Reproducibility of shoulder short range of motion in isokinetic and isometric strength testing

Strength testing of the shoulder external and internal rotators is an important part of the evaluation and follow-up of a patient with a shoulder injury. Several studies have tested the reproducibility of such testing basing their conclusions mainly on intraclass correlation coefficient (ICC) values. The main goal of our research was to assess fully the reproducibility of isokinetic and isometric strength testing using both absolute (ICC) and relative reproducibility indices. A sample of 21 healthy recreational athletes without any history of shoulder injury was tested twice within a 1-week period using the TechnoGym REV900 isokinetic dynamometer. Testing was performed in a short range of motion (30°) at the angular velocities 30°/second and 60°/second in the concentric and isometric (at an angle of 30°) modes of contraction for both muscle groups. External rotators were tested in the eccentric mode of contraction at both testing velocities. Reproducibility analysis included ICC, repeated measures analysis of variance (ANOVA), standard error of measurement (SEM), smallest real difference (SRD), and Bland and Altman analysis. Mean ICC ranged from 0.80 to 0.94; repeated measures ANOVA did not show any difference between the two test sessions (p > 0.05). In general, SEM% ranged from 6.0% to 9.9% and SRD% from 17% to 28%. Based on our findings, we conclude that the strength testing protocol used in our study showed an excellent reproducibility and can be safely used in clinical settings to monitor the strength changes in a group of individuals or in a single individual.