Isokinetic strength, bilateral and unilateral strength differences: variation by age and laterality in elite youth football players / Fuerza isocinética, diferencias de fuerza bilaterales y unilaterales: variación por edad y lateralidad en futbolistas juveniles de élite

SUMMARY: The purpose of this study was to investigate the effect of age on peak torque (PT) relative strength measures of knee extensors (KE) and knee flexors (KF), bilateral strength ratio (Q:Q, H:H), and ipsilateral strength ratio (H:Q) in youth elite male football players.Elite male players (n = 70) from four age categories (U16: n = 20, U17: n = 20, U19: n = 15, U21: n = 15) performed isokinetic strength tests using a Cybex dynamometer. Results show a significant effect of age on PTKE (F3,142 = 4.54, p = 0.005, = 0.09) and PTKF (age: F3,142 = 3.07, p = 0.030, = 0.06). No significant effect of age on unilateral (F3,142 = 1.05, p = 0.375, = 0.02) or ipsilateral strength ratio (F3,142 = 2.63, p = 0.053, = 0.06) was found. Results revealed significant differences in PTKE for nondominant limbs between U16 and U21. Higher bilateral differences were detected for flexors (H:H = 7.94-11.47 %, Q:Q = 7.97-9.29 %) compared to extensors. Our study showed that 17-year-old players have levels of strength similar to U19 players. A higher bilateral difference of knee flexors than extensors was found, suggesting that more attention should be paid to knee flexors. No effect of laterality was found for strength and strength asymmetries. Future research should also focus on ipsilateral strength ratio in late stage of maturation.

RESUMEN: El objetivo de este estudio fue investigar el efecto de la edad en las medidas de fuerza relativa de torque máximo (TM) de los extensores de rodilla (ER) y flexores de rodilla (FR), la relación de fuerza bilateral (Q: Q, H: H) y la relación de fuerza ipsilateral (H: Q) en jugadores de fútbol masculino de élite en 70 sujetos de cuatro categorías de edad (U16: n = 20, U17: n = 20, U19: n = 15, U21: n = 15). Se realizaron pruebas de fuerza isocinética utilizando un dinamómetro Cybex. Los resultados mos- traron un efecto significativo de la edad en TMER (F3,142 = 4,54, p = 0,005, = 0,09) y TMFR (edad: age: F3,142 = 3,07, p = 0,030, = 0,06). No se observó efecto significativo alguno de la edad sobre la fuerza unilateral (F3,142 = 1,05, p = 0,375, = 0,02) o relación de fuerza ipsilateral (F3,142 = 2,63, p = 0,053, = 0,06). Los resultados indicaron diferencias significativas en TMER para miembros no dominantes entre U16 y U21. Se detectaron mayo- res diferencias bilaterales para los flexores (H:H = 7,94-11,47%, Q:Q = 7,97-9,29 %) en comparación con los extensores. Nues- tro estudio mostró que los jugadores de 17 años tienen niveles de fuerza similares a la de los jugadores sub-19. Se observó una mayor diferencia bilateral de flexores de rodilla que de extensores, lo que sugiere que se debe prestar más atención a los flexores de rodilla. No se encontró un efecto de lateralidad para fuerza y asimetrías de fuerza. Los estudios futuros también deben centrarse en la relación de fuerza ipsilateral en la etapa tardía de maduración.

Effect of Muscular Strength, Asymmetries and Fatigue on Kicking Performance in Soccer Players.

The aim of this study was to determine the effect of muscular strength, strength asymmetries, and fatigue on the speed and accuracy of an instep kick in soccer players. We measured ball velocity (BV) and kicking accuracy (KA) in the preferred (PL) and non-preferred leg (NPL) before (PRE) and after (POST) physical load in the PL. Maximum peak muscle torque of the knee extensors and flexors in the PL and NPL as well as ipsilateral knee flexors and knee extensors ratio (H:Q ratio) for both legs were assessed. BV was significantly decreased in POST physical load (5.82%, BVPRE=30.79±1.70 m·s-1, BVPOST=29.00±1.70 m·s-1, t19=3.67, p=0.00, d=1.05). Instep kick accuracy after the physical load worsened by an average of 10% in the most accurate trials. Results revealed a significant decrease in instep kick accuracy after physical loading (KAPRE=2.74±0.70 m, KAPOST=3.85±1.24 m, t19=-3.31, p=0.00, d=1.10). We found an insignificant correlation between H:Q ratio and KA in PRE test value, whereas a lower ipsilateral ratio (higher degree of strength asymmetry) in the POST physical load significantly correlated with KA in all angular velocities (r=-0.63 up to -0.67, p=0.00).