Scapulohumeral Rhythm Overview

Scapulohumeral rhythm refers to the coordinated movement between the scapula and humerus during shoulder abduction. Scapulohumeral rhythm ratio is 2:1, meaning that for every 2 degrees of humeral movement, the scapula moves 1 degree.

This interaction between the glenohumeral (shoulder) joint and the scapulothoracic joint is critical for full and effective shoulder elevation. Understanding this rhythm is essential for assessing shoulder function, especially in the diagnosis and treatment of shoulder pathologies. In clinical practice, observation of scapulohumeral rhythm helps in evaluating muscular imbalances, joint instability, and range of motion limitations.

During full shoulder abduction (0°–180°), there is a roughly 2:1 ratio between the movement of the humerus and the scapula. Specifically, 120° of movement occurs at the glenohumeral joint (GHJ), and 60° at the scapulothoracic joint (STJ). However, variations exist depending on individual anatomy, speed of movement, and specific shoulder conditions.

When evaluating scapulohumeral rhythm, clinicians should consider using a 3-kg (6.6-lb) weight to better observe dynamic muscle function during active movements. This added load may highlight weaknesses or instabilities that are otherwise subtle during unloaded motion. Moreover, it is critical to assess the rhythm not only during the elevation phase but also during the descent phase, as scapular control issues often manifest more clearly during the latter.

See Also: Shoulder Range of Motion

Scapulohumeral Rhythm Phases

Phase 1 (0°–30° of Abduction): This is often referred to as the “setting” phase. In this stage, the scapula may show minimal movement, slightly rotating in or out, or even remaining stationary. The clavicle may elevate slightly at the sternoclavicular (SC) and acromioclavicular (AC) joints, but overall, there is no established 2:1 movement ratio at this phase.

Phase 2 (30°–90° of Abduction): During this phase, the scapula rotates upward by about 20°, while the humerus elevates by 40°. A 2:1 movement ratio begins to manifest in this phase. The clavicle elevates due to scapular rotation, and posterior tilting of the scapula starts. At the SC and AC joints, retraction and medial rotation occur, allowing the scapulothoracic joint to contribute to the upward rotation.

Phase 3 (90°–180° of Abduction): In the final phase, the 2:1 ratio continues, with further scapular elevation and rotation. The clavicle rotates posteriorly by 30°–50° and elevates up to 15°. During this stage, the humerus undergoes lateral rotation to avoid impingement with the acromion process.

Variability and Clinical Significance

One should note that scapulohumeral rhythm can vary significantly among individuals. Factors such as muscle strength, joint mobility, and speed of movement can influence the exact distribution of movement between the joints. Importantly, in clinical practice, it is more relevant to compare the movement of the affected shoulder with the unaffected side, rather than focusing on exact measurements. Asymmetry in scapulohumeral rhythm often points to underlying pathologies, such as joint instability, muscle weakness, or compensatory mechanisms from previous injuries.

See Also: Scapular Dyskinesis

Abnormal Scapulohumeral Rhythm

In conditions such as frozen shoulder (adhesive capsulitis), abnormal scapulohumeral rhythm is commonly observed. In this case, the scapula moves excessively to compensate for restricted humeral movement, a condition referred to as reverse scapulohumeral rhythm. Instead of a smooth abduction, patients may show shoulder “hiking,” elevating the entire shoulder complex to achieve motion—a compensatory mechanism that sacrifices functional movement quality.

Other pathologies, such as shoulder instability or rotator cuff tears, can alter the normal scapulohumeral rhythm. For example, an instability “jog” or “hitch” may be observed during the descent phase of abduction as the patient loses control over the scapular stabilizers.

References & More

1. Orthopedic Physical Assessment by David J. Magee, 7th Edition.
2. Scibek JS, Carcia CR. Assessment of scapulohumeral rhythm for scapular plane shoulder elevation using a modified digital inclinometer. World J Orthop. 2012 Jun 18;3(6):87-94. doi: 10.5312/wjo.v3.i6.87. PMID: 22720268; PMCID: PMC3377910. Pubmed