muscles of the shoulder

Question 1

deltoid

Answer 1

• origin: lateral clavicle, acromion, the lower lip of spine of the scapula
• insertion: deltoid tuberosity of the humerus
• action: abducts arm
  clavicular portion: medially rotate the arm and help pectoralis major flex arm at shoulder
  spinous portion: laterally rotate and help latissimus dorsi extend arm at shoulder
• principle abductor of the arm but due to poor mechanical advantage it cannot initiate this action; it is assisted by the supraspinatus

Question 2

teres major

Answer 2

• origin: dorsal surface of the inferior angle of the scapula
• insertion: crest of the lesser tubercle of the humerus (medial lip of intertubercular groove)
• action: adducts the arm, medially rotates the arm, assists in arm extension
• inserts beside the tendon of latissimus dorsi, and assists latissimus in its actions

Question 3

supraspinatus (RC)

Answer 3

• origin: supraspinatus fossa
• insertion: greater tubercle of the humerus (highest facet)
• action: abducts the arm (initiates abduction),lateral rotator
• drawes humerus toward glenoid fossa

Question 4

infraspinatus (RC)

Answer 4

• origin: infraspinatous fossa
• insertion: greater tubercle of the humerus (middle facet)
• action: laterally rotates the arm
• braceshead of humerus in glenoid fossa

Question 5

subscapularis (RC)

Answer 5

• origin: costal surface of the scapula (subscapular fossa) and lower lateral border of scapula
• insertion: lesser tubercle of the humerus
• action: medially rotates the arm, assists in extension of the arm, adduction
• prevents anterior displacement of humerus

Question 6

teres minor (RC)

Answer 6

• origin: upper 2/3 of the lateral border of the scapula
• insertion: greater tubercle of the humerus (lowest facet)
• action: laterally rotates the arm
• RC: fixes the head of the humerus in the glenoid fossa during abduction & flexion of the arm