Posted on 21 Nov 2010 22:25
By Ground Up Strength
The muscles that move the hand are divided into two groups, the extrinsic and intrinsic muscles. The extrinsic muscles of the hand are the powerful flexor and extensor muscles of the forearm. These muscles originate outside the hand and insert within the hand. Since they are located and originate on the forearm outside the hand itself, they are called extrinsic which comes from the Latin word extrinsecus meaning "on the outside".
There are also small muscles located within the structure of the hand itself. Since these muscles both originate and insert within the hand, they are referred to as intrinsic muscles. The word intrinsic comes from the latin word intrinsecus. The intrinsic muscles of the hand can be further divided into four groups, the thenar, hypothenar, interossei (dorsal and palmar), and lumbrical muscles.
The intrinsic muscles that control the thumb, unlike those of the fingers, are actually stronger than the extrinsic thumb muscles.
Thenar Muscles of the Hand
The word thenar refers to the palm of the hand and here, specifically to the fleshy muscular area at the base of the thumb on the palm or volar side of the hand. This area and its muscles are called the thenar eminence, the word eminence referring to an anatomical projection or protuberance. The thenar eminence contains three muscles, the abductor pollicis brevis, flexor pollicis brevis, and the opponens pollicis. The bellies of this muscles form a thick, fleshy area directly proximal to the thumb. These are all supplied by the recurrent branch of the median nerve and together their synergistic actions lead chiefly to opposition of the thumb, which is its most important movement.
Opposition involves the action of a number of muscles which results in the metacarpal joint being drawn forwards and medially towards the fingers, starting with abduction and then medial rotation and finally flexion and adduction of the carpometacarpal joint. Opposition brings the thumb into contact with the other fingers which allows precision gripping tasks using pincer grip, one of the single-most important skills of the human hand.
Abductor Pollicis Brevis
The abductor pollicis brevis (APB) is a large, flat and thin muscle and is just under the skin of the lateral part of the thenar eminence. Of the three muscles of the thenar it is the most superficial. It gets its name from its action as an abductor of the thumb. The word abductor comes from the Latin roots ab, meaning "away from" and ducere meaning "to draw" So to abduct means to draw a part of the body away from another part, usually the midline. The word pollicis is derived from the Latin word pollex which refers to the thumb, and from the word brevis which means short. Therefore the muscle's name indicates it is a "short thumb abductor".
Its chief role is to assist in abduction of the thumb which is movement of the thumb perpendicularly away from the palm. In this action it abducts the thumb at both the carpometacarpal and metacarpophalangeal joints. Its action is important in opposition of the thumb. It also assists with with extension of the thumb, which is moving the thumb anteriorly, toward the direction of the back of the hand.
It is difficult to precisely palpate and locate all of the muscles of the thenar eminence due to their tightly packed arrangement. However if movement of the thumb is resisted and the area carefully palpated the abductor pollicis brevis can be felt during abduction. Place your finger on the lateral aspect of the thenar while resisting abduction of the thumb by placing it against the underside of a table. The APB, being the most superficial is the most easily palpated in this fashion.
Abductor Pollicis Brevis Muscle
The APB originates from the front of the flexor retinaculum and onto the ridge of the trapezium and tuberosity of the scaphoid bone. It inserts on the base of the lateral proximal phalanx of the thumb via a short tendon. This muscle is supplied by the median nerve. Among peripheral nerve entrapments, entrapment of the median nerve is the most common which occurs at the wrist. Since this entrapment occurs from swelling and inflammation of the tissues in the "Carpal Tunnel" this entrapment condition is known as Carpal Tunnel Syndrome. During severe and prolonged cases atrophy of the abductor pollicis brevis can occur along with the other muscles of the thenar eminence supplied by the median nerve, severely limiting movement of the thumb.
