Ch.11 note

Actions of Skeletal Muscles

Skeletal muscles generally do not work in isolation; rather, they work together to produce movements. Muscles are grouped according to their primary actions into three types: agonists, antagonists, and synergists

scapular muscles

four rotator cuff muscles (subscapularis, supraspinatus, infraspinatus, and teres minor) provide the strength and stability of the glenohumeral joint (figure 11.22). These muscles attach the scapula to the humerus

muscle move glenohumeral joint

pectoralis major latissimus dorsi

extend the forearm ( elbow )

triceps anconeus

flex the forearm

brachioradialis biceps brachii brachialis

flex the arm

brachioradialis biceps brachii brachilis

glenohumeral joint

crossed by 11 muscles that insert on the arm (humerus) or forearm (radius and/or ulna) (figure 11.21). The latissimus dorsi is a broad, triangular muscle located on the inferior part of the back. Often, it is referred to as the "swimmer's muscle," because its actions are required for many swimming strokes. It is the prime arm extensor and also adducts and medially rotates the arm. The pectoralis major is a large, thick, fan-shaped muscle that covers the superior part of the thorax. It is the principal flexor of the arm and also adducts and medially rotates the arm The latissimus dorsi and pectoralis major muscles are the primary attachments of the arm to the trunk, and they are the prime movers of the glenohumeral joint. These muscles are antagonists with respect to arm flexion and arm extension. However, these same two muscles work together (synergistically) when performing other movements, such as adducting and medially rotating the humerus The triceps brachii and biceps brachii, discussed in detail with the muscles that move the elbow joint, also participate in the glenohumeral joint. Specifically, the long head of the triceps brachii originates on the infraglenoid tubercle, spans the glenohumeral joint, and helps extend and adduct the arm. The long head of the biceps brachii originates on the supraglenoid tubercle of the scapula and assists in flexing the arm

antagonist

♦ Is a muscle whose actions oppose those of the agonist. For example, if the *agonist* produces *extension*, the *antagonist* produces *flexion*. ♦ The contraction of the agonist lengthens the antagonist, and vice versa. As this movement occurs, the lengthened muscle usually does not relax completely. ♦ Instead, the tension within the muscle being lengthened is adjusted to control the speed of the movement and ensure that it is smooth. ♦ When the triceps brachii acts as an agonist to extend the forearm, the biceps brachii muscle on the anterior side of the humerus acts as an antagonist to stabilize the movement and to produce the opposing action, which is flexion of the forearm.

agonist

♦ also called a prime mover, is a muscle that contracts to produce a particular movement, such as extending the forearm. ♦ The triceps brachii of the forearm is an agonist that causes extension of the forearm

deltoid

♦ functions as a prime abductor of the arm and forms the rounded contour of the shoulder. ♦ deltoid originate from three different points, and these different fiber groups all perform different functions: (1) The anterior fibers flex and medially rotate the arm. (2) The lateral fibers abduct the arm; in fact, the deltoid is the prime abductor of the arm. (3) The posterior fibers extend and laterally rotate the arm. ♦ The coracobrachialis is a synergist to the pectoralis major in flexing and adducting the arm. ♦ The teres major works synergistically with the latissimus dorsi by extending, adducting, and medially rotating the arm.

synergist

♦ is a muscle that assists the agonist in performing its action. ♦ The contraction of a synergist usually either contributes to tension exerted close to the insertion of the muscle or stabilizes the point of origin. ♦ Usually, synergists are most useful at the start of a movement when the agonist is lengthened and cannot exert much power. ♦ Examples of synergistic muscles are the biceps brachii and the brachialis muscles of the arm. ♦ Both muscles work synergistically (together) to flex the elbow joint. ♦ Synergists may also assist an agonist by preventing movement at a joint and thereby stabilizing the origin of the agonist. ♦ In this case, these synergistic muscles are called fixators