![shoulder abduction muscles shoulder abduction muscles](https://images.squarespace-cdn.com/content/v1/5c0446f19d5abbbbab6263ea/1574640970668-F29NXUOCAICV3L6E83LK/ke17ZwdGBToddI8pDm48kFWxnDtCdRm2WA9rXcwtIYR7gQa3H78H3Y0txjaiv_0fDoOvxcdMmMKkDsyUqMSsMWxHk725yiiHCCLfrh8O1z5QPOohDIaIeljMHgDF5CVlOqpeNLcJ80NK65_fV7S1Uf_TT3g97i6_XmO-qcQt4zAfXqdI_5B_HjrxMT8d5xsm3WUfc_ZsVm9Mi1E6FasEnQ/the-rotator-cuff-2696385-FINAL1-474e476cc4554dbd97995610f4402577.png)
Pectoralis major and latissimus dorsi act as antagonists. These muscles include the latissimus dorsi and posterior fibres of the deltoids, with both acting as the prime mover. Muscles that have their origins in the posterior (back of) shoulder joint extend the arm. Pectoralis major is the muscle which acts as prime mover in shoulder flexion.The biceps brachii assist this movement. Muscles that have their origins in the anterior (front of) shoulder joint tend to flex the arm (pectoralis major, coracobrachialis and anterior fibres of the deltoid). Movements possible at the shoulder joint include flexion, extension, horizontal flexion, horizontal extension, abduction, adduction, medial rotation, lateral rotation and circumduction. The last two muscles, the teres major and coracobrachialis, cross the shoulder joint, but do not reinforce it. They also, however, act as synergists for angular or rotational movements of the arm. Their main function is to reinforce the shoulder joint capsule to help prevent dislocation of the humerus. The other six are used as synergists or fixators.įour of the nine muscles, the supraspinatus, infraspinatus, teres minor and subscapularis muscles, are collectively known as rotator cuff muscles. Only three of the nine muscles act as prime movers for arm movements – pectoralis major, latissimus dorsi and the deltoid muscles. All of these muscles have their origin on the shoulder girdle. Stability has to be provided by ligaments and muscles, and a total of nine muscles cross each shoulder joint to insert on the humerus.
![shoulder abduction muscles shoulder abduction muscles](https://www.muscleandmotion.com/wp-content/uploads/2019/07/V2-4.png)
![shoulder abduction muscles shoulder abduction muscles](https://image.slidesharecdn.com/upperlimb-bones-joints-muscles-dr-161220123457/95/upper-limb-bonesjointsmusclesdrbbgosai-22-638.jpg)
The shoulder joint is the most mobile joint in the body, but also one of the most unstable because the shallow cavity gives little support to the head of the humerus. The shoulder joint is a ball and socket joint, with the head of the humerus fitting into a very shallow cavity on the scapula, called the glenoid fossa.