Background and Purpose
Football is the most widely played sport worldwide, engaging approximately 4% of the global population. Among football-related injuries, anterior cruciate ligament (ACL) injuries are prevalent, with incidence rates ranging from 0.06 to 3.7 per 1,000 hours of participation. Such injuries result in prolonged absence from sports, diminished quality of life, and decreased athletic performance. ACL injuries are multifactorial, influenced by intrinsic factors including neuromuscular and biomechanical characteristics, anatomical predispositions, and fatigue, alongside extrinsic factors such as equipment and playing surface conditions. Fatigue is a critical contributor, as most ACL injuries occur during the latter stages of matches or training, affecting lower limb kinematics by decreasing knee flexion and ankle dorsiflexion while increasing knee valgus. Players with a history of ACL injury face elevated reinjury risk due to persistent biomechanical alterations and psychological concerns. This study investigates the impact of football match-induced fatigue on knee and ankle kinematics in healthy players compared with those who have undergone ACL reconstruction, addressing previous research gaps.
 
Methods
Forty male amateur football players from the Tehran Premier League participated and were divided into two groups: healthy players (n = 20), randomly selected from two teams, and players with prior ACL reconstruction (n = 20), purposefully recruited. Groups were matched by playing position. An orientation session was conducted with the research team, coaches, and participants to detail study procedures and collect demographic and injury history data. Written informed consent was obtained.
Testing comprised four phases. Pre-match assessments occurred two hours before match commencement after standardized warm-up. Kinematic variables ankle dorsiflexion, maximum knee flexion, and knee valgus were measured during a vertical jump-landing task using inertial measurement unit (IMU) accelerometers (Noraxon, USA), operated by two trained evaluators. The fatigue protocol entailed a full official football match (two 45-minute halves plus extra time and halftime). Post-match assessments occurred immediately after the match or upon player substitution. Fatigue was quantified via the Borg Rating of Perceived Exertion (RPE) scale.
IMU sensors were bilaterally affixed to the thigh, shin, and ankle. System calibration established target joint angles. The vertical jump-landing test required participants to jump down from a 30-cm box onto a marked area and immediately execute a maximal vertical jump, with landing phases analyzed for kinematic variables. Three trials were performed, with the most accurate selected for analysis.
Data were analyzed with SPSS v.23 using mixed 2×2 ANOVA to examine fatigue and group effects, with significance at α ≤ 0.05.
 
Results
Mixed ANOVA results indicated no significant within-group, between-group, or interaction effects for ankle dorsiflexion (p ≥ 0.05). For maximum knee flexion, no significant within- or between-group differences were observed (p ≥ 0.05), but a significant group-by-time interaction was found (p ≤ 0.05), indicating significant fatigue effects between groups. Maximum knee valgus revealed a significant within-group effect (p ≤ 0.05), suggesting fatigue influenced knee valgus regardless of ACL injury history, while between-group and interaction effects were non-significant (p ≥ 0.05).
 
Conclusion
This study compared the effects of football match-induced fatigue on kinematic variables associated with ACL injury risk between players with prior ACL reconstruction and healthy controls. Findings demonstrate that acute fatigue significantly decreases maximum knee flexion in ACL-reconstructed players and increases knee valgus in healthy players during landing. Although knee valgus did not change significantly in the ACL group post-fatigue, its baseline magnitude was higher compared to healthy players. These outcomes underscore the necessity for tailored preparation, recovery, and injury prevention programs, especially among players with ACL history. Coaches and medical staff should implement and routinely review interventions designed to mitigate fatigue-induced biomechanical risks to optimize player safety and performance.
 
Article Message
Players with prior ACL reconstruction require ongoing monitoring by coaching and sports medicine personnel due to persistent functional alterations and fatigue's exacerbating effects on lower limb kinematics. Systematic evaluations enable management of injury risk factors under controlled conditions, enhancing safety and performance.
Ethical Considerations
Ethical approval was obtained from Shahid Beheshti University Ethics Committee (IR.SBU.REC.1403.003). All procedures conformed with institutional guidelines.
Authors’ Contributions
The first author led research design, data collection, analysis, and manuscript drafting. The second author provided research supervision and guidance.
 
Conflict of Interest
No conflicts of interest were declared.
 
Acknowledgments
No external funding was received. The authors gratefully acknowledge participating athletes and supporting personnel who facilitated this research.