Muscles Acting on the Forearm

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The elbow and forearm are capable of four motions: flexion, extension, pronation, and supination (table 10.12). The principal flexors are on the anterior side of the humerus and include the superficial biceps brachii45 and

45bi = two + eeps = head + braehi = arm. Note that biceps is singular, there is no such word as bieep. The plural form is bieipites (by-SIP-ih-teez).

deeper brachialis (see fig. 10.22; table 10.13). In flexion of the elbow, the biceps elevates the radius while the brachialis elevates the ulna. The biceps is named for its two heads, which arise from separate tendons at the scapula. The tendon of the long head is important in holding the humerus in the glenoid cavity and stabilizing the shoulder joint. The two heads converge close to the elbow on a single distal tendon that inserts on the radial tuberosity.

The brachioradialis is a synergist in elbow flexion. Its belly lies in the antebrachium (forearm) beside the radius, rather than in the brachium with the other two flexors (see fig. 10.22a). It forms the thick, fleshy mass on the lateral side of the forearm just distal to the elbow. Its origin is on the distal end of the humerus, and its insertion is on the distal end of the radius. Since its insertion is so far from the fulcrum, the brachioradialis does not generate as much power as the prime movers; it is effective mainly when the prime movers have partially flexed the elbow.

The prime mover of extension is the triceps brachii on the posterior side of the humerus (see figs. 10.2 and 10.22).

Saladin: Anatomy & I 10. The Muscular System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

358 Part Two Support and Movement

Table 10.10 Muscles Acting on the Humerus (see figs. 10.22-10.24)

O = origin, I = insertion, N = innervation (n. = nerve, nn. = nerves) Pectoralis (PECK-toe-RAY-liss) Major

Table 10.10 Muscles Acting on the Humerus (see figs. 10.22-10.24)

O = origin, I = insertion, N = innervation (n. = nerve, nn. = nerves) Pectoralis (PECK-toe-RAY-liss) Major

Prime mover of shoulder flexion; adducts and medially rotates humerus; depresses pectoral girdle; elevates ribs; aids in climbing, pushing, and throwing O: clavicle, sternum, costal cartilages 1-6 I: intertubercular groove of humerus N: medial and lateral pectoral nn.

Latissimus Dorsi (la-TISS-ih-muss DOR-sye)

Adducts and medially rotates humerus; extends shoulder joint; produces strong downward strokes of arm, as in hammering or swimming ("swimmer's muscle"); pulls body upward in climbing O: vertebrae T7-L5, lower three or four ribs, thoracolumbar I: intertubercular groove of humerus N: thoracodorsal n. fascia, iliac crest, inferior angle of scapula

Deltoid

Lateral fibers abduct humerus; anterior fibers flex and medially rotate it; posterior fibers extend and laterally rotate it O: clavicle, scapular spine, acromion I: deltoid tuberosity of humerus

N: axillary n.

Teres (TERR-eez) Major

Adducts and medially rotates humerus; extends shoulder joint

O: from inferior angle to lateral border of scapula I: medial aspect of proximal shaft of humerus

N: subscapular n.

Coracobrachial (COR-uh-co-BRAY-kee-AL-iss)

Adducts arm; flexes shoulder joint

O: coracoid process I: medial aspect of shaft of humerus

N: musculocutaneous n.

Rotator Cuff

All rotator cuff muscles hold head of humerus in glenoid cavity and stabilize shoulder joint in addition to performing the functions below.

Infraspinatus (IN-fra-spy-NAY-tus)

Extends and laterally rotates humerus

O: infraspinous fossa of scapula I: greater tubercle of humerus

N: suprascapular n.

Supraspinatus (SOO-pra-spy-NAY-tus)

Abducts humerus; resists downward displacement when carrying heavy weight

O: supraspinous fossa of scapula I: lesser tubercle of humerus

N: suprascapular n.

Subscapularis (SUB-SCAP-you-LERR-iss)

Medially rotates humerus

O: subscapular fossa of scapula I: lesser tubercle of humerus

N: subscapular n.

Teres Minor

Adducts and laterally rotates humerus

O: lateral border of scapula I: greater tubercle of humerus

N: axillary n.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

10. The Muscular System Text

© The McGraw-Hill Companies, 2003

Chapter 10 The Muscular System 359

Deltoid

Triceps brachii Lateral head Long head

Deltoid

Triceps brachii Lateral head Long head

Coracobrachialis

-Clavicle Sternum

Pectoralis major Coracobrachialis

Medial head Biceps brachii Brachialis— Brachioradialis— ll if. */

-Clavicle Sternum

Pectoralis major Coracobrachialis

Scapula Humerus Clavicle Anatomy Biceps

Lateral head Long head

Latissimus dorsi

Supraspinatus

Spine of scapula

Greater tubercle of humerus Infraspinatus

Humerus Teres minor

Teres major

Triceps brachii ill_

Lateral head Long head

Latissimus dorsi

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