ACL Research Retreat IX March 17–19, 2022 High Point, NC

Abstract

Objective: To critically assess the literature focused on sex-specific trajectories in physical characteristics associated with anterior cruciate ligament (ACL) injury risk by age and maturational stage.Data Sources: PubMed, CINHAL, Scopus, and SPORTDiscus databases were searched through December 2021.Study Selection: Longitudinal and cross-sectional studies of healthy 8- to 18-year-olds, stratified by sex and age or maturation on 1 measure of body composition, lower extremity strength, ACL size, joint laxity, knee-joint geometry, lower extremity alignment, balance, or lower extremity biomechanics were included.Data Extraction: Extracted data included study design, participant characteristics, maturational metrics, and outcome measures. We used random-effects meta-analyses to examine sex differences in trajectory over time. For each variable, standardized differences in means between sexes were calculated.Data Synthesis: The search yielded 216 primary and 22 secondary articles. Less fat-free mass, leg strength, and power and greater general joint laxity were evident in girls by 8 to 10 years of age and Tanner stage I. Sex differences in body composition, strength, power, general joint laxity, and balance were more evident by 11 to 13 years of age and when transitioning from the prepubertal to pubertal stages. Sex differences in ACL size (smaller in girls), anterior knee laxity and tibiofemoral angle (greater in girls), and higher-risk biomechanics (in girls) were observed at later ages and when transitioning from the pubertal to postpubertal stages. Inconsistent study designs and data reporting limited the number of included studies.Conclusions: Critical gaps remain in our knowledge and highlight the need to improve our understanding of the relative timing and tempo of ACL risk factor development.Context: The risk of anterior cruciate ligament (ACL) injury in female athletes increases throughout the stages of maturation. Previous evidence has indicated a lack of dynamic knee control during double-limb jump landings throughout maturation, yet the extent of biomechanical changes that occur throughout maturation during single-limb landings are not well understood. A maturational analysis of high-risk biomechanics during single-limb landings may elucidate and inform injury risk reduction efforts.Objective: To determine differences in single-limb landing biomechanics between pre-pubertal, pubertal, and post-pubertal female soccer players.Design: Cross-sectional (pre-testing component of a randomized controlled trial)Setting: Research biomechanics laboratoryPatients or Other Participants: 151 competitive female soccer playersIntervention(s): Participants were categorized based on percent of adult stature into pre-pubertal (PRE: <84% of adult stature, n=23), pubertal (PUB: 87%-94% of adult stature, n=36), and post-pubertal (POST: >94% of adult stature, n=92) maturational groups.Main Outcome Measure(s): All participants were instrumented for standard three-dimensional motion analysis and completed three trials of an ipsilateral single-limb hop and land over a 4-inch hurdle. Variables of interest included peak landing kinematics (hip flexion, hip adduction, hip internal rotation, knee flexion, knee abduction, ankle dorsiflexion) and peak landing external moments and ground reaction forces normalized to body mass (hip flexion, knee flexion, knee abduction, vertical ground reaction force). Multivariate analysis of variance (MANOVA) was used to compare the biomechanical variables of the dominant limb (preferred kicking limb) between maturational groups (α=0.05). Pearson's correlational analyses identified the relationship between the estimated percent of adult stature and lower extremity biomechanical variables.Results: There were significant differences in lower extremity kinematics (λ=0.79, p<0.001) and kinetics (λ=0.78, p<0.001) between maturational groups. POST landed in less hip internal rotation (POST: -0.3±5.4°) than PRE (3.8±7.0°, d= 0.67, p=0.008) and PUB (3.4±6.0°, d= 0.65, p=0.004), and greater knee abduction (POST: 5.1±4.8°) than PUB (2.4±3.4°, d= 0.66, p=0.009). POST also landed with vertical ground reaction forces 14% lower (POST: 3.20±0.76 xBW) than PRE (3.69±0.78 xBW, d= 0.62, p=0.01) and 13% lower than PUB (3.65±0.47 xBW, d= 0.73, p=0.005). Weak relationships were identified between the estimated percent of adult stature and peak hip internal rotation angle (r=-0.26) and vertical ground reaction force (r=-0.31).Conclusions: Post-pubertal female soccer players performed single-limb landings with lesser hip internal rotation, greater knee abduction, and lower vertical ground reaction forces. These findings are inconsistent with previous evidence that indicates higher risk movement strategies in post-pubertal athletes. The single-limb landing over a standardized hurdle height may not provide a comprehensive biomechanical picture for which to