Understanding unilateral scapular dyskinesis in asymptomatic individuals established by the scapular dyskinesis test
Background: Scapular dyskinesis (SDK) is a controversial phenomenon that is thought to be an impaired movement with altered scapular muscle activity requiring intervention. Clinicians of all levels identify SDK via the Yes/No method of the Scapular Dyskinesis Test (Y/N SDT). Purpose & Methods: To date, the YN/SDT has neither been established as reliable nor valid against the electromyography (EMG) when used in healthy individuals. Also, researchers have not examined scapular muscle activity in asymptomatic individuals with SDK. Experiment 1 determined the reliability of the Y/N SDT in individuals with asymptomatic SDK between student and expert physical therapists via an intra- and inter-rater reliability design. Experiment 2 determined the construct validity of the Y/N SDT in symmetrical and asymmetrical asymptomatic individuals using EMG as the reference standard utilizing known-groups validity design. Experiment 3 characterized the scapular muscle activities of asymptomatic unilateral SDK established by the Y/N SDT through repeated measures design. Results: Experiment 1: The Y/N SDT was reliable when used by either students or experts. Students' reliability averaged 20 percentage points less than experts. Experiment 2: The overall accuracy in identifying shoulder asymmetries in asymptomatic individuals against the EMG reference was poor. Sensitivity and specificity were 56% and 36%, respectively; positive and negative predictive values were 68% and 25%; positive and negative likelihood ratios were 0.87 and 1.22. Experiment 3: There was no difference in EMG activities between subjects based on the Y/N SDT. Overall, high muscle variability was observed during the experiments. Conclusion: The Y/N SDT did not appear to have clinical value, therefore, may not be useful in screening SDK in healthy individuals. Hand-dominance may be considered for shoulder rehab wherein the dominant shoulder might respond with endurance exercises while nondominant may benefit from strength training with priority to the serratus anterior muscle. It appears that scapular muscles are likely not synergists as the study failed to find temporal relationships among the muscle activities. Overall, SDK may not be a movement impairment. It may simply be a normal variability that may be ignored or could possibly be a helpful adaptation to achieve shoulder function that should be encouraged. In light of the results of the study, traditional biomechanical theories in understanding SDK did not appear helpful. Exploration of other models like motor control theories in understanding unfamiliar human movements may be considered.
Ramiscal L. Understanding unilateral scapular dyskinesis in asymptomatic individuals established by the scapular dyskinesis test. Augusta University Theses and Dissertations. Published online May 2021. http://hdl.handle.net/10675.2/624021.