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

Abstract 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.