The ExtRA Capacity Test: Reliability and validity study of a new tool for assessing shoulder muscle performance

Abstract

Objectives: The primary objective was to evaluate the reliability of a new tool for assessing shoulder muscle performance: The ExtRA Capacity Test. The secondary objective was to assess whether this tool was a valid measure for assessing shoulder strength. Methods: The ExtRA Capacity test involves two measures: maximal scapular plane lateral raises to 90° abduction with 2.5kg of external load and the maximal number of prone lying unsupported external rotations with the shoulder at 90° abduction. Both capacity tests are completed to a metronome set to 30 beats per minute. 20 asymptomatic participants were sampled on 2 separate sessions, 1 week apart. The ExtRA Capacity Test was completed by the lead researcher and an independent physiotherapist. Shoulder strength was also measured using isokinetic dynamometry. Results: The test showed excellent inter-rater reliability (mean abduction ICC= 0.969, mean external rotation ICC= 0.822, with a 95% CI). Mean intra-rater variability was 3.96± 4.09 for the abduction measure and 1.70± 1.17 for the external rotation measure. Validity was calculated using Pearson correlation coefficient. The abduction measure showed good/moderate correlation for the majority of strength measurements taken using isokinetic dynamometry however the external rotation capacity test did not correlate closely to isokinetic dynamometry strength measures. Conclusion: The abduction component of the ExtRA Capacity Test is a suitable measure for assessing shoulder strength in clinical practice. The external rotation measure is of suitable reliability however if used in clinical practice, it should not be used to assess shoulder strength, instead it may be suitable to assess movement control of the shoulder.