Soccer is the most popular sport around the world, but it is a vigorous sport event with frequent exposure to injuries, from adolescents to adults (Dahlstrom et al., 2012).

Sports injuries are caused by various intrinsic risk factors such as injury history, body components, and joint flexibility (Bahr and Holme, 2003; Gomez et al., 1998; Krivickas and Feinberg, 1996). Since these intrinsic factors cannot be manipulated voluntarily, sports injuries of athletes have been predicted and prevented by evaluating movement dysfunction (Bond et al., 2019; Plisky et al., 2006).

Movement dysfunction means that muscle and joint movements are not normal while performing a specific motion or this motion is completed by compensation of another muscle and joint. In the past, sports scientists recognized that movement dysfunction in athletes could lead to sports injury during the training and competition, but there were no enough equipment and methods to investigate movement dysfunction. In the 2000s, various evaluation tools were presented to assess the relationship between sports injuries and movement dysfunction, of which Functional Movement Screen (FMS) developed by Cook et al. (2006a) was recommended as the most effective way to predict sports injury of athletes. Kramer et al. (2019) studied relationship between FMS and athletic performance and found a significant correlation in FMS scale and physical performance test such as standing long jump, vertical jump, and agility, suggesting a standardized assessment to predict sports injury and performance of athletes.

In addition to FMS for injury prevention strategy, the analysis of the Quadriceps-angle (Q-angle) during the single-leg squat is also one of the methods to examine knee damages including the muscle, ligament, and tendon (Hewett et al., 2006). Many previous studies suggested that changes in the Q-angle value in single-leg squat test increased the occurrence of sports injury along with movement dysfunction of athletes (Hewett et al., 2005; Zazulak et al., 2005; Zeller et al., 2003).

In studies on FMS and sports injury in soccer players, Chalmers et al. (2018) investigate the relationship between FMS asymmetry and number of games played before first injury and reported to be closely associated with a significant increase in prospective injury risk (Grygorowicz et al., 2013; Miller and Susa, 2019; Sprague et al., 2014).

With these research findings, FMS and Q-angle during single-leg squat tests are absolutely necessary for prediction and prevention of sports injuries in soccer players. But almost all previous studies on asymmetry and physical performance in soccer have mainly focused on senior athletes, and there are few studies on gender in adolescence soccer players. Therefore, the purpose of this study was to investigate the relationship among FMS, Q-angle, and sports injuries according to gender of youth soccer player.

Movement dysfunction is an important indicator of sports injury through imbalance and asymmetry in adult athletes, suggesting that FMS and Q-angle are the most well-known measurement methods to predict sports injury in the training and competition (Kramer et al., 2019; Warren et al., 2018). Thus, we analyzed FMS and Q-angle in single-leg squat movement to youth male and female soccer players, which can instantly examine the dysfunction in the field.

Athletic performance is influenced by several factors including maximum strength, jumping, sprinting, agility, and balance capabilities. Among various assessment tools to predict body imbalance and asymmetry, FMS is the most popular way in the sports field and it is composed of a 4-point scale (0=pain with pattern regardless of quality, 1=unable to perform pattern, 2=perform pattern with compensation, and 3=perform pattern as described) for each of 7 movements. In previous studies related to FMS, Shojaedin et al. (2014) reported that FMS might help screen musculoskeletal injury in university level athletes, and Chapman et al. (2014) suggested the positive relationship between FMS and competitive performance in elite track and field athletes. On the contrary, Dorrel et al. (2015) mentioned that the FMS is not an appropriate field test for predicting risk rate of sports injury and athletic performance. It is still controversial that FMS represents sports injury and performance of athletes. In the present study, we investigated 7 items of FMS, and represented that the only trunk stability push-up test showed significant difference between male and female groups and it was higher in the male group than in the female group (t=3.284, P=0.002). Trunk stability push-up test is closely associated with core stabilization (Cook et al., 2006b), and the hip adductor and rotator muscles, abdominus, erector spinae muscle, and quadratus lumborum muscle that control core functions are relatively higher in male athletes than in female basketball players (Leetun et al., 2004). These findings in previous studies support the reasons why female athletes score low on the trunk stability push-up test, and we believe that training programs to improve core strength and functions should be applied since adolescent.

Q-angle is referred to as the quadriceps angle and it is the angle between two intersection lines from the patella center to the ASIS and the patella center to the tibia tuberosity (Pantano et al., 2005). We confirmed that total values of Q-angle during single-leg squat were significantly lower in male group compared to female group (t=2.666, P=0.009). These differences are because female athletes had larger range of motion of dorsiflexion, ankle eversion, hip flexion, hip external rotation, and lateral trunk flexion during single-leg squat than male athletes (Zeller et al., 2003). Zazulak et al. (2005) demonstrated that women could have a lager Q-angle during single-leg squat through low electromyographic (EMG) activity of gluteus maximus and high EMG activity of rectus femoris compared to men. A larger Q-angle may lead to greater peak knee valgus during physical activity and then result in anterior cruciate ligament (ACL) damage over time (Pantano et al., 2005). In the clinical study on valgus and ACL strain reported by Bjordal et al. (1997), senior and junior female athletes were found to have 3 to 5 times higher incidence of ACL injury than male athletes (Engström et al., 1991).

Taken together, in FMS test according to gender of youth soccer players, female athletes should improve their relative upper extremity muscle strength and scapula stabilization from adolescents, considering that female athletes showed only higher values of trunk stability push-up movement than male athletes. Also, since female athletes have Q-angle values during single-leg squat, specific and scientific training programs to prevent the knee valgus must be performed from adolescence.

Therefore, our results provide evidence that FMS and Q-angle during single-leg squat might be indicators to predict and prevent sports injury in youth male and female soccer players.