Reliability Challenges in 1RM Parallel Back Squat Testing: The Critical Role of Squat Depth

Purpose: This study analyzed the inter-session reliability of the parallel back squat during one-repetition-maximum (1RM) testing in elite youth soccer players and to exploratorily evaluate the consistency with which the prescribed squat depth is achieved. Methods: The sample comprised 47 male soccer players (age: 18.1±2.3 years, body mass: 72.4±7.7 kg, height: 1.83±0.06 m) from German youth soccer academy teams competing in top junior leagues (U16-U19). The 1RM was assessed under standardized conditions, with squat depth monitored using a hybrid marker- and silhouette-based motion capture system. The inter-session reliability of 1RM load was quantified using the intraclass correlation coefficient (ICC). Systematic bias between sessions was evaluated using a paired-samples t-test (with Cohen’s d for effect size). Within-session deviations from the 90° squat-depth target were assessed using a one-sample t-test (with Cohen’s d for effect size). Results: The participants achieved an absolute 1RM of 94.9±13.7 kg (relative: 1.32±0.19). The 1RM test–retest (inter-session) reliability regarding load lifted yielded an ICC of 0.970 (95% CI: 0.948–0.982), with a mean bias of 1.71 kg (P=0.018, d=0.39). An explanatory analysis of the kinetic data from each participant’s best trial (within-session) showed that squat depth deviations (thigh angle 89.7°±4.1°) did not differ significantly from the target depth of 90° (P=0.630, d=0.15), where 0° corresponds to full knee extension. Conclusions: This study’s parallel back-squat 1RM testing protocol in national-level male youth soccer players demonstrated excellent load reliability. Squat depth may confound reliability, and its full impact on inter-session error warrants further study; nonetheless, familiarization, skilled spotters, and practiced participants can help limit its effect.