BIOMECHANICS OF THE MOVEMENT OF THE SHOULDER COMPLEX.pptx

The shoulder complex acts as in coordinated fashion to provide the smoothest and greatest range of motion possible of the upper limb.
Combined motion of GH and ST joint of shoulder complex helps in:
Distribution of motion between other two joints.
Maintenance of glenoid fossa in optimal position.
Maintenance of good length tension
Although some amount of glenohumeral motion may occur while the other shoulder articulations remain stabilized, movement of the humerus more commonly involves some movement at all three shoulder joints.

1. MOVEMENTS AT THE SHOULDER COMPLEX DR. NIDHI SHUKLA

2. INTREGRATED FUNCTION OF THE SHOULDER COMPLEX The shoulder complex acts as in coordinated fashion to provide the smoothest and greatest range of motion possible of the upper limb. Combined motion of GH and ST joint of shoulder complex helps in : Distribution of motion between other two joints. Maintenance of glenoid fossa in optimal position. Maintenance of good length tension Although some amount of glenohumeral motion may occur while the other shoulder articulations remain stabilized, movement of the humerus more commonly involves some movement at all three shoulder joints.

3. MOVEMENTS OF THE SHOULDER COMPLEX Elevation of the humerus in all planes is accompanied by lateral rotation. As the arm is elevated in both abduction and flexion, rotation of the scapula accounts for part of the total humeral range of motion. Although the absolute positions of the humerus and scapula vary due to anatomical variations among individuals, a general pattern persists. This important coordination of scapular and humeral movements, known as scapulohumeral rhythm, enables a much greater range of motion at the shoulder than if the scapula were fixed

4. Scapulothoracic and Glenohumeral Contribution For flexion and abduction(elevation) of the humerus the scapulothoracic joint contributes 60 of range of motion by rotating the glenoid fossa upward while GH joint contributes to 120 flexion and anywhere from 90 to 120 of abduction. The combination of scapula and humeral moment results in 180 degree of elevation with ratio of two 2:1 for GH and ST motion. During initial 60 degree of flexion or 30 degrees of abduction of humerus, the scapula motion is inconsistent with increasing range a scapular contribution reaching to 1:1 with GH ratio. In later phase the GH joint again increases its contribution. This concomitant coordination of GH and ST motion is commonly referred to as scapulo -humeral rhythm. The rhythm also involves the SC and AC joint during 60 degrees of upward rotation of scapula because the ST joint is a part of close chain with SC and AC joint, The rotation of scapula is produced by the force couple of trapezius and serratus anterior muscle which are only muscles capable of upwardly rotating the scapula.

5. Flexion The muscles crossing the glenohumeral joint anteriorly participate in flexion at the shoulder. The prime flexors are the anterior deltoid and the clavicular portion of the pectoralis major. The small coracobrachialis assists with flexion, as does the short head of the biceps brachii. Although the long head of the biceps also crosses the shoulder, it is not active in isolated shoulder motion when the elbow and forearm do not move

6. Extension When shoulder extension is unresisted, gravitational force is the primary mover, with eccentric contraction of the flexor muscles controlling or braking the movement. When resistance is present, contraction of the muscles posterior to the glenohumeral joint, particularly the sternocostal pectoralis, latissimus dorsi, and teres major, extend the humerus. The posterior deltoid assists in extension, especially when the humerus is externally rotated. The long head of the triceps brachii also assists, and because the muscle crosses the elbow, its contribution is slightly more effective when the elbow is in flexion.

7. Abduction The middle deltoid and supraspinatus are the major abductors of the humerus. Both muscles cross the shoulder superior to the glenohumeral joint. The supraspinatus, which is active through approximately the first 110° of motion, initiates abduction. During the contribution of the middle deltoid (occurring from approximately 90° to 180° of abduction), the infraspinatus, subscapularis, and teres minor neutralize the superiorly dislocating component of force produced by the middle deltoid.

8. Adduction As with extension at the shoulder, adduction in the absence of resistance results from gravitational force, with the abductors controlling the speed of motion. With resistance added, the primary adductors are the latissimus dorsi, teres major, and sternocostal pectoralis, which are located on the inferior side of the joint. The short head of the biceps and the long head of the triceps contribute minor assistance, and when the arm is elevated above 90°, the coracobrachialis and subscapularis also assist.

