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Abstract

Background: Athletes who participate in sports that involve cutting and pivoting movements are particularly susceptible to anterior cruciate ligament (ACL) injury. Preventing this injury is the best way to combat its health consequences and costs. There may be a dose-response relationship between adherence and injury reduction. Purpose: We sought to examine whether athletes’ adherence to injury prevention programs (IPPs) is associated with reductions in ACL and lower extremity (LE) injuries. Methods: We conducted a systematic review of the PubMed, EMBASE, and Cochrane Library databases, searching for studies published between 2011 and 2021. Studies were included if they reported on the use of an ACL IPP compared with a control group and recorded the rate of injuries to calculate a rate ratio, as well as adherence to the program as a percentage of sessions performed. For the meta-analysis, the rate ratios were pooled using the DerSimonian-Laird random-effects model. Results: For the 15 studies included (11 randomized controlled trials and 4 cohort studies), the random-effects model grouped athletes’ adherence to an IPP as high (76% or more of the sessions), moderate (51%–75% of the sessions), and low (50% or fewer of the sessions). We found that athletes with the highest level of IPP adherence had a significantly lower incidence of ACL injury. The rate ratios for moderate and low adherence did not demonstrate a reduced incidence of ACL injury. Injury prevention program participation was also associated with a decrease in LE injury rates. Conclusion: This systematic review and meta-analysis found that athletes with high adherence to IPPs had reduced rates of ACL and LE injuries. Our findings suggest that educating coaches and athletes on the dose-dependent benefits of IPPs may promote the routine incorporation of these programs into warm-up sessions to decrease the risk of ACL and LE injuries.

Keywords

ACL injury adherence injury prevention knee injury neuromuscular training

Introduction

Anterior cruciate ligament (ACL) rupture is a common sports-related injury, with an estimated 200,000 to 250,000 injuries per year in the United States alone [2,12]. Athletes who participate in sports that involve cutting and pivoting movements—including soccer, basketball, and football—are particularly susceptible to ACL injury [21]. Despite improvements in ACL reconstruction and rehabilitation, obstacles persist in athletes returning to sports after injury. Returning to play typically takes up to 12 months. A recent meta-analysis reported that while 81% of patients returned to sport following ACL reconstruction, only 55% returned to a competitive level [1]. Moreover, the long-term health consequences of ACL injury may include an increased risk of reinjury, additional ACL surgery in either the ipsilateral or contralateral knee, concomitant meniscal and cartilage injury, and development of osteoarthritis [10,11,19].

Thus, preventing the injury in the first place is the best strategy to combat the health consequences and costs of ACL; this can be accomplished through injury prevention programs (IPPs). The hallmark of IPPs is neuromuscular training programs (such as the FIFA 11+) that include plyometric, strength, flexibility, and mobility exercises and drills for changing speed and direction. In research settings, IPPs have proven to be efficacious in decreasing the rates of ACL injury [13,20,25,28]. A meta-analysis reported that IPP resulted in a 50% risk reduction in all ACL injuries and a 67% reduction in noncontact ACL injuries in female athletes [31].

Although IPPs have demonstrated efficacy in protecting athletes from ACL injuries in controlled research settings, the effectiveness in practice has been limited due to challenges with implementation and adherence. Several obstacles have been identified as barriers to adoption of IPP, including time restrictions, lack of support from athletes and parents, and lack of knowledge about implementing the program [7]. Prior studies have shown that low adherence to program recommendations can hinder the impact, while higher adherence has been shown to be positively correlated with protective effects [15]. In one study, the rate of ACL injuries decreased from 0.48 to 0.33 per team when physical therapists began working with the teams. However, the injury rate rebounded to 0.56 after they stopped working with the teams and players discontinued the IPP.

The objective of this systematic review and meta-analysis was to examine whether adherence to an IPP affects ACL and lower extremity (LE) injury rates. We hypothesized that there would be a dose-response relationship, with greater adherence to IPPs associated with a lower rate of ACL and LE injuries among athletes.

