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

Background/Objectives: Type II superior labrum anterior–posterior (SLAP) lesions of the long head of the biceps (LHB) tendon are associated with excessive tendon loading and are commonly treated surgically using SLAP repair, tenotomy, or tenodesis. These procedures alter musculotendinous length and loading and may affect functional outcomes, including forearm supination strength. Appropriate restoration of tendon tension is critical for favorable muscle adaptation and recovery. Shear wave elastography (SWE) is a non-invasive imaging technique capable of quantifying tissue stiffness as a surrogate for in vivo musculotendinous tension. This study aimed to characterize LHB tendon tension across forearm positions and loading conditions to improve the understanding of functional tendon loading relevant to postoperative activation and rehabilitation. Methods: In this controlled laboratory study, thirteen healthy female volunteers without shoulder pathology were assessed using SWE with the elbow positioned at 90° flexion. LHB tendon tension was measured in forearm pronation and supination under passive, active (unresisted), and weighted conditions. Paired t-tests were used to compare forearm positions within each loading condition. Results: LHB tendon tension was significantly greater during active and weighted conditions compared with passive loading in the pronated position (p < 0.05). During active contraction, tendon tension was significantly lower in supination than pronation (p < 0.05), whereas no positional differences were observed under passive or weighted conditions. Relative to passive loading, tendon tension increased by approximately 18.2% and 89.2% in supination, and 67.0% and 97.9% in pronation during active and weighted conditions, respectively. Conclusions: 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.