# Assessment of Limb Imbalance in Professional Soccer Players
## 4 Discussion
### Tests Measuring Strength  
Soccer requires players to perform repeated moderate and high-intensity movements, such as sudden accelerations and decelerations, rapid changes of direction, and jumping and landing tasks, situations in which players are involved in the struggle to maintain possession or to win the ball [64]. Muscle flexibility, joint stiffness, and the ability to generate muscle strength are essential for soccer performance [65]. Specifically, appropriate flexibility and strength of the hip muscles are necessary to allow proper hip and knee movements, generating the power required for soccer practice [66]. It also requires hip muscle strength to absorb and generate energy and provide stability in the lower limbs and trunk during different actions produced on the soccer pitch [60]. Therefore, an assessment of limb strength imbalance is crucial to detect muscle imbalance in professional soccer players.  
Two studies were found that assessed limb imbalance through strength testing [22, 38]. On the one hand, [PERSON] et al. [22] found greater torque in the external rotators (measured through a handheld dynamometer) and less flexibility in the illipsoas and rectus femoris (measured using the modified Thomas test [67]) of the dominant leg compared to the non-dominant leg (\(p<0.01\)). This reinforces the conclusions drawn from previous studies [65, 68], highlighting the importance of addressing hamstring flexibility to facilitate optimal knee flexion and hip extension. It is important to note that reduced illipsoas flexibility would likely restrict hip extension rather than flexion, while reduced rectus femoris flexibility could limit knee flexion when the hip is extended or restrict hip extension when the knee is flexed. On the other hand, [PERSON] et al. [38] concluded that data obtained from strength tests and functional jumping tests were not comparable and should not be used interchangeably. They emphasized the need for further development and validation of specific functional performance tests, suggesting that extrapolating data from only a strength or power test would not reflect real-world conditions.  
In summary, when assessing strength imbalance together with other qualities, differences in strength values and bilateral flexibility imbalance between the dominant and non-dominant legs of the soccer players were established. Therefore, hamstring flexibility work in professional soccer players will be of great importance in achieving positive functional synergies with strength development.  
This systematic review is not without limitations. First, only articles written in English were included; hence, some equally valid articles in different languages might have been excluded. Second, all the articles except one were composed of male subjects; hence, the data cannot be extrapolated to the female population, with different conditioning factors than those of men. Lastly, the heterogeneity in the study procedures and objectives of the selected investigations, as well as their sample space, which may be somewhat scarce (12 articles), make it difficult to reach universal conclusions.