Muscles Acting on the Shoulder and the Arm
The four rotator cuff muscles—infraspinatus, supraspinatus, subscapularis, and teres minor—hold the head of the humerus in place in a spot called the glenoid cavity. Together they stabilize the shoulder, and each enables particular humerus motions: rotating, extending laterally, resisting displacement when bearing weight, abducting, and adducting.
Muscles Acting on the Forearm
Muscles acting on the forearm determine four varied motions: flexion and extension of the forearm at the elbow, and supination and pronation of the forearm and the hand. The biceps brachii, brachialis, and brachioradialis muscles flex the forearm. The biceps brachii is a prominent two-headed muscle visible on the front of the upper arm. Its heads have separate points of origin, the upper glenoid cavity and the scapula's coracoid process. It inserts at a tuberosity of the forearm's radius bone. The deeper brachialis muscle originates at the humerus and inserts at the forearm's ulna bone. The brachioradialis bulges in the forearm, attaching from the humerus to the styloid process of the radius. The triceps brachii and anconeus muscles extend the forearm. The triceps brachii originates at three points, one at the scapula and two at different spots of the humerus. It inserts at the ulna's olecranon process. The anconeus is in the forearm, from the lateral epicondyle of the humerus to the posterior ulna.The supinator muscle and biceps brachii supinate the forearm. A supinator is a muscle responsible for rotating the forearm so that the palm faces upward or toward the front. The supinator originates at the lateral epicondyle of the humerus and ulna. It inserts at the radius. The pronator teres and the pronator quadratus are the primary pronators of the forearm. A pronator is a muscle that rotates the arm so that the palm faces downward or toward the back. The pronator quadratus attaches from the distal ulna to the distal radius. The pronator teres originates at the medial epicondyle of the humerus and coronoid process of the ulna and inserts at the radius. Innervation of the muscles acting on the forearm is provided primarily by the radial nerve and musculocutaneous nerve.
Supination and Pronation of the Forearm
Muscles Acting on the Wrist and Hand
Hand motions are controlled by extrinsic muscles in the forearm connected to the hand by tendons and by muscles intrinsic to, or inside, the hand. The extrinsic muscles also act on the wrist. Muscles in the forearm are divided by connective tissue into anterior and posterior groups. Anterior group muscles are innervated by the median and ulnar nerves. The radial nerve innervates muscles in the forearm's posterior group. Wrist flexion is achieved by three anterior group muscles, the flexor carpi radialis, flexor carpi ulnaris, and the flexor digitorum superficialis. These each originate fully or in part at the humerus. The flexor carpi radialis inserts at the hand's second and third metacarpal bones and the flexor carpi ulnaris at the wrist bone called the pisiform. The flexor digitorum superficialis inserts at the middle phalange, or finger bone. Wrist extension is accomplished by posterior group muscles: primarily the extensor digitorum, plus the extensor carpi radialis longus, the extensor carpi radialis brevis, and the extensor carpi ulnaris. All originate at the humerus. The extensor digitorum inserts at bases of fingers 2 to 5; the other three insert at the bases of specific metacarpal bones 2 to 5. Wrist abduction is determined by the flexor carpi radialis, the extensor carpi radialis longus, the extensor carpi radialis brevis, and the abductor pollicis longus. The abductor pollicis longus is a posterior group muscle attached from the posterior ulna and radius in the forearm to the base of the first metacarpal. Wrist adduction is accomplished by two anterior forearm muscles, the flexor carpi ulnaris and the extensor carpi ulnaris.
In the hand, finger flexion or bending is accomplished by three anterior forearm muscles: the flexor digitorum superficialis, which also flexes the wrist, the flexor digitorum profundus, and the flexor pollicis longus. The flexor digitorum profundus originates at the ulna and inserts at the distal finger bones of digits 2 to 5. The flexor pollicis longus attaches from the radius to the distal portion of the thumb. Four posterior forearm muscles control finger extension, including the extensor digitorum, which also extends the wrist. The extensor pollicis longus originates at the ulna and inserts at the distal thumb phalanx. The extensor pollicis brevis attaches from the radius to the thumb's proximal phalanx. The extensor indicis originates at the ulna and inserts at the second digit.Intrinsic hand muscles allow for delicate and precise finger movements. These muscles are divided into three groups. The hypothenar group (opponens digiti minimi, flexor digiti minimi brevis, abductor digiti minimi) lies at the base of the little finger, the fleshy thenar group (opponens pollicis, adductor pollicis, flexor pollicis brevis, abductor pollicis brevis) lies at the base of the thumb, and the midpalmar group (interosseous and lumbrical muscles) is located between the two. Thumb opposition is achieved by the opponens pollicis muscle. The adductor pollicis adducts the thumb, pulling it toward the other fingers. The flexor pollicis brevis flexes the thumb, while the abductor pollicis brevis abducts it. In the hypothenar group, the opponens digiti minimi pulls the little finger forward and rotates it to oppose the thumb. The abductor digiti minimi abducts the little finger, pulling it away from the other fingers. The flexor digiti minimi flexes the little finger at the metacarpophalangeal joint, which is the knuckle joint just below the base of the little finger. Eleven midpalmar group muscles are further divided by function into three subgroups. Four dorsal interosseous muscles attach to the metacarpal bones and abduct or spread the fingers. Three palmar interosseous muscles arising from the metacarpal bones adduct the fingers (move them together). Four lumbrical muscles flex the metacarpophalangeal joints—the proximal knuckles—and extend the distal knuckles, the interphalangeal joints.