Muscles come in a variety of shapes and sizes. The shape of a particular muscle determines the degree of muscle contraction, and therefore, the amount of force that can be generated within.
Muscle shapes can be classified according to the position of the muscle fibres, in relation to the tendon which attach the muscle to the skeleton and transmits the muscular pull onto the bones.
Pennate muscle shapes
These look like feathers and occur where muscle fibres are arranged around a single central tendon. The many short fibres enable a high degree of force to be developed within the muscle, and therefore occur where much power and strength is required. They are, however, slower than fusiform muscles. There are many forms (see diagrams):
- Unipennate – the muscle fibres occur on one side of the tendon only
- Bipennate – muscle fibres occur on both sides of the tendon
- Multipennate – muscle fibres are placed around the central tendon
- Circumpennate – muscle fibres are arranged in a circular pattern
These are strap-like and occur where muscle fibres lie parallel to each other and a tendon occurs at either end of the muscle belly. They have fewer fibres that are longer, but cannot achieve as great a force as pennate muscle shapes (although they can generate a higher limb velocity).
These occur where the base is much wider than the insertion, giving the muscle a triangular shape and enables the muscle to contract with great force. An example of this type of muscle is the deltoid (shoulder).
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