Parallel fibres and the line of pull
In a parallel muscle, whose fibres (cells that shorten) run along its long axis, a free insertion, conventionally the more mobile attachment, moves towards the origin, the less mobile one.
Remember
A muscle can only pull, so a free insertion travels towards the origin along the fibres; with the origin steady on the medial side, fibres running medial to lateral move the insertion medially.
A muscle can only pull on its attachments. The origin is conventionally the less mobile attachment, usually proximal (towards the trunk). The insertion is the more mobile one, usually distal (away from the trunk). When the muscle contracts with its origin held steady and the insertion is free to move, the insertion is drawn towards the origin and not away from it.
The direction of that pull is set by the direction of the fibres. In a parallel muscle they all run the same way, along the long axis of the muscle, so the muscle shortens along one straight line. That line is its line of pull (the line along which the muscle's force acts).
If the fibres run from the medial side to the lateral side, the muscle shortens horizontally, and with the origin on the medial side the insertion is drawn medially. If the fibres ran from a superior origin to an inferior insertion instead, the muscle would shorten vertically and pull the insertion upwards.
Pennate muscles (from the Latin for feather) are the usual contrast. Their fibres attach to a tendon at an angle, so each fibre pulls at an angle. Parallel fibres have no such angle, so they pull straight along the muscle. Sartorius, a thin strap-like muscle of the thigh, is a familiar parallel muscle.
Key facts
- Direction of pull
- A muscle can only pull, so a free insertion moves towards the origin and not away from it.
- Origin
- It is conventionally the less mobile attachment, usually proximal.
- Insertion
- It is conventionally the more mobile attachment, usually distal.
- Parallel muscle
- The fibres run in the same direction as the long axis, so the muscle shortens along its length.
- Medial to lateral fibres
- With the origin on the medial side, these fibres shorten the muscle horizontally and pull the insertion medially, whereas fibres running from a superior origin to an inferior insertion pull it superiorly.
- Pennate contrast
- Fibres attach to a tendon at an angle, so each fibre pulls at an angle and the tendon tends to move less than the attachment of a parallel muscle.
Exam traps
- Following the fibres towards the insertion does not drive the insertion away from the origin, because a muscle can only pull and so it cannot push.
- An isometric contraction (tension without shortening) happens when a load or another muscle prevents movement, so it does not mean that a free insertion stays still.
- Pulling at an angle to a tendon belongs to pennate muscles, not parallel ones, because parallel fibres lie along the long axis and so pull straight along that line.
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