Inman, Saunders and Abbott were the first to measure scapulohumeral rhythm (also referred to as Glenohumeral-GH Rhythm) using radiography and suggested what became the widely accepted 2:1 ratio between glenohumeral elevation and scapulothoracic (ST) upward rotation (SUR). Since then imaging modalities (X-ray and magnetic resonance imaging), cinematography , goniometry   , and more recently 3-dimensional tracking systems  have been used to gain a better appreciation of shoulder kinematics.
The scapula on the thorax contributes to elevation (flexion and abduction) of the humerus by upwardly rotating the glenoid fossa 50° to 60° from its resting position. If the humerus were fixed to the fossa, this alone would result in up to 60° of elevation of the humerus. The humerus, of course, is not fixed but can move independently on the glenoid fossa.
The overall ratio of 2of GH to 1of ST motion during arm elevation is commonly used, and the combination of concomitant GH and ST motion most commonly referred to as scapulo-humeral rhythm. According to the 2-to-1 ratio frame-work, flexion or abduction of 90 in relation to the thorax would be accomplished through approximately 60of GH and 30of ST motion. Thus,
- The GH joint contributes 100° to 120° of flexion and 90° to 120° of abduction.
- The combination of scapular and humeral movement results in a maximum range of elevation of 150° to 180°. 
- During the initial 60of flexion or the initial 30° of abduction of the humerus, Inman and coworkers reported an inconsistent amount and type of scapular motion in relation to GH motion. 
- The scapula has been described as seeking a position of stability in relation to the humerus during this period (setting phase). 
- In this early phase, motion occurs primarily at the GH joint, although stressing the arm may increase the scapular contribution. 
- It must also be recognized, however, that elevation of the arm is often accompanied not only by elevation of the humerus but also by lateral rotation of the humerus in relation to the scapula.
- During abduction of the humerus in the plane of the scapula, an average of 43° of lateral rotation from the resting position has been reported, with peak lateral rotation generally occurring between 90° and 120° of humeral elevation. 
Observation and examination of the scapulohumeal rhythm is commonly performed by physical therapists during postural and shoulder examinations. The notion of a proper "rhythm" is routinely used to describe the quality of movement at the shoulder complex.
- Alterations in scapular position and control afforded by the scapula stabilizing muscles are believed to disrupt stability and function of the glenohumeral joint   , thereby contributing to shoulder impingement, rotator cuff pathology and shoulder instability.
- Given the role of the scapula in shoulder function, the ability to monitor the coordinated motion of the scapula and humerus, or scapulohumeral rhythm,  may have clinical implications when dealing with overhead athletes and patients with shoulder pathologies.
Variations In Scapulohumeral Rhythm
- Debate continues about the exact nature of this relationship in terms of static vs. dynamic pattern differences, the effects of various conditions such as external resistance, and the causal associations of "rhythm" dysfunction to specific shoulder pathologies
- A number of studies have investigated this “rhythm,” with ratios reported varying between 1.25:1 and 2.69:1.
- Ratios are often described as nonlinear, indicating changing ratios during dif-ferent portions of the ROM for elevation of the arm. The rhythm varies among individuals and may vary with external constraints.
- Some of the variability in ranges reported by investigators is due to individual structural variations (especially for the GH joint); another factor in variability may be the extent to which trunk contributions were isolated from humeral motions during the measurement.
- ↑ 1.0 1.1 Kibler WB. The role of the scapula in athletic shoulder function. Am J Sports Med 1998;26:325-337
- ↑ Cathcart CW: Movements of the shouLder girdle involved in those of the arm on the trunk. J Anat Physiol 1884; 18:209-218
- ↑ Cleland J: A lecture on the shoulder girdle and its movements. Lancet 1881;1:11-12.
