## 1 Introduction  
Anterior cruciate ligament (ACL) injuries affect recreational, competitive, and professional athletes and, in particular, football players with a negative impact on their quality of life [1]. Indeed, a tear of the ACL is considered a noteworthy injury that commonly requires a prolonged period away from competition and need of an adequate rehabilitation plan [2, 3]. Although 70% of players return to play (RTP) [2], only two-thirds of players with a complete ACL lesion play at the high level 3 years later [4]. Moreover, ACL injury is related to a higher risk of a knee re-injury [5], and long-term medical disability due to early osteoarthritis in half of the individuals 10-15 years later [6, 7].  
There are four main situational patterns of ACL injuries in football players: pressing/ tackling, tackled, regaining balance after kicking, and landing from a jump. In most cases (81%) a mechanical perturbation causes a high knee valgus loading [8]. When a perturbation alters the equilibrium of a system, a preparatory contraction of the musclesstiffens the joint as a protective reaction to prevent injury with a feedforward mechanism. In particular, the anticipatory contraction of quadriceps and hamstrings muscles protect the joint generating dynamic stiffness and higher stability [9] that could reduce the dynamic knee valgus [10].  
Albeit quadriceps strength, lateral-lateral symmetry, and hamstring-quadriceps ratio are important parameters in knee injury [11], neuromuscular control plays a key role in determining the timing of muscle activation [12]. [PERSON] et al. compared muscular pre-activation of selected lower extremity muscles in response to a drop landing, to detect neuromuscular recruitment in young athletes, highlighting the need to improve hamstrings training methods for better neuromuscular control to lower the risk of injury [9].  
In the literature, there is strong evidence on prevention exercise programs demonstrating evident benefits in reducing the risk of ACL injury [1]. Most of all, methods based on the enhancement of neuromuscular and proprioceptive control have been shown to decrease the risk of ACL injury up to 51% [13; 14; 15; 16], improving muscle firing patterns, and increasing dynamic stability of the lower limb [17]. The improvement in muscle firing is the consequence of an improvement of reactive muscle activity due to a more rapid recognition of unexpected perturbations by the muscle spindle [18; 19]. During a sport specific task, co-contraction represents a motor control strategy to dynamically stabilize and protect a joint. Co-contraction improves knee stability when the joint is subjected to high load or shear forces [20]. Specifically, one of the most effective patterns to dynamically stabilize the knee during varus and valgus moment is Quadriceps: Hamstring (Q:H) co-contraction. Unbalanced Q:H co-contraction can contribute to higher knee abduction loads increasing strain on the ACL [21; 22]. To date, the preventive exercises programs proposed in the literature [14; 15; 17; 23] have a duration of several weeks to whole season; however, [PERSON] et al. [24] have recently reported that an ACL exercise program included in a football warm-up could reduce dynamic knee valgus in the short-term, preventing knee injuries in male soccer players [17]. In this scenario, neuromuscular evaluation by surface electromyography (sEMG) showed to play a crucial role providing a deep view of the behavior of the neuromuscular system [25]. Specifically, it is possible to determine the timing of muscle excitation showing when a muscle turns \"on\" [25] and to record muscle activation of the knee stabilizer muscles to determine the recruitment order and the time of pre-activation as the start time of contraction before landing. Therefore, this proof-of-principle study aims to evaluate the short-term effects of a neuromuscular warm-up on the knee stabilizer muscles' activation pattern in a team of professional football players.