9. SCAPULO- HUMERAL RHYTHM PHASE 1: The upper portion of the trapezius muscle elevates the clavicle and the lower portion of the trapezius muscle produce an upward rotatory force on scapula (middle portion of trapezius may also involve) . It would appear that rotation of the scapula is occurring at AC joint but it is prevented by coracoclavicular ligament so the movement occurs at next possible SC joint. The clavicular elevation leads the scapula through 30° of upward rotation as the scapula rides on the lateral end of the rising clavicle while maintaining a relatively fix scapula clavicular angle. Elevation of the clavicle is checked when costoclavicular ligament becomes taut because the ST upward rotation and clavicular elevation occur concurrently with GH motion. The GH joint can be expected under normal condition to simultaneously flexes or abduct about 60 ° so that there will be total 90°-100 ° of the elevation of the arm.

10. SCAPULO- HUMERAL RHYTHM PHASE 2: As the two muscles that is the trapezius and serratus anterior are continue to generate rotatory force while the elevation at SC joint is checked by costoclavicular ligament and upward rotation at AC joint is still restrained by coracoclavicular ligament with no other available motion to dissipate the upward rotatory force treated by trapezius and serratus muscle. Tension in the coracoclavicular ligament builds as the coracoid process of the scapula gets pulled downwards and forward causes the clavicle to rotate posteriorly leads 30° of upward rotation of scapula because the scapula is attached to the lateral end of clavicle. As the scapula finds it final position on the rib cage the AC joint absorb into varying amount of anterior and posterior tipping of medial and lateral rotation.

11. Medial and Lateral Rotation Medial, or inward, rotation of the humerus results primarily from the action of the subscapularis and teres major, both attaching to the anterior side of the humerus, with the subscapularis having the greatest mechanical advantage for medial rotation. Both portions of the pectoralis major, the anterior deltoid, the latissimus dorsi, and the short head of the biceps brachii assist, with the pectoralis major being the primary assistant. Muscles attaching to the posterior aspect of the humerus particularly infraspinatus and teres minor, produce lateral, or outward, rotation, with some assistance from the posterior deltoid.

12. Horizontal Adduction and Abduction The muscles anterior to the joint, including both heads of the pectoralis major, the anterior deltoid, and the coracobrachialis, produce horizontal adduction, with the short head of the biceps brachii assisting. Muscles posterior to the joint axis affect horizontal abduction. The major horizontal abductors are the middle and posterior portions of the deltoid, infraspinatus, and teres minor, with assistance provided by the teres major and the latissimus dorsi.

13. LOADS ON THE SHOULDER Because the articulations of the shoulder girdle are interconnected, they function to some extent as a unit in bearing loads and absorbing shock. However, because the glenohumeral joint provides direct mechanical support for the arm, it sustains much greater loads than the other shoulder joint. Likewise, when analyzing the effect of the positions of body segments on a joint such as the shoulder, we assume that the weight of each body segment acts at the segmental center of mass. The moment arm for the entire arm segment with respect to the shoulder is therefore the perpendicular distance between the weight vector (acting at the arm’s center of gravity) and the shoulder. When the elbow is in flexion, the effects of the upper arm and the forearm/hand segments must be analyzed separately.

14. LOADS ON THE SHOULDER Although the weight of the arm is only approximately 5% of body weight, the length of the horizontally extended arm creates large segment moment arms and therefore large torques that must be countered by the shoulder muscles. When these muscles contract to support the extended arm, the glenohumeral joint sustains compressive forces estimated to reach 50% of body weight. Although this load is reduced by about half when the elbow is maximally flexed due to the shortened moment arms of the forearm and hand, this can place a rotational torque on the humerus that requires the activation of additional shoulder muscles. Muscles that attach to the humerus at small angles with respect to the glenoid fossa contribute primarily to shear as opposed to compression at the joint. These muscles serve the important role of stabilizing the humerus in the glenoid fossa against the contractions of powerful muscles that might otherwise dislocate the joint.