Introduction
Ankle sprains are among the most frequent injuries in both sports and daily activities, accounting for 10-28% of all sports-related injuries [1,2]. Over two million people suffer from ankle injuries each year [3], often leading to prolonged absences from athletic activities. Interventions such as exercises, ankle taping, and ankle bracing are commonly employed to reduce the incidence and recurrence of these injuries [4]. Despite the widespread use of ankle taping, it is often considered cumbersome, time-consuming, and prone to loosening during activity, leading to a preference for ankle bracing [5].
Prophylactic bracing can prevent approximately 30 ankle sprains per 1,000 athletic exposures [6]. However, its impact on other joints, particularly the knees, is less understood. According to the kinetic chain theory, motion at one joint affects the movement and stress at other joints [7]. For instance, an ankle brace might alter tibial movement, affecting knee joint kinematics and potentially increasing the forces on the knee [8], thereby raising the risk of knee injuries, including anterior cruciate ligament (ACL) tears [9-13]. Previous studies have found that ankle braces may increase knee axial rotation, which could heighten the risk of ACL injuries given the ACL’s role in limiting knee anterior sliding and axial rotation [12,13].
Cutting movements are particularly relevant in sports due to their strong association with ACL injuries, especially in activities involving sudden changes in direction like basketball and soccer [14,15]. These movements place substantial stress on the knee joint, which can be exacerbated by external factors such as ankle bracing, potentially increasing the risk of injury [16,17]. Given that non-contact ACL injuries frequently occur during cutting maneuvers [18], analyzing these tasks is crucial for understanding ACL injury mechanisms and assessing how ankle bracing might alter knee joint kinetics.
The severity of ACL injuries further underscores the importance of this research. ACL injuries are common and can occur in athletes of all ages and levels, leading to significant long-term consequences, including an increased risk of developing knee osteoarthritis and substantial financial costs associated with surgical reconstruction [19,20]. Moreover, these injuries can result in lost playing time and practice opportunities, deeply affecting an athlete’s career.
Although some studies report no significant difference in knee injury incidence with ankle bracing [21], others suggest that bracing may increase knee joint loading and, thus, ACL injury risk during dynamic activities [22,23]. Therefore, the purpose of this study was to investigate the impact of prophylactic ankle bracing on knee joint kinetics and ground reaction forces (GRFs) during 90° side-step cutting tasks, with a particular focus on assessing the potential increase in ACL injury risk. Although previous studies have explored related topics [22-25], there remains a need for a more detailed examination of the specific mechanisms by which ankle bracing may contribute to ACL strain. It was hypothesized that participants would exhibit greater knee moments and GRFs during foot contact when using ankle bracing compared to no bracing, potentially increasing ACL strain and injury risk.
Methods
Participants
Thirty participants (15 males: age = 21.6 ± 2.1 years, height = 180.1 ± 7.8 cm, weight = 82.6 ± 11.1 kg; 15 females: age = 20.2 ± 1.2 years, height = 165.2 ± 5.4 cm, weight = 65.7 ± 9.7 kg) were recruited for this study. Prospective participants were screened for current lower extremity or back injuries and were excluded if they had previously undergone surgical intervention on the lower limbs, used ankle braces as a preventive measure, or were distance runners. Participants were required to be physically active, defined as participating in either intramural athletics or regular moderate to vigorous exercise three or more days per week, with a body mass index (BMI) no greater than 30 kg/m2. Participants were instructed to wear tight-fitting athletic apparel and their own athletic shoes during the trials. Ankle Stabilizing Orthosis braces (ASO; Medical Specialties Inc., Charlotte, NC) were used bilaterally in the “braced” condition. All participants were informed of the study’s purpose and provided written informed consent, approved by the Institutional Review Board of Texas Tech University (IRB: 503196).