Methods

A medical librarian performed a systematic search of the PubMed (Medline), EMBASE, and Cochrane Library databases from January 2011 to February 2021 in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (https://www.prisma-statement.org/). Search terms included “compliance,” “patient compliance,” “adherence,” “neuromuscular training,” neuromuscular intervention,” “prevention,” “knee injury,” and “ACL injury” (Supplemental Table 1). A manual search of reference lists of included studies and relevant review articles was performed, as well.

All studies underwent title and abstract screening followed by full-text screening. Each study was independently screened for inclusion by 2 authors using the online software Covidence (Veritas Health Innovation Ltd). Conflicts were resolved by the senior author. Studies were included if they reported on the number of ACL or LE injuries, the amount of athlete exposures to an IPP, and the percentage of sessions in which athletes participated in an IPP. Studies were excluded if they were not published in English or were conference abstracts.

The quality of included studies was assessed using the Physiotherapy Evidence Database (PEDro) scale for randomized controlled trials (RCTs) and the Newcastle-Ottawa scale for cohort studies. The PEDro scale uses 11 binary questions to assess characteristics including eligibility criteria, random allocation, blinding, reporting of key outcomes, intention-to-treat analysis, between-group statistical comparison, and measure of the treatment effect size. The Newcastle-Ottawa Scale scores each study on selection, comparability, and outcome. For both scales, quality was determined by summing the individual component scores.

The following data were extracted from full-text articles: study design, study participant characteristics (ie, age, sex, level of sport), who led the IPP, number of injuries by type (ie, ACL or LE), total number of athlete exposures, adherence rate, and how adherence was recorded (self-report by the person leading the warm-up or direct observation by study staff). The rate ratio (RR) was calculated as the rate of injury in the IPP group divided by the rate of injury in the control group. A value of less than 1 indicated that the IPP reduced the rate of injury.

Some studies reported data stratified by injury type (ACL only or any LE injury). As these data were extracted separately when possible, some articles contributed multiple comparisons or “studies” to the meta-analysis. In addition, other studies reported on injury rates for multiple levels of adherence (eg, low and high adherence). These data were also extracted separately and contributed multiple “studies” to the meta-analysis.

Data from each study were pooled using the DerSimonian-Laird random-effects model (REM) and subgrouped according to the following adherence categories: high (76% or more of the sessions), moderate (51%–75%), and low (50% or fewer). The REM gives more conservative estimates with wider confidence intervals (CIs) compared with fixed-effects models because it assumes that the meta-analysis includes only a sample of all possible studies. In addition, the REM accounts for both within-study variability (random error) and between-study variability (heterogeneity). The RR from each study and the 95% CI are displayed in forest plots. A sensitivity analysis was also performed with categories of adherence based on tertiles of the data distribution.

Heterogeneity among studies was detected using the I² test, which measures the variability in RR due to heterogeneity. Further reasons for the heterogeneity were investigated by meta-regression analysis due to the different study populations and settings. In this linear regression model, the studies served as the unit of analysis and the RR was the dependent variable. The independent variables were the covariates potentially associated with variability in the RR and were determined a priori: study design (RCT vs cohort), sex (female athletes or both male and female athletes), age (adolescent vs adult), and reporting of adherence (self-report vs direct observation). All analyses were conducted in Stata SE, version 14 (StataCorp., LLC).

Results

We identified 3344 articles through the database search; after duplicates were removed, the remaining 2276 articles underwent title and abstract screening. A total 2165 articles were determined to be irrelevant to the study aims and 111 articles were assessed with full-text review. Among those, 96 articles were excluded because of wrong study design or outcomes. An additional 38 articles were identified through manual review of systematic reviews and the reference lists of included studies. From this manual search, 11 additional articles were included in the meta-analysis. A total of 15 articles (11 RCTs and 4 cohort studies) met the criteria for data extraction (Fig. 1, Table 1) [3,4,6,8,9,14,16 –18,20,22,25,26,29,30].

Fig. 1.

