Multipennate muscle: the deltoid
A multipennate muscle has its fibres (the parts that shorten) in several regions, set at an angle on tendinous strands, which are cords of tendon.
Remember
Force depends mainly on physiological cross-sectional area (the total area of all the muscle fibres added together when the muscle is cut at right angles to the fibres), and a multipennate muscle such as the deltoid, especially its middle part, makes that area large by packing many short fibres on to tendinous strands.
Pennate describes how muscle fibres meet the tendon. In a unipennate muscle they lie on one side of the tendon, as in extensor digitorum, and in a bipennate muscle they lie on both sides of a central tendon, as in rectus femoris. The word pennate comes from the Latin word for a feather.
A multipennate muscle has its fibres in several separate regions, each set obliquely (at an angle) on tendinous strands, which are cords of tendon running into the muscle. The deltoid, which covers the shoulder, is the classic example, and its middle part is especially strongly multipennate. The whole muscle ends in a single tendon on the humerus, but its fibres wrap around that tendon and form separate fascicles (bundles of muscle fibres).
This packing fits many fibres into a given volume of muscle. Force depends mainly on the physiological cross-sectional area, which is the total area of all the muscle fibres added together when the muscle is cut at right angles to the fibres, so a larger area means more force for the muscle's size. The price is movement: each short fibre shortens only a small distance and pulls on the tendon at an angle, so the tendon moves less far than in a parallel muscle, whose fibres run straight from end to end, along the line of pull. A multipennate muscle favours force over range of movement, which suits the powerful abduction (lifting the arm out to the side) of the middle deltoid.
The deltoid does not behave as one block. Its multipennate middle part does most of the lifting, while its more strap-like anterior and posterior parts help to flex and extend the shoulder.
Key facts
- Definition
- the fibres sit in several regions, set obliquely on tendinous strands, which are cords of tendon running into the muscle.
- Classic example
- the deltoid, especially its middle part, is the classic multipennate muscle.
- Benefit
- a large physiological cross-sectional area, so high force.
- Cost
- each short fibre shortens only a small distance, so the tendon moves less far than in a parallel muscle.
- Naming
- fibres on one side of a tendon make a unipennate muscle, fibres on both sides make a bipennate muscle, and fibres on several strands make a multipennate muscle.
- Parallel muscle
- the fibres run along the line of pull, which favours range of movement over force, as in sartorius.
Exam traps
- A larger angle between the fibres and the tendon reduces the fraction of each fibre's force directed along the tendon, and it does not increase it; being pennate increases whole-muscle force by allowing more fibres to be packed into a given volume.
- A muscle whose fibres fan out from a broad attachment and converge on one narrow tendon, as in pectoralis major, is triangular (convergent) and not multipennate, because a multipennate muscle is defined by oblique fibres on tendinous strands, not by a fan shape; convergence describes the shape of the muscle and does not rank it as strong.
- A muscle whose oblique fibres lie on one side of a tendon only, as in extensor digitorum, is unipennate, the simplest pennate design, and not multipennate, because it lacks the several regions on tendinous strands that define a multipennate muscle.
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