assign injury risk profiles.Context: Researchers have identified modifiable biomechanical anterior cruciate ligament (ACL) injury risk factors in laboratory settings with change of direction tasks. To preserve athlete-environment relationship, it has been advocated to assess movement strategies on the field rather than only in the laboratory. However, the kinematic differences between laboratory and on-field settings have never been investigated.Objective: To investigate the knee kinematics of female soccer players during agility movements performed both in laboratory and in soccer field environments.Design: Observational.Setting: Research laboratory and field.Patients or Other Participants: Twenty healthy female soccer players (14.9 ± 0.9 years, height 167.9 ± 4.8 cm, mass 56.4 ± 7.3 kg) participated.Intervention(s): Unanticipated sidestep cutting tasks with the dominant leg (towards the non-dominant leg) were collected both in the laboratory and on the field. In the laboratory, the players used a 5m approach run followed by a 1-foot landing and a 40°-50° change of direction followed by running through a gate 5m away. On-field tasks were recorded during a game performed as part of their regular training session. Kinematics were collected through wearable inertial sensors (Xsens Technologies, Enschede, The Netherlands). One-way ANOVA was used to compare knee joint kinematics between the conditions, with level of significance set at p<.05. Waveform consistency was investigated through Pearson's correlation coefficient and standardized z-score vector.Main Outcome Measure(s): For both conditions, 3D joint angles were defined using the Euler sequence ZXY and exported from the Xsens software (Xsens MVN Analyze 2020.0.1) to a customized Matlab (The MathWorks 2019a, Natick, Massachusetts, US) script. The ultimate foot contact before the change of direction was used and data were processed in a time-normalized interval from 50ms prior to the initial contact (0%) to 25ms after (100%). For the field, 40°-50° changes of direction were extracted for comparison with the laboratory condition.Results: On-field agility yielded vastly different knee kinematics compared to lab agility. At initial contact, the average knee flexion angle on-field was 31.96° - 36.92° versus 19.47° - 21.15° in the laboratory (p=.004). For the frontal plane at initial contact, we found -1.08° - -2.31° knee abduction in the field and 0.29° - 0.67° knee adduction in the laboratory (p=.007). The peak knee angles in frontal and sagittal planes were comparable among the two conditions. Waveform correlation was poor-to-excellent between laboratory and field (r: 0.20-1.00).Conclusions: Movement strategies used for changes of direction tasks in the laboratory were different from the on-field movement strategies in young female soccer players. This could be attributed to the differences in the environment and support the need for on-field injury risk screening.Context: Increased knee extensor moments relative to hip extensor moments during landing have been proposed as a risk factor for ACL injury in females. Diminished strength of the hip extensors relative to the knee extensors has been shown to predict an increased knee-hip extensor moment ratio during the deceleration phase of landing.Objective: To determine if decreased strength of the hip extensors relative to the knee extensors predicts re-injury in females who have returned to sport following ACL reconstruction (ACLR). Design: Retrospective, case-controlSetting: Patient clinicIntervention(s): Maximum isometric strength of the hip and knee extensors was measured using a motor-driven dynamometer (BTE Primus; BTE) as part of return to sport testing following ACLR. Patients were contacted to participate in an online survey concerning their return to sport and ACL injury status at least 12 months following testing.Patients or Other Participants: 345 females post ACLR who had previously undergone baseline strength testing were surveyed. The survey response rate was 54%. Respondents who sustained an ipsilateral ACL re-injury (non-contact) within 36 months of returning to sport were considered as cases.Main Outcome Measure(s): Twelve cases (14-22 years) were identified and matched with 2-4 non-injured controls (n = 41) based on sport level, athletic exposures, age, and graft type. Peak isometric strength for the hip and knee extensors was identified for each case and control from patient records. The hip-knee extensor strength ratio was calculated by dividing the peak strength of the hip extensors by the knee extensors (expressed as percentage). Logistic regression was performed to determine if the hip-knee extensor strength ratio predicted ACL re-injury, adjusted for known confounders (athletic exposures, age, graft type, months ACLR to return to sport). Receiver operator characteristic (ROC) curve analysis was conducted to determine the cutoff for the hip-knee extensor strength ratio that distinguished between