Flexor Pollicis Brevis
The flexor pollicis brevis (FPB) is the most medial of the three thenar muscles and is located under the abductor brevis. It gets its name from the Latin word flexus, meaning bent, and flexor, meaning that which bends. With the words pollicis and brevis, both discussed above, the name indicates the muscle is a "short thumb flexor".
It causes flexion of the thumb which bends the thumb at both the metacarpophalangeal and carpometacarpal joints. During continued action it produces medial rotation of the thumb since flexion of the carpometacarpal involves this motion.
Flexor Pollicis Brevis Muscle
The FPB has both a superficial and deep part and it is usually partly fused with the opponens pollicis. Its superficial part originates from the distal border of the flexor retinaculum and the tubercle of the trapezium. The deeper part originates from the capitate and trapezoid bones. Both parts converge into a single tendon containing a sesamoid bone which inserts onto the radial side of the base of the proximal phalanx of the thumb.4
To palpate the flexor pollicis brevis place a finger on the medial part of the thenar eminence (closer to the middle of the palm than to the thumb) and, placing your thumb against your thigh to provide resistance, strongly flex it, bending it at both joints. You should be able to feel the FPB contracting. Move your palpating finger around until you can feel the muscle working.
Also covered by the abductor pollicis brevis, the opponens pollicis brevis (OP) is another important muscle in opposition of the thumb. The Latin word opponens means movement against an opposing structure. With pollicis, discussed above, its name indicates it is an "opposer of the thumb", the opposing structure being the fingers.
Opponens Pollicis Muscle
It flexes and abducts the first metacarpal joint with medial rotation at the hand. It originates from flexor retinaculum and tubercle of trapezium bone and inserts onto the entire length of the lateral half of the anterior surface of the first metacarpal. It is extremely difficult to palpate this muscle and differentiate its action from the other thenar muscles.
Hypothenar Muscles of the Hand
The hypothenar eminence contains the muscles on the opposite side of the palm from the thenar. These three muscles control the actions of the little finger or the digiti minimi. They are the abductor digiti minimi, the flexor digiti minimi brevis, and the opponens digiti minimi. The prefix "hypo" is of Greek origin meaning "less than". These muscles are supplied by the ulnar nerve and this part of the palm is also referred to as the ulnar border.
Abductor Digiti Minimi
As the above explanations imply, this muscle gets its name because it is an abductor of the little finger (digiti minimi), drawing the digit away from its neighbors. It also assists with flexion of the fifth metacarpal joint (so flexing the proximal phalanx). This muscle gives the hypothenar eminence its shape, being the most superficial of the three muscles. It can be easily palpated with a finger while resisting abduction of the little finger.
Abductor Digiti Minimi Muscle
The abductor digiti minimi originates on the pisiform bone of the wrist, the pisohamate ligament, and the flexor retinaculum. It inserts on the medial side of the base of the proximal phalanx of the little finger.
Flexor Digiti Minimi Brevis
The flexor digiti minimi brevis is located on the lateral side of the abductor digiti minimi. This muscle is sometimes absent from the hypothenar eminence. It assists in flexion of the metacarpophalangeal joint of the little finger. It originates from the hook of the hamate bone of the wrist and the retinaculum. It inserts, together with the abductor digiti minimi, on the proximal phalanx of the little finger.
Flexor Digiti Minimi Brevis
Opponens Digiti Minimi
The opponens digiti minimi (ODM) is located deep to the abductor digiti minimi and flexor digiti minimi. As its name implies it is a opposer of the little finger, permitting the fifth digit to be brought into opposition with the thumb, which is a combination of abduction, flexion and lateral rotation of the finger. This motion is sometimes also referred to as supination of the little finger. This muscle does not (normally) act on the proximal phalanx of the finger like the other muscles of the hypothenar eminence.
Opponens Digiti Minimi Muscle
The ODM originates on the hook of the hamate bone of the wrist, the adjacent pisohamate ligament, and the adjacent part of the palmar surface of the flexor retinaculum. It is wider distally than proximally, forming a wide insertion along the whole length of the fifth metacarpal bone, on its ulnar (ulnopalmar) surface.