PRISMA study selection flow diagram. The numbers of screened, excluded, and included studies are shown. PRISMA Preferred Reporting Items for Systematic Reviews and Meta-Analyses.

Table 1.

Characteristics of articles included in the meta-analysis.

Study	Study type	M/F	Age	Sport	Level	Who led the warm-up	How was adherence reported?	Adherence rate	Injury type
Gilchrist et al [3]	RCT	F	18–22	Soccer	College	AT	Self-report	72%	ACL, LE
Hägglund et al [4]	Cluster RCT; secondary analysis	F	12–17	Soccer	Club	Coach	Direct observation, self-report	63%, 82%, 89%	ACL, LE
Hewett et al [6]	Prospective study	F	13–18	Soccer, Basketball, Volleyball	High School	AT	Self-report	67%	ACL, LE
Kiani et al [8]	Intervention trial	F	13–19	Soccer	Club	Coach	Self-report	75%	ACL, LE
LaBella et al [9]	Cluster RCT	F	14–18	Soccer, Basketball	High School	Coach	Direct observation, self-report	80%	ACL, LE
Myklebust et al [14]	Prospective intervention	F	Adult	Handball	Elite	Coach	Self-report	26%, 29%	ACL
Olsen et al [16]	Cluster-RCT	Both	15–17	Handball	Club	Coach	Self-report	87%	ACL, LE
Omi et al [17]	Prospective intervention	F	18–22	Basketball	College	AT	Self-report	89%	ACL
Pasanen et al [18]	Cluster RCT	F	Mean age: 24	Floorball	Elite	Coach, player, or PT	Self-report	74%	ACL, LE
Silvers-Granelli et al [20]	Prospective RCT	M	18–25	Soccer	NCAA Division I and II	ATC	Self-report	47%	LE
Slauterbeck et al [22]	Cluster RCT	Both	13–18	Football, Soccer, Basketball, Lacrosse	High School	Coach	Self-report	32%	LE
Steffen et al [26]	Cluster RCT	F	13–17	Soccer	Club	Coach	Direct observation, Self-report	52%	ACL, LE
Steffen et al [25]	Cluster RCT	F	13–18	Soccer	Youth	Coach	Direct observation, Self-report	81%, 86%	LE
Van Beijsterveldt et al [29]	Cluster RCT	M	18–40	Soccer	Amateur	Coach	Direct observation, Self-report	73%	LE
Waldén et al [30]	Cluster RCT	F	12–17	Soccer	Club	Coach	Self-report	50%	ACL, LE

ACL anterior cruciate ligament, AT athletic trainer, ATC certified athletic trainer, LE lower extremity, NCAA National Collegiate Athletic Association, PT physical therapist, RCT randomized controlled trial.

Participants were 12 years of age or older, and most articles (73%, 11/15) included only female athletes. The articles reported on several sports (soccer, basketball, volleyball, handball, football, floorball, and lacrosse) at various levels (club, high school, college, and elite). The IPPs were led by a coach in 10 articles, an athletic trainer (AT) in 4 articles, and either coach or AT in 1 article. The adherence rates ranged from 26% [14] to 89% [4,17].

Participation in an IPP showed a protective effect against ACL injury in athletes, with an overall RR of 0.59 (95% CI: 0.46–0.77, P < .001). The test for heterogeneity was P = .62 and the I² test was 0%. Athletes with the highest adherence to IPP reported the lowest ACL injury rates: RR = 0.36 (95% CI: 0.21–0.62, P < .001). The RR for moderate and low adherence suggested a dose response, although both estimates crossed the null value: moderate RR of 0.65 (95% CI: 0.38–1.09, P = .11) and low RR of 0.72 (95% CI: 0.50–1.04, P = .08) (Fig. 2).

Fig. 2.

The incidence rate ratios (RRs) by study and adherence level are shown for ACL injuries. The studies are ordered by ascending level of adherence. Myklebust et al [14] included 2 “studies” with low adherence (26% and 29%). Hägglund et al [4] included 2 “studies” with high adherence (82% and 89%) and 1 with moderate adherence (63%). ACL anterior cruciate ligament.

