Relationships Between Isometric Belt Squat Force and Vertical Jump Performance Across Knee Joint Angles.

Rebelo, A, Ripley, N, Weakley, J, and Comfort, P. Relationships between isometric belt squat force and vertical jump performance across knee joint angles. J Strength Cond Res XX(X): 000-000, 2026-The aim of this study was to examine the relationships between vertical jump performance and isometric belt squat force assessed at 2 knee joint angles (120° and 90°) and to compare outcomes across age groups in male and female athletes. One hundred and twenty subjects (20.3 ± 6.5 years, 1.77 ± 0.12 m, 71.0 ± 13.3 kg) performed countermovement jumps, squat jumps, and isometric belt squat tests at 120° and 90° on dual force platforms. Between-group differences were analyzed using 1-way or Welch's analysis of variance with Bonferroni-adjusted post hoc analyses. Associations were examined using Pearson's correlations with Bonferroni-adjusted significance thresholds (p < 0.002), while omnibus tests used corrected alpha levels where applicable. Isometric belt squat net peak force demonstrated weak-to-moderate associations with jump height across tasks (r = 0.36-0.55). Associations between net force at 200 milliseconds and jump height were moderate at 90° (r = 0.45-0.52), whereas net force at 100 milliseconds showed negligible associations to jump height. Older athletes produced significantly higher absolute isometric net peak force than younger groups only at 90° knee flexion (up to ∼860 N difference), with moderate-to-large effects (g = 0.67-0.80), whereas relative net peak force did not differ between age categories. These findings indicate that the isometric belt squat can be confidently used to monitor maximal lower-limb force capacity across knee angles, with the 90° position offering greater sensitivity for distinguishing strength levels between age groups.