1.    Introduction
Anterior cruciate ligament (ACL) injury is one of the most common and debilitating sports injuries, affecting millions worldwide. In the U.S., ACL reconstruction surgeries have significantly increased across all age groups (1). Globally, over two million ACL injuries occur annually, with nearly 40% of individuals unable to return to their pre-injury sports performance even two years post-reconstruction (2,3). Soccer, the world’s most popular sport, is also among the riskiest for ACL injuries, with non-contact mechanisms such as landing from a jump being major causes (4-6).
ACL plays a crucial role in knee stability, not only as a mechanical restraint but also through its proprioceptive function (8). While ACL reconstruction restores anatomical integrity, deficits in proprioception and neuromuscular control often persist, increasing the risk of re-injury (9). This concern is particularly relevant for athletes returning to high-demand sports like soccer, where the reconstructed knee, especially with a hamstring graft, faces a higher risk of subsequent injuries (10). Traditional rehabilitation programs primarily focus on restoring motor abilities such as strength, power, and coordination. However, recent studies suggest ACL injuries also impair sensory input due to mechanoreceptor damage in the ligament and joint capsule, altering cortical activation patterns and affecting motor control (11-13). Knee braces are commonly recommended in early rehabilitation to limit excessive knee motion and provide additional support (14). Research indicates that braces can reduce excessive tibial rotation under high rotational loads and partially restore normal kinematics (14). However, prolonged brace use may lead to muscle atrophy and decreased proprioception due to restricted joint movement (15). Despite being widely used in dynamic sports, studies on their effectiveness remain inconclusive. While some research suggests braces significantly reduce ACL injuries in soccer players, others report increased knee injury risk among high school and collegiate athletes (16-20).
Most previous studies have focused on simple tasks like walking or landing, rather than sport-specific movements such as jumping and heading, which are critical for injury prevention and performance optimization. Soccer involves high-stress maneuvers like kicking, jumping, and landing under unstable conditions, placing substantial loads on the knee and ankle joints (21,22). Reduced knee flexion during landing has been identified as a key biomechanical risk factor for ACL injuries, increasing strain on the ligament (23-26). Moreover, dual-task activities, such as jumping to head the ball, have been shown to further elevate ACL loading and disrupt landing mechanics (29-32). Despite extensive research on knee braces, most studies have examined their effects during low-impact daily activities. There is a notable lack of studies evaluating the combined effects of knee and ankle braces during sport-specific tasks like jump-landing with heading. This research gap highlights the need to investigate whether bracing can optimize joint kinematics, reduce re-injury risk, and enhance performance in athletes returning to competition after ACL reconstruction. This study aims to assess the effects of knee and ankle braces on lower limb kinematics during a jump-landing task with heading in elite soccer players with a history of ACL reconstruction. By analyzing the biomechanical characteristics of these movements, the study seeks to determine whether bracing can mitigate sensorimotor deficits, improve landing mechanics, and reduce the risk of re-injury in post-ACL reconstruction athletes.
2.    Methods
This study included 12 professional football players who had returned to sport after ACL reconstruction (ACLR) as the experimental group (age: 25.70±1.30 years, height: 1.75±0.02 m, weight: 80.08±3.61 kg, BMI: 26.02±0.96 kg/m²) and 12 healthy professional football players with no history of major injuries as the control group (age: 24.90±1.65 years, height: 1.76±0.02 m, weight: 74.00±3.61 kg, BMI: 23.96±0.70 kg/m²). The required sample size was estimated using G*Power software, considering α = 0.05 and an effect size of 0.80. Height and weight were measured using a stadiometer and weighing scale. Kinematic data were collected using a Vicon motion capture system (Oxford, UK) with four high-speed cameras (200 Hz). Sixteen spherical markers (14 mm in diameter) were attached bilaterally at specific anatomical points following the Plug-In Gait model. Each participant performed four successful barefoot jump-landing trials. Starting 50 cm before a force plate, they jumped over a 20 cm foam obstacle, striking a suspended ball at peak height, and landed with each foot on a separate force plate. Participants were required to maintain balance for 30 seconds post-landing. Trials were repeated if participants lost balance or took an extra step. Before testing, participants warmed up for five minutes and performed 10 practice trials. The test was conducted under three conditions: (a) without knee or ankle braces (WS), (b) with a knee brace (KS), and (c) with both knee and ankle braces (KAS). The braces were provided by OPPO Medical Inc. Kinematic data were recorded using Nexus software and saved as C3D files, which were processed in Mokka and OpenSim-3.1. Analyzed variables included pelvic tilt, hip flexion, knee flexion, and ankle dorsiflexion/plantarflexion across five phases: preparation, takeoff, peak, landing, and post-landing. Data were filtered using a 6 Hz Butterworth low-pass filter. The Shapiro-Wilk test assessed data normality. Mean values of four successful trials were used for statistical analysis. MANOVA was applied to compare movement differences between groups across phases, while repeated measures ANOVA assessed within-group effects of the knee brace. Statistical significance was set at P < 0.05.
