Background: Movement dysfunction of the scapula has been recognized as a contributor to many shoulder disorders. The strength of the scapular stabilizing muscles has been determined to be a factor in controlling the position and movements of the scapula. However, few reports are available on the realiability of objective measures for scapular muscle strength. Objective: To determine the reliability of four new methods of measuring the maximal isometric strength of key scapular stabilizing muscles for the actions of protraction and retraction, both with and without the involvement of the glenohumeral (GH) joint. Methods: The Isobex stationary tension dynamometer was used to measure the maximal isometric force on thirty healthy females. Three measures were taken for each method that was sequentially randomized for three separate testing sessions on three non-consecutive days. Results: Intraclass correlations for intrasession reliability and for intersession reliability ranged from 0.95 to 0.98, and 0.94 to 0.96 respectively. The standard errors of measurement were narrow. Scatter grams for both protraction and retraction testing methods demonstrated a signficant relationship, 0.92 for protraction, and 0.93 for retraction. The Bland-Altman plots indicated good agreement between the two methods for measuring protraction strength but a weaker agreement for the two methods measuring retraction strength. Conclusion: The four new methods assessed in this study indicate reilable options for measuring scapular protraction or retraction isometric strength with or without involving the GH joint for young healthy females.
