The glenohumeral joint (colloquially called the shoulder joint) is the highly mobile ball and socket joint between the glenoid cavity of the scapula and the head of the humerus. Lacking the passive stabilisation offered by ligaments in other joints, the glenohumeral joint is actively stabilised by the rotator cuff, a group of short muscles stretching from the scapula to the humerus. Little inferior support is available to the joint and dislocation of the shoulder almost exclusively occurs in this direction.
The large muscles acting at this joint perform multiple actions and seemingly simple movements are often the result of composite antagonist and protagonist actions from several muscles. For example, pectoralis major is the most important arm flexor and latissimus dorsi the most important extensor at the glenohumeral joint, but, acting together, these two muscles cancel each other's action leaving only their combined medial rotation component. On the other hand, to achieve pure flexion at the joint the deltoid and supraspinatus must cancel the adduction component and the teres minor and infraspinatus the medial rotation component of pectoralis major. Similarly, abduction (moving the arm away from the body) is performed by different muscles at different stages. The first 10° is performed entirely by the supraspinatus, but beyond that fibres of the much stronger pectoralis major are in position to take over the work. Furthermore, to achieve the full 180° range of abduction the arm must be rotated medially and the scapula most be rotate about itself to direct the glenoid cavity upward.
The large muscles acting at this joint perform multiple actions and seemingly simple movements are often the result of composite antagonist and protagonist actions from several muscles. For example, pectoralis major is the most important arm flexor and latissimus dorsi the most important extensor at the glenohumeral joint, but, acting together, these two muscles cancel each other's action leaving only their combined medial rotation component. On the other hand, to achieve pure flexion at the joint the deltoid and supraspinatus must cancel the adduction component and the teres minor and infraspinatus the medial rotation component of pectoralis major. Similarly, abduction (moving the arm away from the body) is performed by different muscles at different stages. The first 10° is performed entirely by the supraspinatus, but beyond that fibres of the much stronger pectoralis major are in position to take over the work. Furthermore, to achieve the full 180° range of abduction the arm must be rotated medially and the scapula most be rotate about itself to direct the glenoid cavity upward.
Elbow joint:
The elbow joint is formed by three bones, the humerus, radius, and ulna. Articulations between the trochlea of the humerus with the ulna and the capitulum of the humerus with the head of the radius comprise the joint. The elbow is an example of a hinge joint, or a joint that moves in only one direction.
Wrist:
composed of the carpal bones, articulates at the wrist joint (or radiocarpal joint) proximally and the carpometacarpal joint distally. The wrist can be divided into two components separated by the midcarpal joints. The small movements of the eight carpal bones during composite movements at the wrist are complex to describe, but flexion mainly occurs in the midcarpal joint whilst extension mainly occurs in the radiocarpal joint; the latter joint also providing most of adduction and abduction at the wrist.
How muscles act on the wrist is complex to describe. The five muscles acting on the wrist directly — flexor carpi radialis, flexor carpi ulnaris, extensor carpi radialis, extensor carpi ulnaris, and palmaris longus — are accompanied by the tendons of the extrinsic hand muscles (i.e. the muscles acting on the fingers). Thus, every movement at the wrist is the work of a group of muscles; because the four primary wrist muscles (FCR, FCU, ECR, and ECU) are attached to the four corners of the wrist, they also produce a secondary movement (i.e. ulnar or radial deviation). To produce pure flexion or extension at the wrist, these muscle therefore must act in pairs to cancel out each others secondary action. On the other hand, finger movements without the corresponding wrist movements require the wrist muscles to cancel out the contribution from the extrinsic hand muscles at the wrist.
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