscular System

The muscular system constitutes about 45 percent or our total body mass. Without muscles all actions, including movements as simple as maintaining a normal posture, breathing or walking, would not be possible.

There are three different types of muscle in the human body:

  1. Skeletal or voluntary muscle is so called because it is under our direct control and the muscles are attached to the skeleton.
  2. Smooth or involuntary muscle is outside our voluntary control and is found, for example, in the digestive tract, circulatory system and respiratory system.
  3. Cardiac muscle is a highly specialised type of muscle found only in the heart.

This module focuses on the voluntary skeletal muscles as they are the muscles that affect movement of the body.

Muscles are a collection of long fibres grouped in bundles. Each bundle is separately wrapped in a sheath of tissue that holds the muscle together and protects it.

Skeletal muscles are capable of producing large movements of the entire body (eg jumping) down to small, very fine movements (eg eye movements), or a combination of both.

Muscles act in different ways to produce movement around a joint or joints. There is always a balance between opposing muscle groups: one muscle group called the agonist contracts concentrically (shortening); the opposite muscle group called the antagonist contracts eccentrically (lengthening). The agonists, also known as the prime movers, are the muscles that actually produce the movement, while the antagonists oppose and regulate the speed and power of the movement.

For example, flexing the elbow requires a coordinated contraction between the biceps (the agonist) and the triceps (the antagonists). The opposite applies when extending the elbow.

A third muscle group (while not directly involved in producing the movement) contracts to stabilise the joint around which the other muscles act. They are called stabilisers.

The brain and nervous system control the muscles. The synchronised contraction of the various muscle groups is important for producing smooth, controlled movement thereby stabilizing joints and preventing injury. Therefore, it is also important that these muscles are balanced in strength because if one muscle in a pair is stronger than the other injury may occur.

Muscle Fibres


There are two types of muscle fibres, fast twitch and slow twitch. In any one muscle there is a mixture of fast and slow twitch muscle fibres. The individual make-up of fast and slow twitch fibres in the muscles is believed to be genetically determined, however some change is possible through training.

  • Fast twitch muscle fibres produce a very quick, forceful contraction and are more predominant in sprint or power players. These fibres use mainly high-energy anaerobic fuel sources and fatigue very rapidly.
  • Slow twitch muscle fibres do not produce a very fast or powerful contraction, however they are very slow to fatigue. Slow twitch fibres use mainly oxygen for fuel and are more predominant in players who do endurance-type activities.