## 4 Discussion  
The first hypothesis in this study, that regular badminton practice reduces the anterior stability of the knee joints in badminton players between the ages of 10 and 11, was not confirmed. During the period of our research, the group of subjects did not experience any trauma causing damage to the ACL and the stability of the knee joints did not decrease in the subjects. It can even be observed that practicing badminton on a regular basis in the training macrocycle increased the frontal stability in the group of subjects. When analyzing the results of the stability index, there is a significant difference in the experimental group (\(p<0.05\)) between the first and second tests, which shows the positive effect of the badminton training on the anterior stability of the knee joint. It is significant that such a situation was not observed in the control group. Based on the literature review, it can be presumed that improved stability of the knee joints is most likely due to the improvement of the neuromuscular control of the trunk and lower limbs as a result of regular badminton training [20; 21; 22]. There is a strong correlation between extensors, abductors and external rotators of the femur and the injuries of the ACL. Researchers claim that disturbed work of these muscles can cause valgus of \begin{table}
\begin{tabular}{l l l l} \hline \hline  & **Body Height (cm)** & **Body Weight (kg)** & **BMI** \\ \hline Experimental group & & & \\ \hline Stability II-I & 0.10 & \(-\)0.04 & \(-\)0.04 \\ Stability II-I (\%) & 0.04 & 0.00 & 0.00 \\ FMS II-I & 0.05 & 0.15 & 0.15 \\ FMS II-I (\%) & 0.05 & 0.15 & 0.15 \\ \hline Control group & & & \\ \hline Stability II-I & \(-\)0.21 & \(-\)0.01 & \(-\)0.01 \\ Stability II-I (\%) & \(-\)0.22 & \(-\)0.03 & \(-\)0.03 \\ FMS II-I & \(-\)0.18 & \(-\)0.03 & \(-\)0.03 \\ FMS II-I (\%) & \(-\)0.20 & 0.00 & 0.00 \\ \hline II-I–difference between measurement I and II; II-I (\%)—percentage difference between measurement I and II. & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Correlation coefficients between body weight, body mass, BMI and stability, and FMS test. the knee joints during dynamic movement with load, which contributes to the damage of the ACL [4; 23; 24]. The muscles acting on the knee joint can also affect the anterior cruciate ligament by increasing or reducing its tension. For example, the quadriceps muscle of the thigh generates forces that act in opposition to the ACL, increasing its tension, which is associated with overloading and the risk of damage to the ACL [25]. On the other hand, the group of hamstring muscles supports the work of the ACL, reducing its tension. This is confirmed by the studies of [PERSON] et al. [26], [PERSON] et al. [13], and [PERSON] et al. [27], according to which it is a group of muscles that interact with the ACL, preventing excessive forward shift of the tibia. Taking into account the results of the research by [PERSON] et al. [14], muscle fatigue during training affects the dynamic control of posture, which may be associated with an increased risk of knee joint injury. Movements which were examined in this study were lunges which are similar to the movements performed during a badminton game. Our second assumption was that practicing badminton on a regular basis affects the functional assessment of badminton players between the ages of 10 and 11. Analyzing the data on the value of functional assessment, it can be observed that the results showed an upward trend in the group of girls and boys after the training macrocycle. There was a statistically significant (\(p<0.05\)) increase in the number of points in the functional assessment in measurement II compared to measurement I. It is worth noting that the movement pattern, which is the squat, improved by 1 point on the 0-3 scale in the majority of people in the study group. This is significant progress in performing this particular motor task, which positively influences the overall result of the assessment.