Reliability of single-leg maximal dynamic strength performance and inter-limb asymmetries in pre-pubertal soccer players. The influence of maturity in asymmetries

The aims of this repeated measures study were (I) to determine the reliability of single-leg squat 1RM and inter-limb asymmetries in pre-pubertal athletes, and (II) to assess associations between bilateral and/or unilateral squat 1RM performance, inter-limb asymmetries, and participants' chronological and biological age. Thirty-one soccer players (age [mean ± standard deviation] = 8.48 ± 0.47 years; body mass = 28.32 ± 3.47 kg; body height = 121.70 ± 4.11 cm) practiced two familiarization and two experimental sessions. Bilateral and unilateral squat 1RM and asymmetries between both legs were assessed. Biological age was obtained through the age at peak height velocity (12.87 ± 0.38 years) and maturity offset (3.92 ± 0.44 years). Absolute and relative reliability and Pearson's correlations were calculated. Right and left leg 1RM and asymmetries presented good relative reliability (ICCrange = 0.84-0.85) levels, but non-acceptable absolute reliability (CV ~ 12 %). Asymmetries presented trivial to small correlations with strength (Range = 0.01-0.11). Similarly, chronological, and biological age presented small correlations with strength and asymmetries (Range = 0.01-0.22). Considering the good reliability, the bilateral and unilateral 1RM back half squat test can be considered in youth soccer players. Finally, lower-body mechanical performance and asymmetry between legs show trivial to small correlations, with maturation not affecting asymmetry in male youth soccer players. In conclusion, strength and power activities and training should be placed on younger soccer players with high asymmetry scores regardless of their stage of maturation.