Background and Purpose
Sleep, a vital biological need, occupies about 35% of human life and plays a crucial role in maintaining optimal physical, cognitive, and psychological health. For athletes, adequate sleep is essential as it enhances performance, promotes effective recovery, strengthens immune function, and facilitates learning and memory consolidation, while simultaneously reducing fatigue, mental stress, and risk of injury. However, athletes frequently face sleep disturbances due to factors such as high training loads, competition stress, frequent travel across time zones, and irregular daily schedules. These conditions often necessitate athletes to obtain 9–10 hours of sleep nightly to sustain optimal functioning. Sleep disorders including insomnia, sleep apnea, or circadian rhythm disruptions affect 30–45% of the global population and negatively influence neurological, endocrine, and motor functions, leading to impairments in balance, decision-making abilities, reaction times, and muscle strength.
This study focuses specifically on poor sleep quality as a potential and modifiable risk factor for secondary anterior cruciate ligament (ACL) injuries in athletes. ACL injuries significantly compromise knee stability, proprioception, and lower-limb movement kinematics, which cumulatively increase the risk of reinjury during dynamic sports activities. Additionally, factors such as gender, age, incomplete or inadequate rehabilitation, and persistent fatigue can further elevate these risks. Post-ACL reconstruction, altered lower-limb kinematics characterized by increased knee valgus and reduced flexion angles might predispose athletes to secondary injuries. Given the severe physical, psychological, and economic consequences of failed ACL repairs, identifying modifiable risk factors such as sleep quality and disturbances is critically important. This research investigates the extent to which sleep disturbances exacerbate deficits in kinematics, balance, and proprioception, thereby contributing to the risk of secondary ACL injuries. The findings are expected to provide valuable insights for athletes, coaches, and clinicians and inform the development of targeted preventive strategies to reduce reinjury incidence and improve long-term athlete health and performance.
 
Materials and Methods
This study employed a descriptive, applied, and within-group design to investigate the relationship between sleep quality and functional outcomes including kinematics, balance, and proprioception in male athletes with a history of anterior cruciate ligament reconstruction (ACLR). The target population consisted of men residing in Tehran who had undergone ACLR at least six months prior, experienced non-contact injury mechanisms, and consistently played football at least twice weekly. Using G*Power software with a power of 0.80 and an effect size of 0.60, a calculated sample size of 44 participants was determined and selected purposively based on strict inclusion criteria to ensure homogeneity.
Inclusion Criteria consisted of males aged between 25 and 35 years, with a normal BMI range of 18–25 kg/m², who had received hamstring autograft ACLR, exhibited positive dynamic knee valgus during single-leg squat assessments, did not use sedatives or psychiatric medications in the preceding three months, had engaged in a minimum of 50 hours of sports activity annually before ACLR, completed standardized post-ACLR rehabilitation protocols, and had no history of prior lower-limb surgeries other than ACLR, nor any multi-ligament or osteochondral injuries.
Exclusion criteria included any discomfort, pain, or injury occurring during testing sessions or inability to complete the full assessment protocols. Participants were actively recruited through collaboration with physiotherapy clinics and knee surgeons in Tehran, ensuring access to relevant patient populations. This carefully controlled sample allows for focused examination of how sleep quality correlates with critical functional parameters relevant to athletic performance and reinjury risk following ACLR.
After signing informed consent, participants underwent a comprehensive battery of assessments designed to evaluate multiple dimensions of sleep quality and lower-limb functional performance.
1.     Pittsburgh Sleep Quality Index (PSQI): This validated self-report questionnaire was used to assess various aspects of sleep disturbances and overall sleep quality in the preceding month. Scores equal to or greater than 6 were considered indicative of poor sleep quality, reflecting issues such as latency, duration, efficiency, and disturbances during sleep.
2.     Single-Leg Squat Test: This functional movement test was employed to evaluate dynamic knee valgus, a critical biomechanical factor linked to lower-limb injury risk . Participants performed controlled single-leg squats while clinicians observed and recorded knee alignment deviations, which serve as indicators of neuromuscular control deficits.
3.     Landing Error Scoring System (LESS): To analyze jump-landing kinematics, this standardized observational tool quantified common movement errors during drop landings. The LESS provides insight into biomechanical factors that may contribute to injury risk by identifying faulty landing mechanics, such as excessive knee valgus or inadequate trunk control.
4.     Y-Balance Test (YBT): This dynamic balance test assessed participants’ ability to maintain stability while reaching in multiple directions with one leg, reflecting neuromuscular coordination and proprioceptive integration critical for athletic performance and injury prevention.
5.     Joint Position Sense Test (at 30° and 60° knee flexion): Proprioceptive accuracy was evaluated by measuring participants’ ability to actively replicate targeted joint angles without visual feedback. Angle reconstruction errors quantified deficits in joint position sense, which have important implications for motor control and stability following ACL reconstruction.
Data normality was confirmed via skewness/kurtosis tests. Simple linear regression (SPSS v27, p ≤ 0.05) examined relationships between sleep quality (predictor) and functional outcomes (dependent variables).
 
