Muscle · Hand

Flexor pollicis brevis

Musculus flexor pollicis brevis

A short two-headed muscle of the thenar eminence that bends the thumb at its main knuckle and helps drive it across the palm. It is the exception in the thumb pad: its superficial head answers to the median nerve while its deep head answers to the ulnar, and the balance between them varies from person to person. That shared supply is clinically useful — it is part of why some thumbs retain a little flexion power even when the median nerve is badly compressed at the wrist, and why thumb weakness alone never localises a lesion on its own.