As core musculature provides stability for effective control of body motions and force production in the lower extremities, its deficiency or imbalance can increase fatigue and decrease muscular endurance, leading to greater susceptibility to injuries [11]. Therefore, most current conditioning programs in schools also include exercises strengthening core musculature, in addition to traditional resistance exercises. Research so far has been aimed at investigations of the relationship between core stability and athletic performance, as well as the assessment of the effectiveness of core strengthening and stabilization exercises in children and adolescents [3; 4; 5; 7; 8; 9; 12; 13; 14; 15; 16; 17; 18]. Nevertheless, there are only a few studies dealing with young athletes. For instance, a 6-week core conditioning intervention enhanced endurance of the core muscles (lateral and prone plank, parallel dynamic back extension on a roman chair, static and dynamic curl-ups) in school-aged children [19]. Similarly, a 10-week Quad-Core training improved strength and endurance of the core muscles in school football players [20]. Furthermore, an 8-week core training using body weights along with handball training improved vertical vault, flexibility, 20 m speed, balance, right hand clutch power, agility, back-leg strength, dribbling, quick passes, and point shooting in 9-10-year-old male handball players [21]. The trunk muscle strength and dynamic balance was also improved after one year of simple core muscle training in pediatric male soccer players [22]. Moreover, one-leg, but not bipedal, vertical jump height was increased after a 4-week integrated program focused on core stabilization in prepubertal athletes [4]. Postural balance control, but not jump ability, was improved after an 8-week integrated neuromuscular training aimed at dynamic balance, core stability, and plyometrics in junior alpine skiers [8]. Throwing velocity was increased after progressive isolated unstable core stability training in young handball players [6]. Similarly, maximal throwing velocity was increased after a 6-week sling training based on core stability in young female handball players, indicating that rotational velocity in multisegmental movements can be improved following core stability training including unstable, closed kinetic chain motions due to a more stable and stronger lumbopelvic-hip complex [4]. Ball-exit velocity, but not throwing velocity, was increased after a 6-week isolated resistance training program in young baseball players [7]. Core muscle stability was improved after a 6-week Swiss ball training, whereas there were no significant changes in myoelectric activity of the back and abdominal muscles, running posture, running economy, and treadmill \(\text{VO}_{\text{2 max}}\) in young male athletes [3]. However, in comparison with a predominant number of studies that conducted research on competitive athletes, a scarce amount of research deals with core strengthening exercises and their efficiency in improving overall fitness in school-age athletes. In order to partly fill this gap in the literature, we have investigated the effect of 12-week core strengthening training and weight training on physical fitness among school-age athletes. Both training programs have been assumed to be effective in improvement of their physical performance; however, their effectiveness is assumed be to different extents regarding the endurance, flexibility, and abdominal strength.