- ↑ Codman EA: The Shoulder,Boston: G.Miller &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp; Company,1934
- ↑ Inman VT, Saunders JB, Abbott LC. Observations of the function of the shoulder joint. 1944. Clin Orthop Relat Res 1996; 330: 3-12
- ↑ Poppen NK, Walker PS. Normal and abnormal motion of the shoulder. J Bone Joint Surg Am 1976;58: 195-201
- ↑ Bagg SD, Forrest WJ. A biomechanical analysis of scapular rotation during arm abduction in the scapular plane. Am J Phys Med Rehabil 1988; 67: 238-245
- ↑ Doody SG, Freedman L, Waterland JC. Shoulder movements during abduction in the scapular plane. Arch Phys Med Rehabil 1970; 51: 595-604
- ↑ Doody SG, Waterland JC, Freedman L. Scapulo-humeral goniometer. Arch Phys Med Rehabil 1970; 51:711-713
- ↑ Johnson MP, McClure PW, Karduna AR. New method to assess scapular upward rotation in subjects with shoulder pathology. J Orthop Sports Phys Ther 2001;31: 81-89
- ↑ An KN, Browne AO, Korinek S, Tanaka S, Morrey BF. Three-dimensional kinematics of glenohumeral elevation. J Orthop Res 1991; 9: 143-149
- ↑ Johnson G, Stuart P, Mitchell S. A method for the measure-ment of three-dimensional scapular movement. Clin Biomech 1993; 8: 269-273
- ↑ Meskers CG, Fraterman H, van der Helm FC, Vermeulen HM, Rozing PM. Calibration of the “Flock of Birds” elec-tromagnetic tracking device and its application in shoulder motion studies. J Biomech 1999; 32: 629-633
- ↑ McClure P: Direct 3-dimensional measurement of scapular kinematics during dynamic movements in vivo.J Shoulder Elbow Surg 2001;10:269–277.
- ↑ Rundquist P, Anderson DD, Guanche CA, et al. Shoulder kinematics in subjects with frozen shoulder. Arch Phys Med Rehabil 2003; 84:1473–1479.
- ↑ Barnes CJ, Van Steyn SJ, Fischer RA: The effects of age, sex, and shoulder dominance on range of motion of the shoulder. J Shoulder Elbow Surg 2001; 10:242–246.
- ↑ Inman B, Saunders J, Abbott L: Observations of function of the shoulder joint. J Bone Joint Surg Br 2004; 26:1.
- ↑ Dvir Z, Berme N: The shoulder complex in elevation of the arm: A mechanism approach. J Biomech 1978; 1:219
- ↑ Doody S, Waterland J: Shoulder movements during abduction in the scapular plane. Arch Phys Med Rehabil 1970; 51:595.
- ↑ Ludewig P, Cook T: Alterations in shoulder kine-matics and associated muscle activity in people with symptoms of shoulder impingement. Phys Ther 2000; 80:276–291.
- ↑ McQuade KJ,Smidt GI. Dynamic Scapulohumeral Rhythm: The Effects of External Resistance During Elevation of the Arm in the Scapular Plane. J Orthop Sports Phys Ther 1998; 27(2): 125
- ↑ Itoi E. Scapular inclination and inferior stability of the shoulder. J Shoulder Elbow Surg 1992;1:131-139
- ↑ Weiser WM, Lee TQ, McMaster WC, McMahon PJ. Effects of simulated scapular protraction on anterior glenohumeral stability. Am J Sports Med 1999; 27: 801-805
- ↑ Ludewig PM, Reynolds JF. The association of scapular kinematics and glenohumeral joint pathologies. J Orthop Sports Phys Ther 2009;39: 90-104
- ↑ Codman E. Chapter II: Normal motions of the shoulder.Boston, MA 1934, 32-63
- ↑ Lockhart RD. Movements of the Normal Shoulder Joint and of a case with Trapezius Paralysis studied by Radiogram and Experiment in the Living. J Anat 1930; 64: 288-302
- ↑ McQuade K, Smidt G: Dynamic scapulohumeral rhythm: The effects of external resistance during elevation of the arm in the scapular plane. J Orthop Sports Phys Ther 1998; 27:125–133.
Learn about the shoulder in this month's members learn topic with 5 chapters from textbooks such as Magee's Orthopedic Physical Assessment, 2014 & Donatelli's Physical therapy of the shoulder 2012.