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Muscles cause movement by contracting across joints. Muscles are attached to the skeleton by tendons in two places:

• the origin
• the insertion

The origin is the end of a muscle which is attached to a fixed bone. The insertion is the other end of the muscle that is attached to the bone which moves.

Muscular contractions

Muscular contractions are defined as the change in the length of the muscle under contraction. Muscles contract in different ways to produce a range of movements:

• Isotonic contraction – involves the muscle producing tension and controlling the speed of the muscular contraction. This movement can be a concentric or eccentric muscular contraction.
  • Isotonic concentric contraction – this involves the muscle shortening. The origin and insertion of the muscle move closer together and the muscle becomes fatter.
  • Isotonic eccentric contraction – this involves the muscle lengthening whilst it is under tension. The origin and the insertion move further away from each other. An eccentric contraction provides the control of a movement on the downward phase and it works to resist the force of gravity.

• Isometric contraction – this involves a muscle producing tension but staying the same length. This occurs when the body is fixed in one position.

Here, the muscles are contracting isometrically to hold this gymnast in the crucifix position and are not changing length. When the isometric contractions end, isotonic contraction will occur.