Extended Abstract
1.    Introduction
Anterior cruciate ligament (ACL) injuries are among the most common non-contact sports injuries, particularly during landing and pivoting movements, often requiring 6–12 months of rehabilitation (1,2,3). Biomechanical risk factors include increased knee abduction and internal rotation with limited flexion during landing (4,5). Studies have shown that reduced knee flexion and greater lateral trunk lean elevate ACL strain during single-leg landings (6,7). Leppänen et al. (2017) reported that every 10° increase in knee flexion reduces ACL injury risk by 45%, while each 100 N rise in ground reaction force (GRF) increases it by 26% (8). Dempsey et al. (2007) also found that external rotation of the foot and knee, hip abduction, and trunk lateral flexion increase valgus and internal rotation loads at the knee, predisposing athletes to ACL injury (9). Additionally, factors such as higher jump height, body weight, and reduced hamstring-to-quadriceps activation ratios have been associated with increased vertical GRF and joint loading (10,11).
In volleyball, with an injury incidence of about 4.21 per 1000 playing hours, ACL tears are particularly common during spike landings (14,15). Fatigue has been identified as a key contributor to altered landing mechanics; some studies indicate that fatigue leads to a more upright landing posture with reduced lower limb flexion, thereby increasing ACL injury risk (19,20), while others suggest compensatory mechanisms that may decrease risk (21,22). Discrepancies also exist regarding whether the dominant or non-dominant leg is more prone to injury (26–28). Therefore, the present study aimed to analyze the kinetic characteristics of volleyball spike landings—specifically the ground reaction forces, vertical loading rate, and lower limb joint moments—and to examine the effects of fatigue and leg dominance on these variables, in order to better understand potential mechanisms underlying ACL injury risk.
2.    Methods
Twenty-eight professional male volleyball players (age: 24.56 ± 2.14 years; height: 1.81 ± 0.15 m; weight: 76.14 ± 9.09 kg) from Iranian leagues participated voluntarily. All trained ≥5 days per week and had no history of musculoskeletal injury or medication use in the past year. Sample size was calculated using G*Power (α = 0.05, power = 0.80). Participants provided written informed consent, and the study was approved by the university ethics committee in accordance with the Declaration of Helsinki.
Three-dimensional motion capture (Vicon T20, Oxford, UK; 200 Hz) with sixteen 14-mm reflective markers (Plug-in Gait model) and two synchronized Kistler force plates (Type 9281; 1000 Hz) were used to collect kinematic and kinetic data during volleyball spike landings. After a standardized 15-minute warm-up and familiarization trials, each player performed three approach steps, a spike jump, and landing on the plates. The dominant leg was identified using a ball-kicking test. Jump height was determined as the vertical displacement of the anterior superior iliac spine marker between standing and peak jump.
Fatigue was induced using the Bosco squat-jump protocol, involving continuous 90° squat jumps for 60 seconds. The post-test was performed immediately after fatigue when the Sargent jump height decreased by ≥30% compared with baseline. Kinematic data were filtered using a fourth-order Butterworth filter at 6 Hz, and kinetic data at 20 Hz, normalized to body weight.
Peak ground reaction forces (vertical, anterior–posterior, and medial–lateral), loading rate, and lower limb joint moments were calculated using inverse dynamics. Statistical analysis was performed with SPSS 21, applying two-way repeated-measures ANOVA (time × leg) and paired t-tests for pre- and post-fatigue comparisons, with significance set at p < 0.05.
3.    Results
The results showed a significant decline in jump height following fatigue, with mean values decreasing from 57.20 ± 9.22 cm before fatigue to 50.76 ± 8.98 cm after fatigue (t = 5.77, p < 0.001). Analysis of ground reaction force (GRF) variables revealed that fatigue significantly influenced only the peak posterior ground reaction force (FPmax; p = 0.017), showing a reduction of about 12%, whereas other GRF components and loading rate did not change significantly (p > 0.05). The foot factor was significant only for the peak lateral GRF (FLmax; p = 0.017), with higher values observed in the non-dominant leg. No significant fatigue × foot interaction effects were found for any GRF variable (Table 1).

In terms of joint kinetics, fatigue significantly affected the hip extension moment (p = 0.044), which decreased by approximately 21% after fatigue, while other hip moment variables remained unaffected. The foot factor had a significant impact on hip joint moments (p < 0.05), except for the external rotation moment in the horizontal plane (p = 0.596). Abduction and adduction moments were higher in the dominant leg, whereas other components showed greater values in the non-dominant leg. The interaction between fatigue and foot dominance was not significant for hip moments (Table 2). Moreover, fatigue significantly reduced several knee joint moments, including extension (≈10%), flexion (≈62%), adduction (≈26%), and external rotation (≈44%) moments (p < 0.05), while abductor and internal rotation moments remained unchanged. At the ankle, significant reductions were also found in plantar flexion (≈20%) and inversion (≈38%) moments after fatigue (p < 0.05). Foot dominance influenced most joint moments, with the dominant leg showing higher eversion, dorsiflexion, and internal rotation moments, whereas the non-dominant leg exhibited greater plantar flexion. Overall, the results suggest that fatigue notably reduced jump height and selectively affected posterior GRF and hip, knee, and ankle joint moments, indicating compromised lower-limb neuromuscular control under fatigue conditions. 

4.    Discussion
This study examined the effects of fatigue on ground reaction forces (GRF), loading rate, and lower limb joint moments during volleyball spike landings. Fatigue selectively influenced the posterior GRF component, which decreased by approximately 12% after fatigue, while other GRF components and the loading rate remained unchanged. The observed 13% reduction in jump height likely contributed to this decrease, as previous studies have shown a direct relationship between GRF magnitude and jump height (50). These findings suggest that fatigue may increase the risk of lower-limb injury by altering the forces transmitted through the kinetic chain (51). Although GRF and loading rate are widely used to quantify external loads on the musculoskeletal system (33), the current study found no significant fatigue-induced change in loading rate, supporting the results of James et al. (54), who also reported stable vertical GRF and loading rate following fatigue. This consistency may indicate the body’s adaptive capacity to maintain impact control despite neuromuscular fatigue (52,53).
Fatigue significantly affected the hip joint kinetics, with a 21% decrease in hip extension moment after fatigue. This reduction may reflect decreased activation of hip extensor muscles, which are essential for maintaining stability and controlling the body’s center of gravity (55). Similarly, fatigue reduced knee and ankle joint moments across multiple planes, aligning with prior findings that reported lower joint angles and moments during fatigued landings as adaptive mechanisms to maintain stability (56,57). Leg dominance also influenced joint kinetics, with higher hip adduction and knee abduction moments in the dominant leg—factors associated with greater ACL injury risk (59,60). Previous research confirms that inter-limb asymmetry contributes to altered joint mechanics and may predispose the dominant leg to ACL injury (61–63).
In conclusion, fatigue reduced jump height and several joint moments but did not substantially alter GRF loading patterns. However, asymmetry between dominant and non-dominant legs appears to be a more influential risk factor for ACL injury. Therefore, implementing strength and balance programs that enhance inter-limb symmetry may help mitigate injury risk and improve athletic performance in volleyball players.

Ethical Considerations
Compliance with ethical guidelines
All ethical considerations in this study were fully observed, and the research was conducted in accordance with the principles outlined in the Declaration of Helsinki.
Funding
This research did not receive any financial support from the government, private, or non-profit organizations. 
Authors' contributions
All authors contributed equally to preparing the article.
Conflicts of interest
The authors declare that there are no conflicts of interest associated with this article.