In Vivo Long Head of the Biceps Tendon Stiffness Varies with Forearm Position During Active Contraction: Implications for Personalized Rehabilitation After SLAP Lesions

: Forearm position selectively influences LHB tendon tension during active, unresisted contraction. Forearm orientation affected LHB tendon stiffness primarily during active, unweighted contraction, where pronation resulted in higher stiffness than supination. On the other hand, stiffness outcomes measured during passive and weighted positions were comparable between forearm orientations, indicating that positional effects are most evident when tendon loading is primarily muscle-driven. These findings highlight the relevance of forearm positioning during early postoperative activation and provide normative in vivo reference data to inform personalized rehabilitation strategies and future investigations of postoperative tendon loading following SLAP lesion treatment.