Test–Retest Reliability and Agreement of Postural Control Variables Within and Between Single-Leg Squat Variations

Single-leg squats are commonly used to assess lower-limb strength and alignment; however, their application for evaluating postural control remains underexplored. This study assessed the reliability and agreement of postural control measures within and between unipedal squat variations. Twenty-eight physically active adults performed a conventional single-leg squat (CSLSQ), the anterior excursion of the Y-Balance Test (ANYBT), and a forward step-down (FRSTD) with both limbs on two occasions, 5–7 days apart. The mean values of five trials were analyzed for center-of-pressure (COP) 95% confidence ellipse area (95%CEA), path length (PL), velocity (VL), and mediolateral and anteroposterior variability (RMS-X and RMS-Y). Most COP variables demonstrated good-to-excellent reliability (ICC = 0.780–0.948), whereas RMS-X showed lower reliability (ICC = 0.367–0.803) and higher measurement error across limbs. The FRSTD demonstrated high ICCs (0.780–0.948) and low measurement error, comparable to the CSLSQ (0.794–0.940) and generally higher than the ANYBT (0.790–0.895), regardless of limb. Overall, the dominant limb exhibited higher ICCs and lower measurement error than the non-dominant limb. Inter-task agreement was greatest between the CSLSQ and FRSTD, primarily on the dominant limb, indicating greater potential interchangeability for selected COP metrics (95% CEA, VL, and RMS-Y). These findings may assist clinicians and sports scientists in selecting appropriate single-leg squat tasks and COP measures for assessment.