Biomechanics of the thumb
*Structure and function of the MP and IP joints are similar to those of the other digits.
The thumb also has an extensor mechanism.
Charles Eaton MD Thumb home page
Clinical Kinesiology lists nine thumb muscles:
Charles Eaton MD
hand muscles page
Review: innervation of thumb muscles
courtesy of Charles Eaton MD
Synergies that prevent shortening of the thumb's multi-articular extrinsic muscles
- Rapid flexion of the thumb's IP joint involves the flexor pollicis longus (FPL), and is often accompanied by palpable activity in the abductor pollicis longus. The first CMC joint may even abduct. The APL activity counters FPL's tendency to flex the CMC joint in the plane of the palm.
- Rapid extension of the thumb's IP joint involves the extensor pollicis longus (EPL), and is often accompanied by palpable activity in the thenar muscles. The first CMC joint may even flex in the plane of the palm. Activity in the opponens pollicis counters the EPL's tendency to extend the first CMC joint or adduct it in a plane perpendicular to the palm.
- The abductor pollicis longus (the APL, a snuffbox muscle) acts in synchrony with any action of the thenar muscles. Its activity stabilizes the first metacarpal so that the thenar muscles do not produce movement at the first CMC joint. Since the APL also produces radial deviation of the wrist, the extensor carpi ulnaris (ECU) may also act to prevent wrist motion.
Extensor Mechanism of Thumb
The first digit's extensor mechanism receives fibers from the:
- abductor pollicis brevis (median nn.)
- adductor pollicis (ulnar nn)
- ulnar head of flexor pollicis brevis / first palmar interosseous (ulnar nn)
- extensor pollicis longus (radial nn.)
Because a variety of nerves supply these muscles, thumb extension is possible even if one of the peripheral nerves is injured.
This arrangement permits the kind of balance between flexion and extension in the thumb that the lumbricales provide in the other digits.
Last updated 3-12-01 ©Dave Thompson PT
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