## 1 Introduction  
The knee joint is crucial in lower limb biomechanics but, at the same time, is often prone to injuries [1; 2; 3; 4]. Therefore, developing comprehensive methods that could be useful for understanding knee biomechanics and being applied for the primary and secondary injury prevention of lower extremities is crucial in sports medicine. These assessments and considering various factors are vital in monitoring rehabilitation and evaluating return to sport to ensure patients representing different levels of sports engagement have regained the necessary abilities [5; 6; 7; 8; 9; 10; 11]. Implementing injury prevention programs and systematic approaches can significantly mitigate injury risks, for example, in football players [12; 13]. Effective knee injury prevention and rehabilitation strategies depend on thoroughly understanding neuromuscular coordination and knee stability during dynamic movement [14; 15]. One valuable approach to assessing these factors can be considered analyzing muscle coactivation, which provides useful insights into the simultaneous activation of agonist and antagonist muscles around the studied joint. The so-called SEBT group, including the Star Excursion Balance Test, its modified version, and the Lower Quarter Y-Balance Test (YBT-LQ), is widely used to assess stability limits, an essential aspect of postural stability [16]. These limits are defined by the body's ability to shift its center of gravity in various directions without losing balance [17]. The YBT-LQ, in particular, is a widely recognized measure used to evaluate an individual's ability to maintain stability while performing reaching tasks in three directions: anterior, posteromedial, and posterolateral. The results from this test are used for assessing injury risk, evaluating intervention outcomes, and determining criteria for returning to sports after musculoskeletal injuries [18; 19; 20]. The YBT-LQ has been shown to have excellent inter- and intra-rater reliability in healthy adults [21]. This means it consistently produces repeatable results whether assessed by different evaluators (inter-rater reliability) or at other times by the same evaluator (intra-rater reliability). This makes it a reliable tool for clinical decision-making. Surface electromyography (sEMG) is a non-invasive method of assessing muscle activity, including the evaluation of muscle coactivation [22; 23; 24]. The main advantage of sEMG is its ability to provide real-time data, which is particularly useful for assessing the activity of muscles during dynamic tasks, such as the YBT-LQ. While sEMG is commonly used to analyze the amplitude of muscle activation, it can also assess muscle fatigue, an important aspect that can impact muscle function during dynamic activities. Fatigue is commonly assessed through the median frequency or zero-crossing rate of the EMG signal [25]. An sEMG index is a quantifiable measure derived from sEMG data to assess the coactivation between muscles surrounding a joint. Coactivation refers to the simultaneous activation of agonist and antagonist muscles, which is crucial for joint stability during movement. In this study, an sEMG-based index was designed to capture the coactivation, sometimes called co-contraction or co-working, between knee extensor and flexor muscle groups, offering a way to evaluate knee muscle function. Establishing a reliable sEMG-based index would ensure consistent and objective measurements of muscle coactivation. This study seeks to address this gap by developing a reproducible metric that can be used in clinical and research settings, especially for prevention in athletes. Improving the reliability of measurement tools used in orthopedics, traumatology, sports medicine, and rehabilitation measures is crucial for raising evaluation standards in clinical studies, especially now, when technology is advancing rapidly in diagnostic and patient monitoring devices in these areas.