# Assessment of Rotator Cuff External Rotation: Isometric vs. Isotonic Testing Modes

## TL;DR

This study compares the reliability and outcomes of isometric and isotonic shoulder external rotation strength tests in healthy adults.

## Contribution

The study reports new 5 RM shoulder external rotation strength values and evaluates reliability and gender differences in test outcomes.

## Key findings

- All tests showed excellent reliability with CV from 1.9 to 3.1% and ICC from 0.970 to 0.994.
- Males outperformed females in isometric tests but not in isotonic Seated 5 RM tests.
- Prone isometric strength predicted 52.4% of the variance in Seated 5 RM ER strength.

## Abstract

Objectives: To assess intra-session reliability of isometric and isotonic shoulder external rotation (ER) strength tests and to compare their outcomes. Methods: Thirty-eight healthy subjects (19 females; 19 males; 25.7 ± 6.0 years; 175 ± 9 cm; 70.3 ± 11.4 kg) completed a shoulder ER strength assessment including Prone and Standing ER Isometric tests and Seated 5 repetition maximum (RM) ER tests. Normality was checked with the Shapiro–Wilk test. Reliability was assessed using the CV and ICC (3, k, 95% CI). Linear mixed models examined sex and dominance effects. Correlations and multiple regression tested associations between tests (p < 0.05). Results: All tests performed displayed “excellent” reliability scores (CV from 1.9 to 3.1% and ICC from 0.970 to 0.994). No significant effect of dominance was observed in any strength test. Males showed significantly higher values than females in both Prone (3.8% higher, p < 0.001) and Standing (2.7% higher, p = 0.003) isometric ER strength tests. Prone and Standing isometric tests were moderately correlated (r = 0.62, 95% CI [0.46, 0.74], p < 0.001). A regression model explained 52.4% of the variance in Seated 5 RM ER strength (R2 = 0.524, p < 0.001), with Prone isometric strength emerging as a significant predictor (β = 0.612, p < 0.001). Conclusions: This study provides previously unreported 5 RM shoulder ER strength values in healthy adults, with all included tests showing excellent reliability. Isometric measures did not fully capture isotonic ER strength. Males outperformed females in isometric tests, but no gender difference was observed in Seated 5 RM strength.