# Assessment of Limb Imbalance in Professional Soccer Players
## 1 Introduction  
The concept of limb imbalance is about the comparison of the performance of one limb in relation to the other and has been extensively researched in recent years [1, 2]. Sports in which a greater number of sporting actions are performed with the dominant leg could lead to this imbalance [3]. Limb imbalance is common in soccer, as players often favor one leg for kicking and cutting skills. Moreover, the high training and competition volumes throughout the season contribute to this imbalance [4, 5, 6]. Many of the skills involved in soccer training and competitions require soccer players to frequently repeat the execution of unilateral and asymmetric movements (e.g., defensive actions, changes of direction, and ball striking) [5, 7, 8]. Over time, these movements can lead to differences in strength, power, and range of motion (ROM) due to loading patterns and coordination of movement [2, 9]. Although injuries have a multifactorial origin, the limb imbalance produced is a potential risk factor for injury in soccer players, especially related to hamstring injury or injuries sustained during landing, both phenomena of great incidence in soccer [10, 11]. Conceptually, numerous classifications have been established to quantify this limb imbalance. These include dominant and non-dominant limb [12], strongest and weakestlimb [2; 13], right and left [14], bilateral and unilateral asymmetry (i.e., asymmetries between limbs or between the antagonist and agonist muscles of the same leg, respectively) [1], and injured and non-injured limbs [15], although they are sometimes just defined as the kicking limb = dominant, versus the stance limb in kicking = non-dominant. However, this variety of classifications has considered that there is no single way to quantify limb imbalance between the lower limbs to date, with the exception of considering a percentage difference of one limb relative to the other [16]. In research, lower limb imbalance has been examined across a wide range of physical qualities, including strength [17], power [18], changes of direction [19], leg flexibility or stiffness, and dynamic balance [20; 21]. These qualities are assessed through various tests. Muscle strength is evaluated using isokinetic dynamometry (hip and knee strength, Nordic curls, and hip abductor and adductor eccentric strength) [22; 23], isometric squats, and mid-thigh pull-ups [24; 25]. Functional performance is measured with change of direction tests and jump tasks (e.g., countermovement and single-leg jumps) [26]. Flexibility and dynamic balance are assessed with tests like the Y Balance Test and straight leg raise tests [20; 27; 28]. All of the above methods have been shown to be sensitive in identifying limb differences in the athlete and non-athlete population, and they have been shown to be a more practical and economical alternative to the data derived from force platforms, which have a high economic cost and are difficult to transport for fieldwork. In addition, it has been argued that limb imbalance may be a product of the time spent competing in the same sport [3]. While it is logical to assume that it is desirable to minimize these differences, it is not yet clear whether this has an actual measurable effect on physical or athletic performance [16; 29]. Sport training increases the likelihood of limb imbalance [30]. It has been shown that playing a sport as well-known and practiced as soccer has led to limb imbalance [24]. The most recent reviews are based on studying the effects of limb imbalance on performance in sports [16; 29], identifying the limb strength characteristics of the dominant and non-dominant legs in soccer players [31], understanding the imbalance produced in quadriceps and hamstring ratios in soccer players [32], and even identifying whether limb imbalance increases the risk of injury in sport [33]. However, there has been no consensus or systematic review of which tests to use and how to assess limb imbalance in soccer players.