1.    Introduction
Volleyball is an Olympic sport played in over 200 countries worldwide. Players use complex movements in both attack and defense, with the spike being one of the most important techniques used to score points (1). The spike requires high skills in jumping and landing. Offensive players aim to achieve a high jump to make their next move unpredictable and to hit the ball with greater speed, which correlates with their level of competition (2). By applying biomechanical principles, coaches and scientists can identify techniques to improve performance, reduce injury risk, and extend athletic careers. This knowledge is especially important in sports involving jumping, like volleyball, where precise body positioning and landing techniques play a significant role in preventing injuries (3, 4). Studies have shown that game-induced fatigue can decrease landing quality and increase the risk of both acute and chronic injuries (5-7). Fatigue negatively impacts athletic performance by reducing neuromuscular precision and motor control, particularly in sports that require high movement accuracy and repeatability (8, 9). Fatigue disrupts the distribution of forces on the body, increases stress on sensitive joints such as the knee and ankle, and reduces muscular resistance to sudden pressures (10, 11). This emphasizes the need for proper training programs and recovery strategies to mitigate fatigue and prevent injuries (12-15). Most studies on landing have assumed that the lower limbs are symmetrical and have only analyzed unilateral data (16-20). However, this study examines the kinetic differences between the dominant and non-dominant legs during landing after a spike and following the onset of fatigue (21-23).
2.    Methods
The This study targeted professional volleyball players aged 18-32 from reputable Iranian leagues. Using G*Power software with α = 0.05 and 80% power (24), 28 male players volunteered, with an average age of 24.56 ± 2.14 years, height of 1.81 ± 0.15 meters, weight of 76.14 ± 9.09 kg, and BMI of 23.71 ± 7.27. None had lower or upper limb injuries in the past year or were on medication in the month before testing. They completed consent forms, and the testing procedures were explained. A 3D motion analysis system (Vicon Peak, Oxford, UK) with six T20 cameras at 200 Hz and markers on the lower limbs was used to capture movements during tasks (25). Synchronized Kistler force plates at 1000 Hz recorded kinetic data, normalized to body weight (26). The volleyball net was set at 2.43 meters in a calibrated lab area. Participants warmed up for 15 minutes, performed five trial spikes, then executed six spikes each before and after a fatigue protocol, landing on force plates. The fatigue protocol, from Bosco (27), involved four 60-second vertical jump sets with 10-second rests, with increasing effort (50%, 70%, and 100%). Fatigue was marked by a 30% drop in maximum jump height. Variables measured included ground reaction forces (vertical, anterior-posterior, medial-lateral) and loading rates (28, 29), normalized to body weight, analyzed using SPSS with a significance level of P < 0.05.
3.    Results
The results of comparing ground reaction forces before and after fatigue showed that the vertical ground reaction force at the moment of impact, the maximum posterior force, and the medial-lateral force at the moment of impact in the dominant leg had significant differences (P < 0.05). Fatigue did not have a significant effect on other variables (Table 1).

4.    Conclusion
This study examines the impact of fatigue on ground reaction forces (GRF) in three main axes during landing from a volleyball spike. The research aims to provide a deeper understanding of how landing kinetics change under fatigue and its effects on the safety and performance of professional volleyball players. Given the importance of kinetic variables and the influence of fatigue, this study offers valuable insights into injury prevention. The results indicate that fatigue leads to a 15% reduction in vertical GRF in the dominant leg during landing (30-32). Previous research suggests that fatigue can increase the risk of lower limb joint injuries by affecting GRFs (33, 34). Although fatigue may cause asymmetric force distribution between dominant and non-dominant legs, this study found equal GRF levels in both legs post-fatigue (35-37). The dominant leg, typically used for precise and powerful movements, may experience more stress and thus higher GRF during landing (38). Conversely, the non-dominant leg, being less conditioned, might be less capable of absorbing GRF under fatigue, leading to imbalance and injury risk. Fatigue can impair effective force absorption, as suggested by previous studies, which might explain the reduced vertical GRF due to a cautious landing strategy to minimize injury risk. Fatigue also significantly affects posterior and medial-lateral forces, with a 13% reduction in maximum posterior force and a 37% increase in medial-lateral force in the dominant leg post-fatigue (39, 40). These forces are crucial for movement control and balance, and excessive medial-lateral loading can be particularly dangerous for knee ligaments like the ACL (41, 42). Studies have noted differences in limbs regarding various anatomical and physiological aspects, which may contribute to increased ACL injury risk in the dominant leg due to these asymmetries and fatigue (43). Consequently, understanding these dynamics is essential for developing effective training and injury prevention strategies (44).

Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be addressed in this research.
Funding
This research did not receive any grants from funding agencies in the public, commercial, or non-profit sectors.
Authors' contributions
All authors contributed equally to preparing the article.
Conflicts of interest
The authors declared no conflict of interest.