high-risk and low-risk outcomes.Results: Cases had a lower hip-knee extensor strength ratio compared to controls (0.83 ± 0.16 vs. 0.98 ± 0.17, p = 0.007). The hip-knee extensor strength ratio significantly predicted ACL re-injury (p = 0.036, adjusted OR = 0.944, 95% CI: 0.894, 0.996). For every 1% increase in the hip-knee extensor strength ratio, there was 5.6% lower odds of re-injury. ROC curve analysis revealed an area under the curve of 74.8% [95% CI: 58.7%, 90.9%] (p=0.010), indicating fair prediction accuracy. The cutoff for the hip-knee extensor strength ratio to define high risk was <= 97.1% (sensitivity: 91.7%, specificity: 51.2%).Conclusions: Female athletes with a lower hip-knee extensor strength ratio following ACLR are at greater risk of ACL re-injury (ipsilateral limb). These results suggest that return to sport testing to assess risk for ACL re-injury should consider the inclusion of hip and knee extensor strength.Context: Upwards of 30% of youth athletes sustain a repeat anterior cruciate ligament (ACL) injury after ACL reconstruction. While specific risk factors for re-injury and predictive models exist, these models are: 1) Limited in scope, failing to consider the large number of variables related to re-injury, 2) Overly simplistic, overlooking the interaction amongst variables, 3) Too broad, identifying risk factors within a group context and 4) non-actionable, utilizing risk factors that cannot be altered in recovery.Objective: The goal of this study was to develop an ACL re-injury prediction model capable of evaluating each patient's individual risk, identifying modifiable risk factors and ranking these factors on the order of importance and ability to be modified.Design: Retrospective database study of 432 patients (mean age 15.0, Female 50.7%) who underwent ACL reconstruction. This dataset represented a targeted extraction from a larger dataset and had a reinjury rate of 30%.Setting: The dataset included variables across the following categories: Demographics, injury information, family history of ACL injury, surgical variables, rehabilitation & performance testing and re-injury information.Intervention(s): Machine learning (ML) techniques leveraging clinician domain knowledge were utilized to develop a model capable of determining a patient's risk for repeat ACL injury. Two highly experienced surgeons and a physical therapist independently assessed risk factors and ranked their contribution to injury and ability to be modified during recovery. These classifications were integrated into the modeling, allowing clinician expertise to improve upon standard ML methodologies. After patient risk classification, the model ranks the most significant risk factors according to impact and ease of modification.Main Outcome Measure(s): Multiple weighting methods and hyperparameter schemes were evaluated to obtain the highest accuracy classifying patients into high, medium or low risk categories, and weighting of modifiable risk factors. Model accuracy was determined using a 5-fold cross validation process over a 20% holdout dataset. An a-priori goal of classifying double the rate of re-injury across the entire dataset into high-risk patients (54%) and half this rate (13.5%) into low-risk.Results: The final model included 23 modifiable variables represented mostly by performance factors (e.g. normalized quad peak torque, quad/hamstring strength ratio, hop testing performance or time to return to sports). The model adjusts risk factor weight on a case-by-case basis. Performance goals were achieved with ACL re-tear rates of 58% in high-risk and 7% in low-risk categories, with sensitivity of 89% and specificity of 66% and verified within the cross-validation process.Conclusions: This ACL-Reinjury Risk Prediction Model can improve clinical care through accurate risk stratification and identification of patient specific modifiable risk factors, that can inform ongoing reinjury reduction or rehabilitation programs. Continued improvements in model accuracy will incorporate additional features, developing more intelligent labeling functions and adding confidence metrics.Context: The high incidence of anterior cruciate ligament (ACL) injuries in young female soccer players compared to their male counterparts has led to a large body of work aimed at reducing injury.Objective: To determine the effects of neuromuscular training using biomechanical biofeedback in order to reduce the risk of ACL injuries in adolescent female athletes.Design: Randomized controlled clinical trial; Level of evidence, 1.Setting: Research laboratoryPatients or Other Participants: 150 (age:13.3±2.2yrs; height: 156.1±10.6cm; mass:50.2±11.3kg) female soccer players.Intervention(s): A prospective, randomized, active comparator, open blinded, end-point trial was conducted. Participants were randomized into one of three study arms where each received neuromuscular training plus 1) sham biofeedback as an active control (NMT), 2) knee-focused