Can professional soccer practice lead to muscle strength imbalances?

In this study, we compared the knee strength parameters tested in professional football players at the preparatory and competitive seasons, separated by 12 weeks. Knee extensors and flexors peak torque, hamstring to quadriceps conventional ratio (H/Qcon), and bilateral asymmetry index (AI) on the dominant and non-dominant limbs of 10 football players were evaluated in an isokinetic dynamometer. Paired t-test was used to compare all variables between the preparatory and competitive seasons. A one-sample t-test was used to compare the H/Qcon and AI with theoretical means defined as 0.6 and 10%, respectively. The results showed a significant peak torque improvement in the dominant side for both knee extensors (p=0.013) and flexors (p=0.003), whereas on the non-dominant limb there was no significant difference. Regarding the H/Qcon, dominant and non-dominant sides were below 0.6 during the preparatory and competitive seasons (p<0.001). The AI was significantly lower than 10% for the knee extensors during the competitive season (p=0.001) and for the knee flexors during the preparatory season (p=0.005), without difference between the preparatory and competitive seasons for the knee extensors and flexors. These results indicate that professional football practice potentially increases interlimb strength asymmetries, therefore, lower extremity limb dominance should be considered when designing resistance training and injury prevention programs for professional football players, as well special attention should be taken regarding the strengthening of the knee flexor muscles