Trunk function: the core of mobility performance in wheelchair tennis.

Introduction: Classification in Paralympic sport aims to create a competitive and fair environment by reducing the impact of impairment on the ability to perform sport-specific activities. In wheelchair tennis (WT), current classification criteria largely rely on expert opinion rather than empirical evidence, particularly regarding trunk function. This study investigated the relationship between upper-body strength (arm, trunk) and wheelchair mobility performance in elite WT players to attain evidence-informed classification. Methods: Fifty-one WT players (men, women, and quad division) were assessed during standardized field tests and match play. Upper-body strength was measured using isometric arm and trunk-related force tests, while mobility performance was quantified using inertial sensors, capturing speed, acceleration, and rotational metrics. Associations between strength and mobility performance outcomes were assessed using Pearson/Spearman correlations. Differences between classification-based trunk function groups (0, 1, 2) were explored using T-tests and effect sizes. Results: Correlations between strength and mobility performance were modest (r = 0.26-0.62). Push and pull forces showed the highest associations with sprint and rotational performance (push up to r = 0.54; pull up to r = 0.62). Comparisons between trunk function groups revealed substantial differences, particularly in acceleration and rotation, with large effect sizes (ES = 1.18-2.43) between athletes with full vs. minimal trunk control. Discussion: Trunk function plays a critical role in WT mobility and is underrepresented in the current classification system. Future classification systems should include functional measures that reflect the impact of impairment on sport-specific activities. Particular attention should be given to dynamic trunk movements during acceleration and rotation to ensure a more evidence-based and functionally relevant approach.