Extended Abstract
1.    Introduction
Landing is a critical phase in sports because it forms the final component of many athletic movements and is strongly associated with lower-limb injuries, particularly anterior cruciate ligament (ACL) tears. These injuries are predominantly complete (80.5%) and occur more frequently in the dominant leg, most often through non-contact mechanisms (74.6%, 63.6%) (1). Several factors influence landing-related injury risk, including landing technique (2), landing height (3), gender and surface characteristics (4), footwear (5), sport experience (6), and the use of knee or ankle braces (7). Another influential factor is muscle fatigue, defined as a temporary reduction in muscular performance capacity. Fatigue has been shown to alter biomechanical parameters such as ground reaction forces and joint flexion angles (8), although findings remain inconsistent due to variations in fatigue induction protocols (9). Research examining fatigue effects across different ages and genders has revealed neuromuscular control differences that may contribute to a higher risk of knee injuries in female athletes. Various fatigue protocols have been used in the literature, including cycling, squats, step tests, repetitive jumps, and simulations of real competition. Given that landing is a major contributor to lower-limb injuries, most studies have focused on how fatigue modifies landing mechanics. However, an important yet often overlooked factor is the complexity of the pre-landing task. The majority of previous research has relied on simple tasks such as drop jumps and countermovement jumps, which do not reflect the cognitive and motor demands of more intricate sport-specific movements. Complex skills like taekwondo jump kicks require greater cognitive load (10) and frequently end with unilateral landings, a situation associated with a high risk of ACL injury. Considering that most taekwondo injuries involve the lower extremities (65.5%) (13), understanding landing mechanics in this context is essential. Accordingly, the present study compares the kinetic and kinematic characteristics of a simple task (countermovement jump) with those of a complex task (taekwondo jump front kick) under both rested and fatigued conditions. It is hypothesized that fatigue will influence landing mechanics differently depending on task complexity.
2.    Methods
Six highly skilled female welterweight taekwondo athletes participated in this study. Kinematic data were captured using a three-dimensional motion analysis system equipped with six high-speed cameras operating at 120 Hz. Sixteen reflective markers were placed on key anatomical landmarks of the lower limbs to track joint motions. Ground reaction forces were collected using a portable force platform. Blood lactate concentration was assessed with a handheld lactate analyzer to determine fatigue thresholds. After a standardized warm-up, participants performed two tasks—the countermovement jump (CMJ) and the taekwondo jump front kick (TJFK)—using both the dominant and non-dominant legs. Each task involved both the jumping and landing phases, with strict technical criteria applied to ensure proper execution of the taekwondo kicks. Following the initial task performance, athletes completed a fatigue protocol consisting of repeated step-up and step-down movements on a 30 cm platform, performed as rapidly as possible for three minutes per cycle. This protocol was repeated four times consecutively without rest until volitional exhaustion, resulting in an average total workload duration of approximately 27 minutes. Kinematic and kinetic data were collected from the supporting leg during each landing. 
Statistical analyses were performed using dependent t-tests to compare pre- and post-fatigue values as well as differences between the two task conditions, with the significance level set at p < 0.05. Percent changes and between-task differences were calculated. Bilateral asymmetry was assessed using a Symmetry Index, where the direction of asymmetry was determined based on sign conventions. This approach enabled direct comparison of asymmetry patterns between the simple and complex tasks under both rested and fatigued conditions.
