The ExtRA Capacity Test: Reliability, validity and normative data of a new clinical tool for assessing shoulder muscle performance

Abstract

Objectives: Introduce The ExtRA Capacity Test, a measure for assessing shoulder muscle performance. Assess its reliability, validity and present normative scores in a large sample of asymptomatic adults. Design: Cross-sectional observational study with test–retest. Setting: Community. Participants: Volunteers (n = 344, age 20–90 years). Interventions: The ExtRA Capacity Test involves two capacity tests completed to a 30 beats per minute metronome: maximal scapular plane lateral raises to 90° abduction with 2.5 kg of external load, and maximal external rotations in unsupported prone lying with the shoulder at 90° abduction. Reliability was assessed in 30 asymptomatic participants, tested by two raters over two sessions, one week apart, using Bland–Altman analysis to determine mean bias and 95 % limits of agreement (LoA) as measures of error. Criterion validity was evaluated in 20 participants using Pearson correlation to examine the relationship between ExtRA and isokinetic dynamometry measures. A normative dataset was also established from 344 asymptomatic individuals across a range of ages, physical activity levels, and both sexes. Results: The intra-rater and inter-rater agreement for the ExtRA Capacity Test was assessed in a sample of 30 participants. The 95 % LoA for abduction and external rotation measurements ranged from 2.9 to 13.1 repetitions. In a sample of 20 participants, the abduction test showed good/moderate correlation with muscle strength measures but not with the external rotation test. Older age, female sex and not achieving the WHO activity guidelines have a negative impact on ExtRA performance. Conclusions: Within the caveats discussed in this paper, ExtRA can be considered a reasonably reliable tool for assessing shoulder strength and control in a clinical setting. The normative database will help clinicians set rehabilitation or return-to-play targets based on sex, age, and physical activity level. Contributions of Paper: • This study introduces the ExtRA Capacity Test as a reliable tool for assessing shoulder muscle performance in both sporting and non-sporting populations. • The test demonstrates clinically acceptable intra- and inter-rater reliability, with the abduction component showing a strong correlation with strength measures from Isokinetic Dynamometry. • The normative database established in this study facilitates the evaluation of shoulder performance relative to reference values stratified by age, sex, and physical activity level. • Given its high reliability, the ExtRA Capacity Test can be used to monitor performance changes over time, providing valuable insights for clinical practice and rehabilitation planning.