Bipennate muscle: fibre arrangement and force
A bipennate muscle has its fibres (the contractile muscle cells) attached obliquely, at an angle, to both sides of a central tendon, which favours force over range of movement.
Remember
A bipennate muscle fits more fibres into its volume by setting them obliquely on both sides of a central tendon, giving greater force at the cost of a smaller range of tendon movement.
Memory aid
penna is Latin for feather, so a bipennate muscle is a feather with fibres on both sides
Picture a feather with its barbs lying on both sides of the middle, just as the fibres of a bipennate muscle lie on both sides of the central tendon.
The word bipennate describes how the fibres of a muscle meet its tendon. Penna is the Latin word for feather, and the fibres of a bipennate muscle attach obliquely (at an angle) to both sides of a central tendon, like the two sides of a feather. Rectus femoris, part of the quadriceps at the front of the thigh, is the usual example.
Compare the neighbouring patterns. In a unipennate muscle the oblique fibres attach to one side of a tendon only, as in flexor pollicis longus. A multipennate muscle, such as the deltoid, has several oblique regions of fibres attaching to tendinous strands (internal bands of tendon). In a parallel muscle, such as sartorius, the fibres run in the same direction as the long axis of the muscle.
Angled fibres generally allow more of them to be packed into a given volume of muscle. That raises the total contractile fibre area, known as the physiological cross-sectional area (the combined cross-section of all the fibres). This total fibre area is what sets the force a muscle can produce, so a bipennate muscle can generate relatively more force for its size.
The price is movement. Each fibre is relatively short and pulls on the tendon at an angle, so the tendon moves through a smaller distance. Fibre length influences how far a muscle can shorten (its range of shortening), so a muscle with long parallel fibres is better suited to a large range of movement, but it is weaker for its size.
Key facts
- Arrangement
- oblique fibres attach to both sides of a central tendon, like the two sides of a feather.
- Example
- rectus femoris is a familiar bipennate muscle.
- Advantage
- more fibres fit into the same volume, so the total fibre area is larger and the muscle produces relatively more force for its size.
- Cost
- the tendon moves through a smaller distance, so the range of movement is smaller.
- Unipennate
- oblique fibres attach to one side of a tendon only, as in flexor pollicis longus.
- Multipennate
- several oblique regions of fibres attach to tendinous strands, as in the deltoid.
Exam traps
- A greater range of shortening does not come with a pennate design, because pennate fibres are short and pull at an angle, so the tendon is not moved far; a parallel-fibred muscle has the greater range of shortening.
- A higher contraction velocity (the speed of shortening) does not come with bipennate fibres, because contraction velocity falls as fibres get shorter, so a parallel-fibred muscle is the faster shortener.
- Oblique fibres attached to a tendon on one side only describe a unipennate muscle, not a bipennate one, which has its oblique fibres on both sides of a central tendon.
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