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

<b>Objectives</b>: To assess intra-session reliability of isometric and isotonic shoulder external rotation (ER) strength tests and to compare their outcomes. <b>Methods</b>: 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 (<i>p</i> < 0.05). <b>Results</b>: 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, <i>p</i> < 0.001) and Standing (2.7% higher, <i>p</i> = 0.003) isometric ER strength tests. Prone and Standing isometric tests were moderately correlated (r = 0.62, 95% CI [0.46, 0.74], <i>p</i> < 0.001). A regression model explained 52.4% of the variance in Seated 5 RM ER strength (R² = 0.524, <i>p</i> < 0.001), with Prone isometric strength emerging as a significant predictor (β = 0.612, <i>p</i> < 0.001). <b>Conclusions</b>: 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.