Protocol
Participants completed 14 successful trials of a 90° cutting task in both braced and unbraced (control) conditions. Each set of 14 trials consisted of seven cuts to the left and seven cuts to the right, presented in random order. Participants were instructed to approach the force plate as quickly as possible while maintaining control, simulating a “game-like” scenario such as running a “fast break” or moving to receive a passed ball. Approximately 2 meters before reaching the force plate, a lab assistant signaled the direction (right or left) for the participant to cut. Each participant was randomly assigned to one of two counterbalanced conditions: either cutting without the brace first or cutting with the brace first. The average of the first five successful trials for each participant cutting off their dominant limb in each condition was used for data analysis. The dominant limb was defined as the limb the participant preferred to use when kicking a ball. Participants were allowed to complete as many practice trials as needed before starting the experimental trials.
Data analysis
For all trials, kinematics (sampling frequency: 100 Hz) and ground reaction forces (sampling frequency: 2000 Hz) were recorded using a six-camera Vicon motion analysis system (Vicon, Denver, CO) and a force plate (AMTI, Watertown, MA). A 15-marker lower extremity kinematic model (Plug-In-Gait Sacrum) was used to track lower extremity motion during the cutting trials. Markers were placed on the sacrum, right and left anterior superior iliac spine (ASIS), mid-thigh, lateral knee, mid-leg, lateral malleolus, heel, and toe. Three-dimensional marker positions were tracked and filtered using Vicon Nexus’ Woltring filter with generalized cross-validation. GRF data were normalized to body weight in Newtons and reported as units of body weight (BW). GRF, kinematic, and anthropometric data were used to calculate knee moments via standard inverse dynamics. Resultant joint moments were calculated as external moments, representing the net effect of all structures crossing the joint. Peak knee joint moments during the first 30% of the stance phase were analyzed in the frontal, sagittal, and transverse planes, as this period is when most ACL injuries occur [18].
Statistical analysis
The independent variable in this study was the brace condition, while the dependent variables were the peak knee moments and GRFs in three dimensions. Paired t-tests were conducted to statistically compare the selected variables between the braced and unbraced conditions using SPSS 21.0 statistical software (Chicago, IL). The initial alpha level was set at p < 0.05 for statistical significance. After applying the Bonferroni correction to control for Type I error, the adjusted p value threshold was set at p < 0.01.
In addition, a priori relationships between variables were assessed using Pearson’s Product-Moment Correlation Coefficient. To minimize the number of statistical comparisons and avoid redundancy, variables with correlation coefficients of 0.7 or higher were considered redundant. These variables were either dropped from subsequent statistical comparisons or interpreted as behaving consistently with their correlated variables.
Results
Pearson correlation coefficients were calculated to assess the relationships between the dependent variables <Table 1 and 2>. Significant correlations were found between unbraced vertical ground reaction force (V-GRF) and unbraced mediolateral ground reaction force (ML-GRF) (r = 0.88, p < 0.001), as well as between braced V-GRF and braced ML-GRF (r = 0.76, p < 0.001). Additionally, unbraced V-GRF showed a strong correlation with unbraced anteroposterior ground reaction force (AP-GRF) (r = 0.72, p < 0.001), and braced knee internal rotation moment (IRM) was strongly correlated with braced knee abduction moment (AB-M) (r = 0.72, p < 0.001). Due to the strong correlations between V-GRF and ML-GRF in both braced and unbraced conditions, and to avoid redundancy, V-GRF was excluded from further statistical comparisons.
Knee internal rotation moment (IR-M) was significantly greater in the braced condition (0.30 ± 0.12 N∙m∙kg-1, <Table 3>) compared to the unbraced condition (0.23 ± 0.13 N∙m∙kg-1; p = 0.003, Cohen’s d = 0.576). No significant differences were observed in knee flexion moment (FL-M) (p = 0.069), knee abduction moment (AB-M) (p = 0.025), ML-GRF (p = 0.651), or APGRF (p = 0.632).
