25/02/2026
ADHESIVE CAPSULITIS (FROZEN SHOULDER) – BIOMECHANICS EXPLAINED
Adhesive capsulitis is primarily a biomechanical disorder of the glenohumeral joint capsule, where inflammation, fibrosis, and capsular contracture progressively reduce joint play. The normally compliant capsule becomes thickened and adherent to the humeral head, drastically limiting arthrokinematic motion. As capsular elasticity is lost, even low-amplitude movements generate high tensile stress, producing pain and protective muscle guarding.
From a joint mechanics perspective, the inferior and anteroinferior capsule are most critically involved. These regions normally allow inferior glide of the humeral head during shoulder elevation. In frozen shoulder, capsular tightness blocks this glide, forcing abnormal superior translation of the humeral head. This alters the center of rotation of the shoulder and increases compressive forces on the articular cartilage and subacromial structures, worsening pain during elevation and rotation.
Rotational biomechanics are particularly affected. External rotation is usually the first and most severely restricted movement, due to tightening of the anterior capsule and coracohumeral ligament. Loss of external rotation disrupts the normal scapulohumeral rhythm, causing early and excessive scapular elevation and protraction. As a result, surrounding muscles such as the upper trapezius and levator scapulae become overactive, while rotator cuff efficiency declines.
At the muscular level, persistent capsular stiffness leads to altered force–length relationships. Rotator cuff muscles are forced to contract in shortened or mechanically disadvantaged positions, reducing their stabilizing role. The deltoid then produces greater shear forces instead of pure rotation, increasing joint compression and reinforcing the pain–stiffness cycle. This explains why strength loss in adhesive capsulitis is often secondary to biomechanics rather than true muscle weakness.
Functionally, frozen shoulder behaves like a closed, high-resistance system. The joint loses its ability to distribute loads smoothly, so everyday movements such as reaching overhead, grooming, or dressing require compensations from the thoracic spine and scapula. Over time, these compensations may contribute to secondary neck and upper-back discomfort due to abnormal kinetic chain loading.
The biomechanics of adhesive capsulitis revolve around capsular contracture, restricted joint glide, altered axis of rotation, and disrupted scapulohumeral rhythm. Effective management must therefore focus not only on pain relief but also on restoring capsular mobility, normal arthrokinematics, and coordinated muscle activation to break the cycle of stiffness and dysfunction.