Muscles of the Shoulder Joint

• Shoulder Joint


  several muscles cross each shoulder to insert on the humerus

  most muscles acting on the humerus originate from the pecotral girdle

  however, 2 of them - latissiumus dorsi + pecotralis major - primarily originate on the axial skeleton
• Prime Movers of Arm Movements


  pectoralis major

  latissimus dorsi

  deltoid muscles

  9 in total the remaining 6 r synergists + fixators
• Rotator Cuff Muscles


  4 muscles that make up the rotator cuff:

  supraspinatus

  infraspinatus

  teres minor

  subscapularis

  they originate on the scapula + their tendons blend w the fibrous capsule of the shoulder joint en route to the humerus
• Rotator Cuff Muscles - Structure & Function


  altho the rotator cuff muscles act as synergists in angular + rotational movements of the arm, their main function is to reinforce the capsule of the shoulder joint to prevent dislocation of the humerus

  
• Flexion of the Arm


  muscles that originate anterior to the shoulder joint flex the arm (lift it anteriorly)

  these flexors inc the pectoralis major, the anterior deltoid + coracobrachialis
• Extension of the Arm


  muscles that originate posterior to the shoulder joint extend the arm

  these inc the latissimus dorsi, posterior deltoid + teres major

  the pectoralis + latissimus dorsi = antagonists
• Abduction of the Arm


  the medial deltoid is the prime mover of arm abduction

  it extends over the superior + lateral side of the humerus to pull it laterally
• Adduction of the Arm


  the main arm adductors r the pecotralis major anteriorly + latissimus dorsi posteriorly
• Rotation of the Arm


  lateral + medial rotation of the arm r mainly brought about by the rotator cuff muscles

  
• Pecotralis Major - Structure & Function


  large, fan-shaped muscle covering superior portion of chest

  forms anterior axillary fold

  divided into clavicular + sternal parts

  
• Pectoralis Major - Action


  adducts + medially rotates arm against resistance

  clavicular part assists in felxion when arm is extended

  sternal part assists in extension when arm is flexed w scapula + arm fixed, pulls rib cage upward, helping w forced inspiration, pushing, thorwing + climbing
• Deltoid - Origin & Insertion

  thick, multipennate muscle forming rounded shoulder mass

  origin - lateral 3rd of clavicle, acromion + spine of scapula

  insertion - deltoid tuberosity of humerus

  
• Deltoid - Action


  prime mover of arm abduction when all its fibres contact simultaneously

  antagonist of pectoralis major + latissimus dorsi, which adduct the arm

  if only anterior fibres r active, can act powerfully in flexing + rotating arm medially

  if only posterior fibres r active, causes extension + lateral rotation of arm

  = active during rhythmic arm swining movements while walking
• Latissimus Dorsi - Location & Description

  broad, flat, triangular muscle of lower back (lumbar region)

  extensive superficial origins

  covered by trapezius superiorly

  contributes to the posterior wall of axilla

  
• Latissimus Dorsi - Action

  prime mover of arm extension

  powerful arm adductor

  medially rotates arm

  plays important role in lowering arm in a power stroke - striking a blow, rowing ...

  w arms fixed overhead, it pulls the rest of the body upward + forward - chin-ups
• Subscapularis - Description, Location, Origin & Insertion

  forms part of posterior wall of axilla

  tendon of insertion passes in front of shoulder joint

  rotator cuff muscle

  origin - subscpaular fossa

  insertion - lesser tubercle of humerus

  
• Subscapularis - Action


  prime mover of rotating arm medially

  assisted by pectoralis major

  helps head of humerus in glenoid cavity

  stablises shoulder joint
• Supraspinatus - Description, Location, Origin & Location

  posterior aspect of scapula, deep to trapezius

  rotator cuff muscle

  origin - supraspinous fossa fo scapula

  insertion - superior part of greater tubercle of humerus

  
• Supraspinatus - Action


  initiates abduction of arm

  stabilises shoulder joint

  helps prevent downward dislocation of humerus as when carrying a heavy load
• Infraspinatus - Location, Origin & Insertion

  partially covered by deltoid + trapezius muscles

  rotator cuff muscle

  origin - infraspinous fossa of scapula

  insertion - greater tubercle of humerus posterior to insertion of supraspinatus

  
• Infraspinatus - Action


  rotates arm laterally

  helps hold head of humerus in glenoid cavity, stabilising the shoulder joint
• Teres Minor - Description, Location, Origin & Insertion

  small, elongated muscle

  lies inferior to infraspinatus

  rotator cuff muscle

  origin - lateral border of dorsal scapular surface

  insertion - greater tubercle of humerus inferior to infraspinatus insertion

  
• Teres Minor - Action

  shares same function as infraspinatus:

  rotates arm laterally

  helps hold of humerus in glenoid cavity, stabilising the shoulder joint
• Teres Major - Description & Location

  thick, rounded muscle

  located inferior to teres minor

  helps form posterior wall of axilla, along w latissimus dorsi + subscapularis

  
• Teres Major - Action

  extends

  medially rotates

  adducts arm

  synergist of latissimus dorsi
• Coracobrachialis - Descritpion, Location, Origin & Insertion

  small, cylindrical muscle of the anterior compartment of the upper arm

  origin - coracoid process of scapula

  insertion - medial surface of humerus shaft

  
• Coracobrachialis - Action


  flexes + adducts arm

  synergist of pectoralis major