Asymmetries of isokinetic strength and flexibility in young soccer players: a systematic review

Purpose
The study aim was to review the isokinetic strength assessment protocols, the obtained peak torque (PT) values, and the hamstring/quadriceps ratio (H/Q); review the unilateral evaluation protocols and normative values for flexibility; and verify the existence of asymmetries between the limbs, muscle imbalances, or bilateral differences (BD) in the strength and flexibility of young soccer players.

Methods
The PRISMA model was used. Searches were conducted in PubMed, SciELO, and LILACS, divided into soccer, isokinetic, and flexibility. Inclusion criteria: articles with a sample of young elite male soccer players with an average age less than 20 years; isokinetic strength assessment: presentation of values for absolute PT and/or H/Q ratio; flexibility assessment: use of unilateral tests for the hip flexion movement. The quality and risk of bias in each study were also analysed.

Results
In the isokinetic strength, a symmetrical pattern was observed for PT and the H/Q ratio between the limbs and divergent results for BD. Regarding flexibility, it was possible to verify a tendency towards an asymmetry of flexibility, with a more flexible dominant than non-dominant leg. Evaluation protocols and normative values were proposed for the tests of isokinetic strength and flexibility.

Conclusions
Imbalances for the H/Q ratio, asymmetries, and BD for isokinetic strength and flexibility are not natural aspects of the modality and should be avoided in young soccer players. The monitoring of these characteristics and the use of interventions to restore a symmetrical pattern have become essential since the beginning of sports life.