Discussion
This study supports the kinetic chain theory, demonstrating that prophylactic ankle bracing increases knee joint loading, particularly by elevating knee internal rotation moments, which may predispose athletes to ACL injuries. The increase in knee internal rotation moment observed in this study aligns with findings from previous research, where internal rotation has been shown to significantly strain the ACL [26,27]. This suggests that the use of ankle braces during dynamic activities, such as side-step cutting, may increase the strain on the ACL, potentially leading to a higher risk of noncontact injuries. The implications of these findings are particularly important for sports where cutting maneuvers are frequent, and the risk of ACL injury is already elevated.
Despite the hypothesized increase in GRFs due to reduced ankle dorsiflexion with bracing, no significant changes in GRFs were observed in this study. This contrasts with some previous studies [28,29], which reported increased GRFs with ankle bracing. One possible explanation for this discrepancy is the larger and more diverse sample size used in the current study, which included both male and female participants. Additionally, the variability in running speed and individual performance strategies may have influenced the results. For instance, participants were instructed to run at a “game-like” speed, which was not standardized across trials. This approach was chosen to maintain the ecological validity of the task, but it also introduced variability that may have affected the consistency of GRFs across brace conditions.
The knee appears to compensate for limited ankle mobility by acting as a primary “shock absorber,” allowing GRFs to remain consistent across brace conditions. However, the observed increase in knee moments suggests that this compensation comes at a cost, potentially elevating the risk of ACL injury. The data imply that participants may have exhibited reduced knee flexion during landing, which, when combined with increased frontal plane moments, could place the knee in a more vulnerable position [30]. This finding is particularly relevant in the context of sports where quick directional changes are required, and athletes must maintain control while performing complex maneuvers, thereby increasing the risk of injury.
The clinical relevance of these findings remains complex, as individual susceptibility to ACL injury varies. Even small increases in knee internal rotation moments could be critical for individuals with certain anatomical characteristics, such as a steeper tibial plateau slope, or those with weaker neuromuscular control [31]. While this study did not specifically measure these individual differences, the findings underscore the importance of considering such factors in clinical practice. Additionally, while this study did not investigate the effects of muscular fatigue, previous research has shown that fatigue can exacerbate the risks associated with knee abduction and internal rotation moments [32]. Given that athletes often compete in a fatigued state, the combined effects of fatigue and ankle bracing on knee joint loading warrant further investigation.
The trend towards increased knee abduction moments in the braced condition, though not statistically significant (0.01 < p < 0.05), also raises concerns about the potential for multi-directional loading on the ACL. Knee abduction has been implicated in ACL injury mechanisms, and its combination with internal rotation moments could exacerbate the risk [33]. While research has debated the role of abduction versus internal rotation in ACL strain [34], the mechanical coupling between these moments, as observed in this study, suggests that they should not be considered in isolation. Instead, the interaction between these forces in a multi-planar context may better explain the occurrence of ACL injuries during dynamic sports activities [35]. Given these findings, clinicians and coaches should carefully evaluate the use of prophylactic ankle braces, particularly in activities involving high-risk movements like cutting. The potential benefits of ankle bracing in preventing ankle injuries must be weighed against the possible increase in knee joint loading and the associated risk of ACL injury.
Conclusions
In conclusion, this study demonstrates that prophylactic ankle bracing significantly increases knee internal rotation moments during dynamic cutting tasks, potentially heightening the risk of ACL injuries. While ankle braces are effective in reducing the incidence of ankle sprains, their impact on knee joint loading cannot be overlooked. The findings underscore the need for a cautious approach when recommending ankle braces, particularly for athletes engaged in sports that involve frequent cutting maneuvers. Future research should focus on developing and assessing alternative injury prevention strategies, such as neuromuscular training programs, that can enhance joint stability without the potential drawbacks associated with bracing. Clinicians and coaches must balance the benefits of ankle bracing with the potential risks to knee health, ensuring that athletes are provided with the best possible protection against injuries.
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