## 1 Introduction  
Football is a complex sport involving a wide range of motor skills, including agility, balance, coordination, speed, strength, and endurance [1]. As with many other sports, speed and strength are key performance parameters in football, with a particular emphasis on the quadriceps and hamstring muscles in the lower extremities, which play a central role in these abilities [2]. Hamstring muscle activity increases significantly during acceleration and maximal sprint performance, making it essential for effective play during high-speed movements [3]. In particular, hamstring muscle strength is important for preventing injuries, shortening the rehabilitation period after injury, and enhancing athletic performance [4]. In football, the lower extremities are exposed to more stress and impact due to movements such as sudden deceleration, acceleration, short sprints, kicking the ball, and jumping [5, 6], thus leading to more frequent injuries compared to the upper extremities [7; 8; 9]. Such injuries are believed to result from tissue damage caused by forces that exceed the mechanical limits of the tissue [10; 11].  
Eccentric contraction is a training method that involves various types of muscle contractions to enhance hamstring muscle strength and dissipate mechanical energy during deceleration of the body [12], thereby facilitating the conversion of kinetic energy into elastic energy in the tendons [13]. For instance, pylemetric training, which involves rapid muscle stretching followed by contraction, supports explosive movements, whereas strength training with overload and isoinertial methods improves muscular strength and performance by applying variable resistance during movement [14]. Eccentric strength exercises are the most researched and recommended exercises for preventing hamstring injuries [15; 16; 17].  
In recent years, Nordic hamstring curl training (NHCT), a type of training based on eccentric contractions aimed at increasing hamstring muscle strength and performed on flat ground without requiring special equipment, has been accepted as a popular training method [18]. Hamstring muscle activation during NHCT, which involves knee and hip joint movements, requires greater maximal eccentric muscle strength than traditional hamstring exercises [19]. The use of body weight for resistance and the fact that no special equipment is needed are the reasons for the widespread use of this exercise [20]. A literature review showed that NHCT research reports increased muscle activation, eccentric muscle strength, and jump height [21]. Many studies have shown that NHCT reduces the risk of injury, improves muscle performance, and is an important exercise that strengthens the hamstring muscles [22; 23; 24; 25]. Contrary to some findings, other studies suggest that hamstring muscle strength may not be directly correlated with athletic performance [26; 27]. In this context, the present study aimed to evaluate the effect of an 8-week NHCT program on athletic performance (linear sprint, jump, and change of direction speed) of young soccer players by increasing eccentric muscle strength. We hypothesized that the NHCT program would lead to improvements in eccentric muscle strength, jump performance, and change of direction ability, contributing positively to athletic performance.