Assessment of scapulohumeral rhythm for scapular plane shoulder elevation using a modified digital inclinometer

Abstract

Aim: To develop a better understanding of scapulohumeral rhythm during scapular plane shoulder elevation.

Methods: Thirteen healthy, college-aged subjects participated in this study. Subjects were free from any upper extremity, neck or back pathology. A modified digital inclinometer was utilized to measure scapular upward rotation of the subject's dominant shoulder. Upward rotation was measured statically as subjects performed clinically relevant amounts of shoulder elevation in the scapular plane. Testing order was randomized by arm position. Scapular upward rotation was assessed over the entire arc of motion and over a series of increments. The percent contributions to shoulder elevation for the scapula and glenohumeral joint were calculated. Scapulohumeral rhythm was assessed and represented the ratio of glenohumeral motion to scapulothoracic motion (glenohumeral elevation: scapular upward rotation). A one-way ANOVA was used to compare scapular upward rotation between elevation increments.

Results: Scapulohumeral rhythm for the entire arc of shoulder elevation was equal to a ratio of 2.34 :1 and ranged from 40.01:1 to 0.90:1 when assessed across the different increments of humeral elevation. Total scapular motion increased over the arc of shoulder elevation. The scapula contributed 2.53% of total motion for the first 30 degrees of shoulder elevation, between 20.87% and 37.53% for 30(o)-90(o) of shoulder elevation, and 52.73% for 90(o)-120(o) of shoulder elevation. Statistically significant differences in scapular upward rotation were identified across the shoulder elevation increments [F((3,48)) = 12.63, P = 0.0001].

Conclusion: Clinically, we must recognize the usefulness of the inclinometer in documenting the variable nature of scapulohumeral rhythm in healthy and injured shoulders.

Keywords: Biomechanics; Kinematics; Scapula; Shoulder.

Figure 1

Modified digital inclinometer (A) and scapular upward rotation measurement (B).

Figure 2

Scapulohumeral rhythm for 15 shoulder elevation increments (A) and 30 degree shoulder elevation Increments (B). Shoulder elevation increments represent degrees of shoulder elevation in the scapular plane. Values listed in the figure represents degrees of glenohumeral motion for every 1^o of scapular upward rotation.

Figure 3

Glenohumeral and scapular upward rotation contributions to shoulder elevation.