biofeedback (NMT+K), and 3) hip-focused biofeedback (NMT+H).Main Outcome Measure(s): Each participant completed a pre-intervention baseline session and a post-intervention session to determine knee abduction moment (KAM) during a double leg drop vertical jump (DVJ) and an unplanned single leg cutting task at discrete time points and at specific time points during the stance phase. Athletic exposures and ACL injuries were tracked weekly for six months following the post-test session.Results: 140 participants (93.3%) completed the 6-week intervention and post-test. No ACL injuries were reported among any of the intervention groups six months following post-testing. Peak KAM was significantly improved in all three intervention groups from baseline to the post-test during the drop vertical jump (NMT: baseline -21.4Nm [95% CI: -25.4, -17.4], post -16.6Nm [95% CI: -19.7, -13.5], p=0.001, d=-0.48; NMT+K baseline -19.5Nm [95% CI: -23.2, -15.8], post -14.6Nm [95% CI: -17.6, -11.7], p=0.003, d=-0.42; NMT+H baseline -22.2Nm [95% CI: -26.9, -17.8]; post -17.6Nm [95% CI: -21.1, -14.1], p=0.002, d=-0.45). The primary outcome in this RCT, peak KAM, had mean improvement of 22.7% from baseline to post-testing which did not differ between groups during the DVJ (p>0.05). However, statistically significant differences in peak KAM during the unanticipated cutting task were only found in the NMT+H intervention group (NMT: baseline -25.0Nm [95% CI: -30.4, -19.6], post -25.0Nm [95% CI: -29.6, -20.5], p=0.49, d=004; NMT+K baseline -23.6m [95% CI: -28.9, -18.3], post -22.8Nm [95% CI: -28.0, -17.6], p=0.377, d=-0.05; NMT+H baseline -29.5Nm [95% CI: -35.3, -23.8]; post -22.9Nm [95% CI: -27.8, -18.0], p=0.003, d=-0.44).Conclusions: While female soccer players involved in neuromuscular training programs regardless of intervention group exhibit significant improvements in KAM during a double leg landing, those that engage in hip-focused biofeedback compared to knee-focused or sham biofeedback exhibit decreased KAM during an unanticipated cutting maneuver. Neuromuscular training programs that aim to modify the high-risk biomechanics associated with ACL injury may benefit from targeting the underlying components of injury, such as underutilization of hip musculature during dynamic sport related movements.Context: Neuromuscular training (NMT) programs have been shown to modify high-risk biomechanics and ACL injury risk. However, it is unknown which neuromuscular factors are modified following the completion of NMT programs. Hip strength has been suggested to be an important factor in ACL injury risk and is commonly emphasized in NMT programs. Understanding the immediate effects and retention of hip strength following a NMT program will provide insight into the effectiveness of NMT programs in reducing ACL injury risk.Objective: To examine the increase and retention of isokinetic hip strength in female adolescent soccer players following a neuromuscular training (NMT) program.Design: Prospective cohort study.Setting: Research laboratory.Patients or Other Participants: 126 adolescent female soccer players (age=13.2±2.1yrs, height=156.4±10.2cm, mass=49.9±11.0kg) were included in this subgroup analysis that completed all isokinetic hip strength assessments.Intervention(s): Participants volunteered to complete 6 consecutive weeks of a neuromuscular training (NMT) program with a frequency of 3 times a week. The NMT program included progressive exercises that included components of lower extremity strengthening, plyometrics, and core strengthening. Using an isokinetic dynamometer, 5 repetitions of isokinetic concentric (CON) and eccentric (ECC) hip extension (HEXT) torque were measured at 60 deg•s−1 bilaterally prior to (PRE) and following (POST) the NMT program. Retention (RET) of hip strength was assessed 6-months following the completion of the NMT program.Main Outcome Measure(s): The average peak torque (Nm) of the middle 3 repetitions for CON and ECC HEXT strength measures of the right and left limb during each testing session were used for analyses. Separate repeated measures ANOVAs were used to compare differences in HEXT strength between PRE, POST, and RET. Post-hoc pairwise comparisons with Bonferroni correction were performed when appropriate.Results: HEXT strength (CON, ECC) were significantly different between testing sessions. Specifically, POST right (146.9±48.3 Nm, 150.0±57.9 Nm) and left (147.4±47.7 Nm, 144.2±52.7 Nm) were greater than PRE right (138.8±44.0 Nm, P=0.007; 139.6±56.5 Nm, P= 0.026) and left (138.4±45.0 Nm, P=0.009; 139.4±56.2 Nm, P=0.035). RET right (130.2±48.8 Nm, 133.5±50.2 Nm) and left (131.9±46.2 Nm. 133.2±50.2 Nm) were less than POST (Prange= <0.001-0.007). However, neither right or left HEXT strength were different between PRE and RET (Prange=0.094-0.203).Conclusions: In female adolescent soccer players, increases in isokinetic HEXT strength were observed immediately following a 6-week NMT program. However, the