To investigate the in vivo effects of forearm rotation and selective isometric wrist muscle activation on the dorsal scapholunate (SL) interval using dynamic ultrasonography, and to determine whether previously described cadaveric biomechanical mechanisms are reproducible in healthy subjects.

Twenty-four asymptomatic adults underwent ultrasonographic assessment of the dorsal SL interval under six experimental conditions combining forearm position (maximal pronation or maximal supination) and contraction state (rest, resisted isometric radial deviation, resisted isometric ulnar deviation). Measurements were obtained using a standardized dynamic ultrasound protocol. Intra-rater reliability was assessed using ICC(3,1). A 2 × 3 repeated-measures ANOVA with Bonferroni-adjusted planned contrasts was performed.

Ultrasound measurements demonstrated good-to-excellent reliability (ICC = 0.82–0.99). Significant main effects were identified for both forearm position (F(1,23) = 20.15, p <  0.001, η ² p = 0.467) and contraction condition (F(1.32,30.37) = 140.40, p  <  0.001, η ² p = 0.86) on the dorsal SL interval. Resisted isometric radial deviation significantly decreased the SL interval compared with rest (p  <  0.0001), whereas resisted isometric ulnar deviation significantly increased it (p  <  0.0001). Maximal forearm supination resulted in significantly greater SL widening than maximal pronation (p = 0.0002). No significant interaction effect was observed between forearm position and contraction condition.

Selective wrist muscle activation and forearm rotation significantly influence the dorsal SL interval in vivo. Radial deviation appears to exert a protective stabilizing effect on the SL complex, whereas ulnar deviation increases SL separation, likely through ECU-mediated intracarpal pronation. These findings support rehabilitation strategies emphasizing activation of intracarpal supinator muscles while minimizing ECU recruitment during conservative or postoperative management of SL instability.

III.