The forearm muscles can also be divided into flexors, on the side of the palm of the forearm (palmar), and extensors, on the side of the back of the forearm (dorsal). The flexors can also be divided into superficial and deep flexors. The superficial flexors include the pronator teres muscle, the palmaris longus muscle, the radial flexor carpi, the ulnar flexor carpi and the superficilis flexor digitorum.
All originate with at least a portion of the epicondylus medialis (bony protrusion at the lower end of the upper arm) and attach to the forearm or the bones of the hand, thus causing flexion of the forearm. All except the pronator teres extend to the hand and therefore cause flexion of the forearm there as well. The pronator teres, the M. palmaris longus and the M. flexor carpi radialis also cause an inward rotation of the forearm (supination) due to their oblique course from the small finger side at the elbow to the thumb side at the forearm.
The flexor carpi radialis muscle also performs a radial abduction, i.e. the hand is bent towards the radius. The flexor carpi ulnaris muscle performs exactly the opposite movement, namely ulnar abduction (towards the ulna). The M. flexor digitorum superficialis also causes flexion in the metacarpophalangeal and middle joints of the fingers, as it extends to the middle phalanges of fingers 2-5 (all except thumb).
The M. palmaris longus extends to the palmar aponeurosis and tightens it. The deep flexors include the M. flexor digitorum profundus, the M. flexor pollicis longus and the M. pronator quadratus. The M. flexor digitorum profundus pulls from the ulna to the end phalanges of fingers 2-5, thus causing flexion of the wrist and finger joints.
The M. flexor pollicis longus starts at the radius and ends at the thumb end phalanx. It leads to flexion of the thumb joints, opposition (touching the thumb and little finger) and radial abduction. The M. pronator quadratus moves from the ulna to the radius and therefore causes an inward rotation of the forearm (pronation).
The extensors of the forearm can be divided into 3 groups. The radialis group, the superficial extensors and the deep extensors. The radialis group includes the brachioradialis muscle, the extensor carpi radialis longus and brevis.
They all extend from the lateral epicondylus of the humerus (muscle attachment point near the elbow) along the radius. The brachioradialis muscle ends at the lower end of the radius and therefore only affects the forearm. Here it causes the forearm to bend and rotate inwards or outwards.
The other two cause a flexion in the elbow joint and an extension of the hand as well as a radial abduction (towards the spoke) due to their attachment to the metacarpals 2 or 3. The superficial extensors include the extensor digitorum muscle, the extensor digiti minimi muscle and the extensor carpi ulnaris muscle. All begin at the epicondylus lateralis of the upper arm.
The M. extensor digitorum and M. extensor digiti minimi (finger extensors) end in the dorsal aponeurosis of fingers 2-5 and 5 respectively. The extensor carpi ulnaris extends to the middle bone of the little finger. All of them cause an extension of the hand.
The finger extensors additionally cause an extension of the finger joints 2-5 and the extensor carpi ulnaris additionally causes ulnar abduction. The deep extensors are the M. supinator, the M. abductor pollicis longus, the Mm. extensor pollicis longus and brevis and the M. extensor indicis.
The supinator pulls from the epicondylus lateralis to the radius and causes an outward rotation of the arm. The abductor pollicis longus and extensor pollicis brevis begin at the back of the ulna, radius and the membrane between them. The abductor pulls towards the 1st metacarpal and performs hand flexion, radial abduction and extension and abduction (moving the thumb away from the palm).
The extensor ends at the first thumb joint and makes a radial abduction and an extension of the thumb. The M. extensor pollicis longus (thumb extensor) and the M. extensor indici (index finger extensor) originate at the back of the ulna and at the membrane. The thumb extensor pulls towards the end of the thumb joint and provides radial abduction (towards the radius), wrist extension, and extension and adduction of the thumb (pulling the thumb).
The index finger extender ends in the dorsal aponeurosis of the index finger and leads to an extension of the hand and the index finger. The hand muscles can be divided into three groups, the thenar muscles (thumb muscles), the metacarpophalangeal muscles and the hypothenacus muscles (muscles of the little finger). The thenar muscles include the abductor pollicis brevis, opponens pollicis, flexor pollicis brevis and adductor pollicis.
They all start at the palm of the hand and move to different parts of the thumb. The abductor draws to the first thumb link and causes abduction (leading the thumb away from the hand) and opposition (touching the thumb and little finger) in the thumb saddle joint and flexion in the metacarpophalangeal joint. The opponens ends at the first metacarpal bone and causes an opposition, flexion and adduction (leading to the hand).
The flexor pulls towards the first phalanx of the thumb and causes flexion in the metacarpophalangeal joint and opposition. The adductor also pulls to the first thumb limb and causes an adduction, opposition and flexion in the metacarpophalangeal joint. The metacarpal muscles consist of the lumbrical, interosseous palmar and dorsal muscles.
The lumbrical muscles originate at the tendons of the flexor digitorum profundus muscle and move radially to the dorsal aponeurosis of the fingers 2-5 (spoke side). They cause flexion in the basic joints and extension in the finger joints. The Interossei palmares pull from the metacarpals 2,4 and 5 to the dorsal aponeurosis of the fingers 2,4 and 5 and cause flexion in the metacarpophalangeal joints, extension in the finger joints and adduction of the fingers to the middle finger.
The Mm Interossei dorsales begin at the metacarpals 1-5 and end at the dorsal aponeuroses 2 -5, resulting in flexion in the metacarpophalangeal joints, extension in the finger joints and abduction of fingers 2, 4 and 5 away from the middle finger. Hypothenic arm muscles include the abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi and palmaris brevis. They originate either from the palmar aponeurosis or the carpal tunnel.
The abductor ends at the first phalanx of the little finger and causes abduction and flexion in the metacarpophalangeal joint. The flexor also draws to the first phalanx, but only causes flexion in the metacarpophalangeal joint. The Opponens ends at the 5th metacarpal and causes an opposition and a slight flexion of the 5th metacarpal. The M. palmaris moves to the skin of the little finger and its task is to tension the palmar aponeurosis.