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Biomechanics Lecture 03

Glenohumeral rhythm and Loads on Shoulder

Glennohumeral rhythm

Movements of the Shoulder Complex

Humerus movement usually involves some movement at all three shoulder joints

Elevation of humerus is accompanied by 55˚ of lateral rotation

Co-ordination of scapular and humeral movement is known as scapulohumeral rhythm

Scapulohumeral rhythm

A regular pattern of scapular rotation that accompanies and facilitates humeral abduction

u  As the arm is elevated in both abduction and flexion, rotation of the scapula accounts for part of the total humeral range of motion.

u  During about the first 30° of humeral elevation, the contribution of the scapula is only about one-fifth that of the glenohumeral joint.

u  As elevation proceeds beyond 30°, the scapula rotates approximately 1° for every 2° of movement of the humerus.

u  scapulohumeral rhythm, enables a much greater range of motion at the shoulder than if the scapula were fixed.

Movements of the Shoulder Complex

u  35˚-45 ˚ clavicular elevation during first 90˚ of arm elevation (sternoclavicular joint).

u  Rotation of acromioclavicular joint:

  • during first 30 degree of humeral elevation
  • from 135 degree to max.

Muscles of the Scapula

Levator scapula, rhomboids, serratus anterior, pectoralis minor, subclavius, and parts to trapezius.

Functions:

  1. Stabilize the scapula when shoulder complex is loaded
  2. Move and position the scapula to facilitate movement at glenohumeral joint

LOADS ON THE SHOULDER

u  Because the glenohumeral joint provides direct mechanical support for the arm, it sustains much greater loads than the other shoulder joints.

u  when analyzing the effect of body position, we may assume that body weight acts at the body’s center of gravity.

u  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

u  When the elbow is in flexion, the effects of the upper arm and the forearm/hand segments must be analyzed separately

u  Glenohumeral joint provides direct mechanical support to arm, it sustains much load as compare to other shoulder joints.

u  Horizontally Extended arm creates larger torque at shoulder than flexed arm.

u  Large torque must be countered by shoulder muscles

u  When muscles contract to support the extended arm, glenohumeral joint sustains compressive forces estimated 50% of bodyweight.

u  load is reduced by about half when the elbow is maximally flexed due to the shortened moment arms of the forearm and hand.

u  Because of the effect of arm position on shoulder loading, ergonomists recommend that workers seated at a desk or a table attempt to position the arms with 20° or less of abduction and 25° or less of flexiion.

u  Workers who are required to hold the arms in a sustained position overhead are particularly susceptible to degenerative tendinitis in the biceps and supraspinatus

u  During arm elevation deltoid produces upward shear force countered by downward shear force produced by rotator cuff.

u  Maximum shear with arm elevation up to 60˚ approximately.

For efficient flexion or abduction of the arm, the deltoid muscle and the rotator cuff work together. In the early stages of abduction and flexion through 90°, the rotator cuff applies a force to the humeral head that keeps the head depressed and stabilized in the joint while the deltoid muscle applies a force to elevate the arm.

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