increases in hip strength were not retained 6-months following the completion of the NMT program. While the immediate increases in hip strength may contribute to the effectiveness of NMT programs in reducing ACL injury risk, these positive changes appear to decrease over time. This suggests that adolescent athletes may need to continuously engage in NMT programs to maintain the immediate observed increases in hip strength.Context: Despite decades of research and innovation, primary and secondary ACL injury rates remain high. Traditional injury prevention programs focus on biomechanical-related outcomes, neglecting to maximize more cognitively-driven processes that capitalize on motor learning principles. The advent of virtual reality technology allows for seamless integration of targeted motor learning strategies into injury prevention regimens, which may lead to improved acquisition and retention of safe movement patterns.Objective: Determine the efficacy of a brief virtual-reality intervention to deliver specific motor learning principles of autonomy support (AS) or enhanced expectancies (EE) for superior retention of safe frontal plane single-leg squatting kinematics. We hypothesized that AS and EE would result in greater improvements compared to a control group.Design: Quasi-experimental repeated measuresSetting: Three-dimensional motion analysis laboratoryPatients or Other Participants: Our sample included forty-five participants (21 male: 22.2±2.3 yrs, 181.2±9.4 cm, 79.6±16.9 kg; 24 female: 21.8±2.2 yrs, 163.0±7.3 cm, 63.1±12.3 kg) who demonstrated risky frontal plane single-leg squatting patterns. Study inclusion required no history of concussion, vertigo, lower extremity surgery, or lower extremity injury for six months prior. Due to data loss, the final sample was 39.Intervention(s): Participants were quasi-randomized to one of three groups (control, AS, or EE) such that the male:female ratio was constant across groups. All participants wore virtual reality goggles that displayed an avatar (posterior/frontal plane view) performing exemplary single-leg squats. Participants were instructed to mimic the avatar while performing 5 sets of 8 single-leg squats on the left leg. The AS group chose their avatar color prior to each set. The EE group received real-time positive biofeedback in the form of joint-specific green highlights when kinematics remained within predefined injury-resistant constraints.Main Outcome Measure(s): Participants were digitized for three-dimensional motion capture and completed five repetitions of left leg single-leg squats. Peak frontal plane knee, hip, and trunk angles were measured before and immediately following the intervention. Pre-post changes were assessed with 3x2 (group x time) RMANOVAs.Results: A main effect for time indicated that all groups demonstrated increased pre-post hip adduction (p=.01; η2p=.18; control Δ=1.5°, AS Δ=3.2°, EE Δ=0.7°). Group by time interactions revealed that the control group demonstrated greater pre-post increases in knee abduction (Δ=2.3° increase) compared to AS (Δ=0.1° increase) and EE (Δ=0.4° decrease) (p=.003; η2p=.28) and that EE demonstrated greater decreases in ipsilateral trunk lean (Δ=11.7° decrease) compared to AS (Δ=6.3° decrease) and control (Δ=3.3° decrease) (p=.002; η2p=.30).Conclusions: Hip adduction worsened in all groups, suggesting hip-abductor fatigue. Despite this, AS and EE groups maintained baseline knee kinematics, while the control group worsened. Knee maintenance was likely accomplished through decreases in ipsilateral trunk lean. Incorporation of motor learning principles appears to mitigate the detrimental effects of fatigue via increased learner motivation and engagement.Context: Providing choices to athletes during practice increases intrinsic motivation and therefore, positively influences the motor learning process. Autonomy on the timing of feedback (self-controlled feedback) seems promising to optimize movement techniques. The effects of self-controlled feedback on optimizing the technique of sidestep cutting (SSC), a task that is highly related with ACL injury risk, are unknown.Objective: To investigate the effect of self-controlled feedback on SSC technique. It was expected that the self-controlled group progresses more after receiving the training than the yoked group.Design: Randomized controlled trial.Setting: Research laboratory.Patients or Other Participants: Thirty healthy ball team sport athletes (22.6±1.7 years, 185.3±6.6 cm, 78.2±8.3 kg) were recruited from local sports clubs. Based on arrival, participants were assigned to the Self Control (SC) or Yoked (YK) group.Intervention(s): On day 1 and day 2 (one week later), participants performed five anticipated 45° SSC trials as pre-test, immediate-post test and retention test. Training consisted of three randomized conditions: an anticipated 45° SSC task and two conditions in which participants had to catch and throw or receive and pass a ball to the test