Findings
PSQI Scores: Nearly half of participants (mean score ~6, range: 2–14) had clinically significant sleep disturbances.
Correlations:
·       Strongest correlation: Sleep quality and balance (r = +0.962).
·       Negative correlations: Sleep disturbances worsened LESS scores (r = -0.768) and proprioceptive errors at 30° (r = -0.808) and 60° (r = -0.883).
Regression Results: Sleep quality predicted:
·       92% of balance variance (YBT).
·       78% (60°) and 65% (30°) of proprioceptive error variance.
·       59% of landing kinematics (LESS).
Poor sleep quality significantly correlated with impaired landing mechanics, balance deficits, and reduced proprioception in ACLR patients. These findings highlight sleep as a modifiable risk factor for secondary ACL injuries, emphasizing its role in rehabilitation and injury prevention strategies.
 
Conclusion
This study examined the relationship between sleep disturbances in male athletes with ACL reconstruction (ACLR) and dynamic knee valgus, landing kinematics, balance, and proprioception. Results demonstrated that poor sleep quality significantly predicted deficits in landing mechanics, balance, and joint position sense (30° and 60° knee flexion), highlighting its role as a modifiable risk factor for secondary ACL injuries.
Sleep disturbances likely impair neuromuscular, sensory, and motor systems, directly compromising athletic performance. Supporting studies Marwanasari et al., 2024; Erlacher, 2024) found that sleep deprivation negatively affects memory, pain perception, immunity, and metabolism, while improved sleep enhances cognitive and physical performance. Similarly, Fullagar et al. (2023) linked adequate sleep to better skill acquisition and tactical learning in athletes.
Physiologically, sleep is critical for recovery. The glymphatic system clears neurotoxic waste (e.g., beta-amyloid) during deep sleep, while cytokines and growth hormones produced during sleep aid immune function and cellular repair (Souza et al., 2023). Poor sleep disrupts these processes, impairing proprioceptive accuracy, balance, and neuromuscular control. Silva et al. (2021) emphasized extending nighttime sleep or adding naps to boost performance, aligning with our findings.
In ACLR athletes with dynamic valgus, sleep-related deficits may exacerbate sensorimotor dysfunction. The brain integrates visual, vestibular, and proprioceptive inputs to coordinate movement; sleep deprivation likely disrupts this integration, increasing kinematic errors and balance instability. Checa et al. (2020) reported similar gait and balance impairments in sleep-deprived females, consistent with our Y-Balance and LESS results
 
Article Message
This study underscores the critical role of sleep quality in preventing secondary injuries. Routine and seasonal sleep assessments are recommended for athletes to reduce sleep disturbances and injury risk. Future research should explore the relationship between sleep quality and injury risk during functional activities, incorporating laboratory measurements, biomarkers, postural analysis, sex differences, and a range of lower limb disorders across different age groups.
Ethical Considerations
The study was approved by the Research Ethics Committee on 2024-05-25 under the code of IR.IAU.SRB.REC.1403.152.
Authors’ Contributions
Mostafa Jalili Bafrouei: conceptualization, data collection, literature review, data analysis, manuscript writing, project administration.
Mohammadreza Seyedi: data collection, literature review, manuscript editing.
Seyed Hossein Mirkarimpour: data collection, literature review, manuscript revision.
All authors reviewed and approved the final manuscript.
Conflict of Interest
The authors declare no conflict of interest related to this study.
 
Acknowledgments
The authors extend sincere gratitude to all participants and staff at the physiotherapy and rehabilitation centers who supported this research, including Arvand Physiotherapy Center, Sarir Rehabilitation Center, Dr. Mohammad Ali Sazegari Ardakani, and Arad Hospital.