Shoulder Complex Biomechanics

Normal Shoulder Movement: A coordinated timing relationship exists between the scapulothoracic and glenohumeral joints during shoulder abduction.
- Movement typically begins with the glenohumeral joint.
- Scapulothoracic joint movement increases gradually, peaking in the middle of abduction (60-120 degrees).
- Overall ratio: 2:1 (two degrees of glenohumeral motion for every degree of scapulothoracic motion).
Functions of Scapulohumeral Rhythm:
- Orients the glenoid fossa in the direction of movement.
- Maintains optimal length (and force) of the middle deltoid muscle.
  - Without upward scapular rotation, the middle deltoid would be shortened and actively insufficient (weak) at the end range of abduction.

Definition: Elevation of the arm in a plane parallel to the scapula.
Advantage: Minimizes impingement (pinching of tissues) between the humeral head and the acromion process, compared to pure flexion or abduction.

Inferior Glide: The inferior glide of the humeral head during abduction is crucial.
- This glide is NOT automatic.
- It is caused by the downward pull of the rotator cuff muscles.

Definition: Two or more forces pulling in different translational directions but creating torque in the same direction.
Glenohumeral Abduction Example:
- Deltoid: Pulls upward.
- Supraspinatus: Pulls medially.
- Subscapularis, Infraspinatus, and Teres Minor: Pull slightly inferiorly.
- Combined effect: Glenohumeral abduction with downward glide of the humeral head.