Patients with neck pain and discomfort may complain of pain in the thumb region with difficulties in moving the painful thumb muscles.
Bending the tip of the thumb involves primarily the flexor pollicis longus muscle.
The best way to test the action of this muscle is to have the thumb at the side of the index finger in a position of adduction. Now if you bend the tip of the thumb, the motion of the flexor pollicis longus is isolated.
This muscle is important because it can to aid as an adductor of the thumb, when the adductor pollicis is not functioning due to injury to the ulnar nerve.
In a situation where the adductor pollicis is weak, if a piece of paper is placed between the thumb and index finger, you can easily pull away the piece of paper. Normally, the action of the adductor pollicis will be strong enough to prevent the piece of paper from being pulled out. However, when the flexor pollicis longus starts to contract to bend the tip of the thumb, you cannot easily pull out the piece of paper. This is called Froment’s sign.
The flexor pollicis longus has now substituted to perform the action of the adductor pollicis. The action of the flexor pollicis longus can also be aided by the action of the flexor pollicis brevis and the abductor pollicis brevis.