3.    Results
The kinematics of lower limb joints during jump-landing were analyzed under three conditions (WS, KS, KAS) in ACLR and control groups across five phases. No significant differences were found in the preparation, landing, and post-landing phases (P>0.05). In the start phase, hip flexion in the ACLR group was 3° greater in WS than in KS and KAS (P=0.035), while in the control group, hip flexion in WS was 3° greater than in KS (P=0.046). In the peak phase, hip flexion was 3° greater in WS than KS in the control group (P=0.025). Across all jump-landing conditions, the ACLR group exhibited significantly greater pelvic tilt than the control group in all phases (P<0.05). In the preparation phase, pelvic tilt was 15.8° in WS, 18.5° in KS, and 19.7° in KAS. Similar differences persisted in the start, peak, landing, and post-landing phases. Additionally, hip flexion in the ACLR group was significantly greater than in the control group during landing (10.2° in WS, 9.5° in KS, 9.9° in KAS; P<0.05). In KS, ankle plantarflexion was 6.8° greater in the ACLR group (P=0.052). Post-landing, hip flexion remained higher in the ACLR group (13.8° in WS, 11.6° in KAS; P<0.05).
4.    Conclusion
This study examines the impact of knee and ankle braces on lower limb joint movement patterns during jump-heading-landing in professional football players with a history of anterior cruciate ligament reconstruction (ACLR). The findings reveal that braces significantly influence lower limb kinematics during jumping and landing phases, with notable differences between ACLR and control groups.
The ACLR group demonstrated greater pelvic tilt across all phases, likely as a compensatory mechanism to reduce knee stress, though excessive pelvic tilt and hip flexion may increase injury risk (8, 35). Proprioceptive deficits and neuromuscular control impairments post-ACLR surgery persist, leading to altered movement patterns. While kinesiotape enhances sensory feedback (36-38), knee braces have not been shown to offer similar benefits. Glattek (2022) noted that braces provide psychological security but no significant clinical advantage and may cause muscle atrophy and reduced proprioception with prolonged use (41, 42). Choi et al. (2011) found long-term brace use weakens the vastus medialis oblique, questioning their effectiveness (15). Braces also reduce knee flexion angles, range of motion, and angular velocity, potentially increasing ACL injury risk (43-45). Brian et al. (2023) suggested knee braces might lower ACL injury risk by modulating strain and movement in certain planes, but their efficacy in controlling sagittal plane movement and reducing ground reaction forces (GRFs) remains unclear (46). Other studies found no significant brace effects on knee kinematics (47-49). During takeoff, knee braces reduced hip flexion, potentially limiting performance but reducing knee load (50). At landing, increased ankle plantarflexion in the ACLR group wearing braces may help absorb GRFs and lower ACL stress (32). Persistent abnormal movement patterns in ACLR athletes highlight the need for rehabilitation programs focusing on neuromuscular control (9). Reduced knee flexion during landing increases ACL injury risk due to higher anterior shear forces (35, 51-56). Increasing knee flexion and range of motion is protective (26, 57), and some studies suggest braces may aid in this regard (58). Akbari et al. (2023) found that adding a cognitive task, like heading a ball, altered trunk and lower limb kinematics, potentially increasing ACL injury risk. This underscores the importance of training safe jump-landing techniques under cognitive load to prevent injuries (27).
This study demonstrates that elite soccer players with a history of ACL reconstruction exhibit altered lower limb kinematics, even after returning to sport. The use of knee and ankle braces can modify movement patterns, potentially reducing re-injury risk, though they may also impose performance limitations. Rehabilitation protocols should prioritize restoring normal movement patterns and enhancing neuromuscular control, while the use of braces should be carefully monitored. Future research should investigate the long-term effects of brace use and develop more effective rehabilitation strategies for athletes post-ACL reconstruction.

Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be addressed in this research.
Funding
This research did not receive any financial support from 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.