3.    Results
Blood lactate concentration increased significantly from the rested condition (2.2 ± 0.75 mmol/L) to the fatigued condition (7.4 ± 2.2 mmol/L) (p < 0.005), confirming the effectiveness of the fatigue protocol. In the dominant leg, hip flexion at initial contact during the complex task increased from 5.09° to 23.97° (p < 0.005), while ankle angular velocity in the simple task rose from 271.16°/s to 403.87°/s (p = 0.000). Flight height in the simple task also decreased significantly from 16.31 cm to 15.01 cm (p < 0.02). No significant kinetic differences were observed between the simple and complex tasks in the rested state. However, under fatigue, the vertical ground reaction force impulse was significantly greater in the simple task than in the complex task (p < 0.04). The tasks demonstrated more pronounced differences in kinematic parameters. In the rested condition, all hip-related variables—hip flexion at impact, maximum hip flexion, and hip angular velocity—were significantly higher in the simple task compared to the complex task. Under fatigue, in addition to the hip variables, knee peak angle, knee angular velocity, and ankle angular velocity also showed significant between-task differences (Table 1). In the non-dominant leg, only one variable was significantly affected by fatigue: hip angular velocity, which decreased from 72.22°/s to 20.1°/s (p < 0.04). Symmetry index analysis revealed distinct patterns across tasks and conditions. In the rested state, the simple task showed a symmetry index range of 0.25 to 25.47 with a mean of 9.48, whereas the complex task displayed a wider range of 0.2 to 84.89 with a mean of 20.8. Under fatigue, the simple task exhibited a range of 2.14 to 38.1 and a mean of 9.78, while the complex task presented values ranging from 6.01 to 53.3 with a mean of 10.1. Bilateral asymmetries were identified when differences between limbs exceeded 15%. 

4.    Discussion
This study examined how muscle fatigue influences landing biomechanics during simple and complex tasks. The fatigue protocol produced a greater reduction in peak vertical ground reaction force (vGRF) in the simple task (13.7%) compared with the complex task (2.8%). Previous research has reported mixed effects of fatigue on vGRF, with studies showing increases, decreases, or no meaningful changes, largely depending on the type of fatigue protocol and the specific muscle groups targeted (16). 
Interestingly, the impulse—which reflects the change in momentum—showed minimal overall change, consistent with findings from other studies (22). However, impulse increased in the simple task and decreased in the complex task following fatigue, which may be partially attributable to a reduction in jump height. Statistical analyses identified two notable changes in hip kinematics during landing in the complex task. First, hip flexion at initial contact increased in the dominant leg following fatigue. This finding aligns with several studies (24), though it contradicts research reporting a decrease in this variable (25). Second, hip angular velocity in the non-dominant leg decreased, a result that is inconsistent with recent studies reporting either increased hip angular velocity (26) or no significant change (27). Additionally, no significant alterations were observed in knee kinematics during the complex task under fatigue. This outcome is supported by some previous investigations, but stands in contrast to studies reporting increased knee flexion at initial contact (28) or at peak (18), as well as research noting reductions in these parameters (18). To our knowledge, no prior study has reported either a decrease or no change in knee angular velocity; most existing literature indicates an increase in this variable (29).
For the ankle, the present study found a significant increase in ankle angular velocity during the simple task after fatigue, consistent with earlier work (24). However, neither the plantarflexion angle at initial contact nor the maximum ankle flexion angle changed significantly. Previous findings regarding these variables remain inconsistent, with some studies reporting decreases (24) and others reporting increases (18), highlighting a lack of consensus in the literature. The analysis of the symmetry index showed considerable variability across variables, suggesting that the influence of fatigue on biomechanical asymmetry is strongly dependent on task complexity. Overall, the interaction between fatigue and skill complexity shapes landing strategies, joint mechanics, and asymmetry patterns in ways that may have important implications for injury risk. These findings underscore the importance of considering both task complexity and fatigue in injury prevention programs and performance training.

Ethical Considerations
Compliance with ethical guidelines
All ethical considerations were fully observed, and the study was conducted in accordance with the principles of the Declaration of Helsinki.
Funding
This research did not receive any financial from the government, private, or non-profit organization. 
Authors' contributions
The sole author was responsible for the study conception and design, data collection, data analysis and interpretation, drafting of the manuscript, and approval of the final version.
Conflicts of interest
The author declares no conflict of interest.