The thumb, unlike other fingers, is opposable, in that it is the only digit on the human hand which is able to oppose or turn back against the other four fingers, and thus enables the hand to refine its grip to hold objects which it would be unable to do otherwise. The opposable thumb has helped the human species develop more accurate fine motor skills. It is also thought to have directly led to the development of tools, not just in humans or their evolutionary ancestors, but other primates as well. The thumb, in conjunction with the other fingers make humans and other species with similar hands some of the most dexterous in the world.

There are four types of grips related to the thumb: Pinch, hold and full grasp.

Pinch involves the tips of the thumb and second finger (forefinger/index finger). An example is holding a pill or pinching skin.

Hold involves the thumb, second finger and third finger (middle finger). An example is holding a pencil.

Full Grasp involves the thumb and all fingers. An example is holding barbell or a dumbbell, suitcase, or high bar for chin-ups or pull-ups.

False Grip involves exercise when an athlete takes the thumb out of opposition to the fingers in the grip. The athlete places the thumb under the bar (e.g., in bench press) or over the bar (e.g., in a triceps pressdown). Use of the false grip changes the co-action of muscles that are used to perform an action or exercise. In other words, slightly different muscle action occurs for the purpose of isolating or focusing action on a certain muscle or alleviating stress and strain on certain muscles or joints. Also known as the “suicide grip” because in the bench press, the bar is essentially balanced on the palm of the hands and can roll of suddenly and quickly and fall on the athlete’s head or chest causing injury or death. A spotter or spotters cannot counter this accident.

Thumb movements are controlled by eight muscles (each with “pollicis” in the name):

Name                       Location    Nerve  

extensor pollicis longus   forearm     posterior interosseous n.
abductor pollicis longus   forearm     posterior interosseous n.
flexor pollicis longus     forearm     anterior interosseous n.
extensor pollicis brevis   forearm     posterior interosseous n.
abductor pollicis brevis   hand        median nerve n.
flexor pollicis brevis     hand        median nerve
opponens pollicis          hand        median nerve
adductor pollicis          hand        ulnar nerve (deep branch)

The extensor pollicis longus tendon and extensor pollicis brevis tendon form what is known as the  anatomical snuff box (an indentation on the lateral aspect of the thumb at its base) The radial artery can be palpated anteriorly at the wrist(not in the snuffbox) In the hand, the abductor pollicis brevis, adductor pollicis, flexor pollicis brevis, and opponens pollicis form the thenar eminence,  the body of muscle on the palm of the human hand just beneath the thumb. The skin overlying the thenar eminence is the area stimulated when trying to elicit a palmomental reflex.

The muscles of the area of the thenar eminence are usually innervated by the recurrent branch of the median nerve, except for the adductor pollicis, which is supplied by the deep branch of the ulnar nerve. They all control movement of the thumb.

Adductor pollicis draws the 1st metacarpal laterally to oppose thumb toward center of palm and rotate it medially.

Abductor pollicis brevis abducts the thumb. This means that if the hand was laid flat so the palm faced upwards, this muscle would point the thumb upwards. This muscle is the most proximal of the thenar group.

Flexor pollicis brevis, which lies next to the abductor, will flex the thumb, curling it up in the palm.
Opponens pollicis lies deep to abductor pollicis brevis. As its name suggests it opposes the thumb, bringing it against the fingers. This is a very important movement, as most of our dexterity comes from this action.

The innervation of these muscles by the median nerve is unusual, as most of the intrinsic muscles on the palm of the hand are supplied by the ulnar nerve. The lateral two lumbrical muscles are the other exception.

Another muscle that controls movement of the thumb is adductor pollicis. It lies deeper and more distal to flexor pollicis brevis. Despite its name, its main action is mainly rotation and opposition. It is not in the thenar group of muscles, so is supplied by the ulnar nerve.

Palmomental reflex
When the thenar eminence is stroked briskly with a thin stick, from proximal (edge of wrist) to distal (base of thumb) using moderate pressure. A positive response is considered if there is a single visible twitch of the ipsilateral mentalis muscle (chin muscle on the same side as the hand tested). The palmomental reflex (PMR) is an example of a primitive reflex — present in infancy, but disappears during maturation of the brain during childhood. The PMR may reappear due to processes that disrupt the normal cortical inhibitory pathways.

Marinesco and Radivici in their seminal paper hypothesize that both the afferent (receptive) and efferent (motor) arms of the reflex are on the same side (ipsilateral) to the hand stimulated; however this hypothesis remains unsubstantiated.

The PMR has been found to be present more frequently in various neurological conditions both localized and diffuse. These include congenital conditions such as Down’s syndrome where it is unclear whether the reflex persists throughout life or disappears and then re-appears in association with the onset of Alzheimer’s disease pathology. The reflex is common in the elderly population and should not be taken as indicative of a dementing process

Expressions and idioms regarding the thumbs

all thumbs
Lacking physical coordination, skill, or grace; clumsy … as if the person doesn’t know how to use all fingers to perform a complex hand action or grasp.

rule of thumb
A general guide for determining behavior, a quick calculation or a custom. May have originated from the use of the thumb to measure something in the absences of a ruler. Often used in re
ference to emergencies when there isn’t enough time to diligently and carefully calculate a response.

thumb (one’s) nose
To express scorn or ridicule by or as if by placing the thumb on the nose and wiggling the fingers.
thumbs down or sometimes to just stick one’s nose in the air in disgust or condescension to another person.

thumbs down
An expression of rejection, refusal, or disapproval. A negative vote. A signal of bad performance.

thumbs up
“Way to go” — An expression of approval, success, or hope.

under (one’s) thumb
Under the control of someone; subordinate to and often refers to excessive domination or manipulation; abuse of power.