Interossei Muscles of the Hand
The word interossei is a plural form of interosseus which comes from the Latin roots os, for "bone", and inter meaning "between". So the interossei muscles are muscles of the hands that lie between bones, namely the metacarpal bones. There are seven interossei muscles, with two layers: three palmar and four dorsal. These muscles do not have separate names but are simply called Palmar Interossei 1 through 3 and Dorsal Interossei 1 through 4. They are important muscles in moving the fingers, all being supplied by the deep branch of the ulnar nerve.
Palmar Interossei Muscles
There are four palmar interossei muscles between the metacarpals, one going to the thumb, index, ring, and little fingers. These muscles adduct the thumb, index, ring, and little fingers towards the middle finger, or midline of the hand. The first palmar interossei of the thumb also assists the flexor pollicis brevis with flexion of the thumb. The remaining three also assist the lumbricales in flexing the metacarpophalangeal joint and extending both interphalangeal joints.1
Palmar Interossei Muscles
Each of the palmar interossei muscles originate from the metacarpal of the digit that it acts on, the second, fourth, and fifth digits. The first originates from the ulnar side of the second metacarpal bone, inserting onto the proximal phalanx of the index finger. The second arises from the radial side of the fourth inserting on the same side of the ring finger. And the third arises from the radial side of the fifth metacarpal bone, inserting to the same side of the little finger.
Dorsal Interossei Muscles
The dorsal interossei muscles lie superficially between the spaces of the metacarpals. There are four muscle, each originating from two heads, one for each side of the adjacent metacarpal bones of the index, middle, ring and little finger. These muscles abduct the fingers away from the midline of the hand (spread the fingers). They also assist the lumbricals in flexion and extension of the joints.
Dorsal Interossei Muscles
The first dorsal interosseus originates on the radial side of the second metacarpal and the proximal half of the ulnar side of the first metacarpal, inserting on the radial side of the base of the second proximal phalanx (index finger) and the extensor expansion. The second originates from the radial side of the third metacarpal and the ulnar side of the second metacarpal, inserting onto the radial side of the third proximal phalanx (the middle finger) and the extensor expansion. The third arises from the radial side of the fourth metacarpal and the ulnar side of the third metacarpal to insert to the ulnar side of the third proximal phalanx (the middle finger) and the extensor expansion. And the fourth starts from the radial side of the fifth metacarpal and the ulnar side of the fourth metacarpal and inserts onto the ulnar side of the fourth proximal phalanx (the ring finger) and the extensor expansion.
Lumbricals (Lumbricales) Muscles of the Hand
The lumbical muscles get their name form the Greek word lumbricus, which means earthworm. These four slender muscles resemble an earthworm in shape, size and color. Along with the interosseus muscles they flex the fingers at the metacarpophalangeal joints and extending the proximal and distal interphalangeal joints of the fingers to which they insert. These muscles are highly active during any activity requiring active extension of the interphalangeals and are used to place the fingers into the writing position.
Lumbricals (Lumbricales) Muscles of the Hand
The lumbricals originate from the medial three tendons of the flexor digitorum profundus and insert onto the tendons of the extensor digitorum communis in the area of the proximal phalanges.  The fact that these muscles originate from tendons instead of bone makes them unique to the human body. 
Labelled Diagram of Bone and Joints of Hand
Bones and Joints of Hand
image derived from Grays, public domain
For further in-depth reading on the joints, muscles, and movements of the wrist and hand, see Anatomy and Human Movement: Structure and Function by Nigel Palastanga, Derek Field, and Roger Soames
Unless otherwise noted, all images on this page used under license. Images by LifeART (and/or) MediClip image copyright 2010. Wolters Kluwer Health, Inc.- Lippincott Williams & Wilkins. All rights reserved. Images not for reuse.