## 1 Introduction  
Unilateral lower extremity jumps and landings are very common in ground-contact sports [1, 2]. Because of increased load and reduced stability, mechanical stress on the knee joints increases with such movement modalities, augmenting the risk of injuries. For example, single-leg jumps applied in polymetric exercise training experiment evoked as high as 6.2 N/kg vertical ground reaction force [3], and knee joint is stressed by 8.5 N/kg reaction force when volleyball players perform their take-offs [4]. In addition, Tibiofemoral joint contact force can increase to more than ten times the bodyweight when landing from as low as 30 cm [5].  
Knee joint stabilization is often studied by using different landing tasks [6, 7, 8, 9], and the role of knee extensor and flexor muscles to protect passive tissue damage during landings is well understood [10]. The knee flexor co-activation as well as flexor to extensor strength ratio, known as knee joint stability indicators, have been considered important contributors to knee joint protection during unilateral landings on unstable surface [11]. An important limitation, however, is that researchers focused mainly on sagittal plane dynamics, while the dynamic control of knee valgus, which is a frontal plane kinematical phenomenon in the knee, has also been shown to relate to knee ligament injuries [12]. The gluteus medius muscle interests physiotherapists because it functions as a hip abductor, suggesting a potential role in knee valgus control [13; 14; 15]. The closest to the focus of the present work is a study by [PERSON] et al. [9], in which individuals with chronic ankle instability demonstrated smaller gluteus medius activation during landing on unstable surface, compared to healthy controls. Therefore, despite the fact that the gluteus medius is a pelvic muscle, its strength and/or activation seems to be important in the indirect stabilization of the knee joint during landings as it prevents excessive knee valgus.  
In addition to the aforementioned stabilization roles, knee extensor and flexor strength have been shown to contribute to countermovement jump (CMJ) height [16], an important measure of athletic performance, which has been extensively studied [17; 18; 19]. In addition, both knee extensor and flexor activation were found greater when landings were performed unilaterally versus bilaterally [6]. Based on the above pieces of evidence, therefore, it seems reasonable to assume that knee joint stabilization ability and CMJ performance are related. Furthermore, extensive knee valgus may deteriorate CMJ performance through biomechanically disadvantageous force exertion; therefore, it is important to investigate hip abduction strength contribution to CMJ performance. The latter assumption derives from previous evidence showing that gluteus medius malfunction, for example, alters ankle joint kinematics, which is a compensation strategy to maintain posture during a single-leg forward jump [20].  
In the present study we tested the hypothesis that CMJ performance is associated with the ability of knee joint stabilization at landing. We addressed this question through measuring and correlating CMJ impulse and knee extension, knee flexion, and hip abduction torque, as well as the activation of the involved muscles (joint stabilization markers) during unilateral CMJ and landing on unstable surface. If CMJ impulse is a significant predictor of knee joint stabilization ability, then physiotherapists and strength specialists could evaluate knee injury risks with simple CMJ tests.