This study examined reliability and validity of a hand-held isometric strength assessment protocol for the shoulder. Two hundred and eleven elite athletes from four sports completed up to 11 isometric strength tests using a hand-held dynamometer. Inter-repetition and inter-tester reliability were assessed via intraclass correlation coefficients (ICC), mean absolute errors and mean absolute percentage errors, while agreement with isokinetic dynamometry was evaluated in a subsample using concordance correlations (n = 15). Inter-repetition reliability is "good" to "excellent" (ICC = 0.857-0.956), and inter-tester reliability ranged from "moderate" to "very good" (ICC = 0.548-0.944). Isometric strength shows good agreement with moderate speed (60 deg/s) isokinetic measurements, whereas bilateral differences demonstrate only moderate concordance. Principal component analysis reveals three main factors, indicating potential redundancy among tests. The findings support the protocol`s use for field-based monitoring of shoulder strength in elite athletes, with a shortened version recommended for improved test efficiency and athlete compliance.
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