Fusiform muscle
A fusiform muscle is spindle-shaped, wide in the middle and tapering at each end, with muscle fibres (myocytes) running along its length.
Remember
Compared with a pennate muscle (one whose fibres attach at an angle to a tendon) of the same size, a fusiform muscle tends to favour a large range of shortening over force. Its fibres run along the long axis, so they can shorten over a long distance.
Memory aid
fibres in a line give travel, while fibres on a feather give power
Use it to sort range from force: fibres in a line favour travel, and angled fibres on a feather favour power, meaning more force for the size of the muscle.
A fusiform muscle is spindle-shaped: wide in the middle and tapering towards the tendons at each end. The thick middle part is the belly. Biceps brachii is the familiar example, the muscle whose bulge you can see and feel when you flex it.
Its fibres run broadly along the long axis of the muscle, meaning that they lie in the same direction as the muscle itself, so a fusiform muscle is a type of parallel muscle. Fibre length sets how far a muscle can shorten. Long fibres that run the length of the muscle can shorten over a large distance, so the muscle tends to give a wide range of movement at the joint.
The other side of the trade-off is force. Force depends on the physiological cross-sectional area, the combined cross-section of the fibres, and not on how bulky the belly looks. A pennate muscle has fibres that attach at an angle to a tendon, like the barbs of a feather, so more of them fit into the same volume. It makes more force for its size, but its tendon moves less far. For its size, then, a fusiform muscle tends to give less force than a pennate one.
To sort a muscle, first ask which way its fibres run: along the muscle (parallel or fusiform), at an angle to a tendon (pennate), in a ring (a circular muscle, which acts as a sphincter) or converging from a broad attachment on to a narrow tendon (a triangular or convergent muscle). If they run along the muscle, ask next whether the belly is spindle-shaped. Fibres that run along the muscle, together with a spindle-shaped belly, mark it out as fusiform.
Key facts
- Shape
- wide in the middle, with a thick belly that tapers towards the tendon at each end.
- Fibre direction
- broadly along the long axis of the muscle, which makes it a type of parallel muscle.
- Range
- long fibres can shorten over a large distance, so the muscle tends to give a wide range of movement.
- Force trade-off
- for its size it tends to give less force than a pennate muscle.
- What sets force
- the physiological cross-sectional area, not how much the belly bulges.
- Example
- biceps brachii.
Exam traps
- A feather-like muscle with fibres attaching to both sides of a central tendon is a bipennate muscle, not a fusiform one. Packing many angled fibres onto a tendon suits force rather than range, the reverse of the fusiform trade-off.
- A thin, strap-like muscle that keeps one width along its whole length has fibres in the same direction as a fusiform muscle, so it is a near miss. The difference is shape: a fusiform muscle bulges into a belly and tapers at both ends.
- Fibres arranged in rings make a circular muscle, which acts as a sphincter. Contraction narrows an opening rather than pulling two attachments together, which is not the fusiform pattern.
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