Background and Purpose
Anterior cruciate ligament (ACL) rupture is among the most prevalent and functionally disruptive injuries in the lower extremities, particularly in active populations. Surgical reconstruction (ACLR) is often required to restore knee stability, yet biomechanical deficits frequently persist long after the procedure, especially when compounded by pre-existing or concomitant biomechanical abnormalities such as pronated feet. Pronated foot posture, characterized by excessive eversion of the calcaneus and collapse of the medial longitudinal arch, alters the kinetic chain from the foot up to the hip and spine, potentially exacerbating instability and maladaptive loading patterns in ACL-reconstructed knees. Epidemiological data indicate that 20–23% of adult’s exhibit pronated feet, while annual ACL injury incidence ranges from 30 to 70 per 100,000 individuals, with over 200,000 ACL reconstructions performed annually in the United States alone. Despite advances in surgical techniques and rehabilitation protocols, re-injury rates remain alarmingly high (5–25%), particularly within the first 1–2 years’ post-surgery, often attributed to premature return to sport, inadequate neuromuscular control, and unresolved biomechanical asymmetries. Muscular fatigue, a physiological state marked by reduced force-generating capacity and impaired neuromuscular coordination, further compounds these deficits. Fatigue diminishes proprioceptive acuity, delays muscle activation, and disrupts agonist-antagonist co-activation patterns, thereby increasing vulnerability to joint instability and abnormal loading. In individuals with ACLR, fatigue may unmask latent biomechanical deficiencies that are not apparent under non-fatigued conditions. Moreover, the synergistic effect of ACLR and pronated foot posture on plantar pressure distribution particularly under fatigued conditions remains underexplored, despite its critical implications for rehabilitation strategy and injury prevention.
Plantar pressure analysis serves as a sensitive biomechanical indicator of postural control, load distribution, and neuromuscular compensation strategies. Abnormal pressure patterns, especially elevated medial heel and midfoot pressures, are associated with increased pronation and may reflect compensatory strategies to stabilize the knee in the presence of residual ACL deficiency. Therefore, the purpose of this study was to investigate static plantar pressure parameters in male individuals with ACL reconstruction and pronated feet across different post-surgical time intervals (less than 6 months, 6–12 months, and more than 12 months), both before and after induced muscular fatigue, to identify critical windows of vulnerability and inform targeted rehabilitation interventions.
 
Methods
This quasi-experimental study employed a pretest-posttest design with repeated measures and was conducted in the Sports Biomechanics Laboratory at the University of Mohaghegh Ardabili, Iran. Sample size calculation using G*Power 3.1 (P = 0.05, effect size = 0.7, power = 0.8) determined a minimum of 10 participants per group. Forty male volunteers aged
18–45 years were recruited and divided into four groups: Group A (ACLR + pronated foot, <6 months’ post- operative, n=10), Group B (ACLR + pronated foot, 6–12 months’ post- operative, n=10), Group C (ACLR + pronated foot, >12 months’ post- operative, n=10), and Group D (healthy controls, n=10). Inclusion criteria comprised unilateral ACLR using hamstring autograft, absence of lower limb pain or injury, ability to run without restriction, BMI between 18.5–25 kg/m², navicular drop >10 mm, and rearfoot angle >4°. Exclusion criteria included history of lower limb fracture, leg length discrepancy >5 mm, or unwillingness to participate. Ethical approval was obtained (IR.UMA.REC.1404.019, IRCT20220129053865N2), and written informed consent was secured from all participants.
Plantar pressure data were collected using the Footscan pressure plate system (RSscan International, Belgium; 4363 sensors, 300 Hz sampling rate). The system automatically segmented the foot into 10 anatomical regions: medial heel, lateral heel, midfoot, metatarsals 1–5 (M1–M5), hallux (T1), and toes 2–5 (T2–T5). Measurements were taken in static standing posture, both pre- and post-fatigue. Fatigue was induced via a progressive treadmill protocol (Horizon Fitness, Omega GT, USA) starting at 6 km/h, increasing by 1 km/h every 2 minutes until participants reached a Borg Rating of Perceived Exertion (RPE) of 13 or higher. Participants then maintained a steady pace of 12 km/h until RPE ≥17 or heart rate ≥80% of maximum for 2 minutes.
Statistical analysis was performed using SPSS v23. Data normality was confirmed via Shapiro-Wilk test. A two-way repeated-measures ANOVA was used to examine the main effects of “time” (pre- vs. post-fatigue) and “group” (A, B, C, D), as well as their interaction. Post-hoc pairwise comparisons with Bonferroni correction were conducted where significant effects were found. Effect sizes (Cohen’s d) were calculated for all significant outcomes. Statistical significance was set at p < 0.05.
 
Results
Results revealed a significant main effect of time (fatigue) on nearly all plantar pressure variables (p < 0.001), indicating that fatigue universally increased plantar pressures across all groups. The “group” factor also showed significant differences (p < 0.05), with Group A (<6 months’ post-op) exhibiting the highest-pressure values in the medial heel, midfoot, and metatarsal regions suggesting that early-stage rehabilitation is marked by the most pronounced biomechanical compromise.
Critically, a significant time × group interaction was observed for mean plantar pressure (right foot: p = 0.006, d = 0.877; left foot: p = 0.007, d = 0.281), medial heel pressure (right: p = 0.001, d = 0.384; left: p = 0.041, d = 0.280), first metatarsal pressure (M1), and pressure under toes 2–5 (T2–T5) (right: p = 0.009, d = 0.364; left: p = 0.004, d = 0.378). Post-hoc analyses confirmed that the greatest pressure increases post-fatigue occurred in Groups A and B (<12 months’ post-op), particularly in medial heel and forefoot regions.
 
Conclusion
Individuals with ACL reconstruction and concomitant pronated foot posture demonstrate significantly elevated and abnormally distributed plantar pressures during static standing, particularly under fatigued conditions and within the first 12 months’ post-surgery. The most critical period appears to be the first six months, where medial heel and forefoot loading increases dramatically with fatigue, signaling profound neuromuscular instability and compensatory strategies.
 
Article Message
It appears that individuals with anterior cruciate ligament reconstruction and pronated feet, particularly during the early stages of rehabilitation (less than 12 months’ post-surgery), experience a significant increase in plantar pressure under fatigued conditions, indicating greater postural instability and improper load distribution across the foot highlighting the critical need for fatigue-resilient, foot-targeted rehabilitation strategies.
 
 
Ethical Considerations
Ethical approval was obtained (IR.UMA.REC.1404.019), and written informed consent was secured from all participants.
 
Authors’ Contributions
Conceptualization: AmirAli Jafarnezhadgero
Data Collection: Ebrahim Piri
Data Analysis: Ebrahim Piri
Manuscript Writing: Ebrahim Piri
Review and Editing: AmirAli Jafarnezhadgero and Ebrahim Piri
Responsible for funding: AmirAli Jafarnezhadgero
Literature Review: Mahrokh Dehghani and Afsaneh Enteshari-Moghaddam
Project Manager: AmirAli Jafarnezhadgero
Conflict of Interest
According to the authors, this article has no conflicts of interest.
 
Acknowledgments
The researchers sincerely appreciate all the participants, instructors, and managers who collaborated in the implementation of this research program.