leader. All participants received expert video instructions and were asked to ‘copy the movement of the expert to the best of their ability'. The SC group was allowed to ask for feedback after every trial during training. Feedback consisted of 1) the Cutting Movement Assessment Score (CMAS), 2) posterior and sagittal videos of the last trial and 3) an external focus verbal cue on how to improve their technique. The participants' goal was to lower the score. The yoked counterparts received feedback after the same trial on which their partner in the SC group had requested feedback.Main Outcome Measure(s): CMAS scores (range 0-11) between groups and sessions were analysed with repeated measures ANOVA, with level of significance set at p<.05.Results: Pre-test scores between groups were equal (p>.05). Regardless of group, CMAS scores at pre-test (2.8±1.0) were higher compared to training (1.8±1.1), immediate-post (2.0±1.0) and retention test (2.1±1.1) (p<.001). Compared to pre-test (2.8±1.9), the SC group improved technique during training (2.0±1.1), which was maintained in the immediate-post (2.0±1.0) and retention test (1.8±1.0), p>.05. The YK group showed higher scores in immediate-post (2.0±1.0) and retention test (2.4±1.1), compared to training (1.6±1.0) (p<.001).Conclusions: The SC group maintained their improved technique during the retention test, whereas the YK group got back to their baseline level. Self-controlled feedback may benefit optimizing SSC technique and could be considered as a valuable tool to implement in ACL injury prevention programs.Context: Over the last twenty years, researchers have developed evidence-based injury prevention exercise programs (IPEPs) capable of reducing the rate of ACL injury. However, youth sport stakeholders have been slow to implement these programs and injury rates continue to rise. Previous studies have largely focused on the efficacy of IPEPs during controlled conditions. Recent research demonstrates that implementation factors impact the effectiveness of IPEPs in real-world settings. Understanding and intervening on implementation barriers and facilitators may be important for improving the adoption and effectiveness of IPEPs in youth sports.Objective: To systematically review and synthesize the knowledge, attitudes, and beliefs of youth sport stakeholders towards IPEPs.Data Sources: An electronic database search was conducted in November 2021 using PubMed and Google Scholar. A combination of search terms including, “knowledge,” “attitude,” “belief,” “injury prevention program,” “youth,” “sport,” “ACL,” and “lower extremity” were entered. Results were limited to articles printed in English. Addition studies were added following a citation search for those meeting criteria.Study Selection: Studies with partially overlapping data were merged and duplicate studies were removed. Studies that did not evaluate injury prevention in sport were excluded as were abstracts, white papers, dissertation and thesis manuscripts, supplements, book chapters, and conference papers. One author reviewed the remaining studies and removed those which did not include knowledge, attitudes, and/or beliefs, did not partially focus on the knee, and only included populations outside of youth sport (U9-U19).Data Extraction: Nineteen studies comprising responses from 2774 sport stakeholders, 2473 coaches (Age: 18-66 years; Sex: 629 male, 196 female 1648 not reported; Coaching Experience: 1-40 years), were collected. Data pertaining to knowledge (95%), attitude (32%), and beliefs (74%) toward IPEPs were reported in the included studies. Quality of studies was not evaluated.Data Synthesis: Themes influencing barriers to IPEP implementation include lack of knowledge (53%) and lack of access to materials to increase knowledge (21%). Themes influencing facilitators to IPEP implementation include positive attitudes toward IPEPs (42%), belief that youth athletes are at high risk of injury (21%), and belief that IPEPs help reduce injury (58%).Conclusions: The recurring themes identified though this literature review support the need for programs, protocols, and policies to enhance knowledge and support of youth sport stakeholders responsible for implementing IPEPs. Limitations of the evidence contained in the review include: 13 of the 18 studies came from outside the U.S, the majority focused on soccer and/or basketball (84%), and when sex of respondent was identified, the majority (76%) were men. The use of implementation science frameworks such as the Consolidated Framework for Implementation Research and the Theoretical Domains Framework, will be beneficial in identifying constructs that affect implementation and developing programming to meet the needs of individual program deliverers.Context: Sports-related concussion (SRC) increases risk of lower extremity musculoskeletal injury, including ruptures to the anterior cruciate ligament (ACL). Residual deficits in neuromuscular control following SRC impl