Participating in an IPP was also associated with a decrease in LE injuries compared with the control group: RR = 0.75 (95% CI: 0.60–0.93, P = .009) (Fig. 3). However, there was no dose-response relationship between the low, moderate, and high adherence groups: low RR = 0.75 (95% CI: 0.43–1.32, P = .33), moderate RR = 0.73 (95% CI: 0.52–1.02, P = .07), and high RR = 0.77 (95% CI: 0.60–0.93, P = .17). While the RRs were consistent across adherence groups, there was a significant amount of heterogeneity across studies (P < .001, I² =79%).

Fig. 3.

The incidence rate ratios (RRs) by study and adherence level are shown for lower extremity injuries. The studies are ordered by ascending level of adherence. Hägglund et al [4] included 2 “studies” with high adherence (82% and 89%) and 1 with moderate adherence (63%). Steffen et al [25] included 2 “studies” with high adherence (81% and 86%).

In a sensitivity analysis, the cutoffs for adherence were tertiles based on the distribution of values organized so that each category would have an equal number of studies: low: 67% or fewer; moderate: 68% to 75%; and high: 76% or more. There were no substantial differences in results from both types of categorization (Supplemental Figures 1 and 2).

Meta-regression was performed to investigate reasons for heterogeneity for both ACL and LE injuries. The coefficients from this linear regression model showed no significant effects of study design, sex, age, and method of reporting adherence on the RR for each study (Table 2).

Table 2.

Meta-regression results for anterior cruciate ligament (ACL) and lower extremity (LE) injuries.

Variable	ACL		LE
Variable	Coefficient (95% CI)	P value	Coefficient (95% CI)	P value
RCT design	1.03 [0.43–2.46]	.94	3.12 [0.78–12.5]	.10
Female athletes only	1.75 [0.29–10.5]	.49	0.95 [0.50–1.80]	.85
Adult athletes only	1.50 [0.49–14.59]	.43	0.79 [0.41–1.50]	.44
Direct observation	1.03 [0.31–13.46]	.96	1.29 [0.69–2.38]	.39

CI confidence interval, RCT randomized controlled trial.

For the quality assessment, the 11 RCTs scored using the PEDro system had ratings ranging from 6 to 8 (out of 11 total points), and the 4 cohort studies were rated as good quality, each earning a Newcastle-Ottawa score of 7 or higher (maximum score, 9 points) (Supplemental Table 2).

Discussion

This study found that there is a dose-response relationship between greater adherence to IPPs and reduced incidence of ACL and LE injuries in athletes. The highest adherence group showed a reduction of ACL injury risk of 64%. While IPP also showed a protective effect against LE injuries, we found no evidence of a dose-response relationship, and the individual study results were more variable.

A limitation of this systematic review and meta-analysis is that two-thirds of the studies measured adherence using self-report by the person conducting the IPP (eg, coach or AT) rather than direct observation, which is inherently biased and very likely overestimated. For example, in a meta-analysis by Sugimoto et al [27], the authors included a study that reported an adherence rate of 100%, noting that this “could be inaccurate” [5]. Furthermore, they obtained this self-reported rate through correspondence with the author of the publication, and it was not included in the original study. For this reason, we did not include this study in our meta-analysis. In addition, the adherence categories used in our study were based on arbitrary cutoffs. However, our overall findings did not change in the sensitivity analysis when the categories were based on the distribution of adherence rates. In addition, data were pooled from both RCTs and cohort studies.

The strengths of this meta-analysis included a comprehensive search strategy with overlapping databases that was developed in consultation with a medical librarian. In addition, our meta-analysis included an additional 11 studies that reported on adherence to the IPP and injury rate that were identified through a manual search. Seven of these articles, published before the start date of our literature search, were missed by the Sugimoto et al [27] meta-analysis, which did not include a medical librarian on the team. We also extracted and analyzed data separately for ACL and LE injuries when possible. Aside from using an REM that gives more conservative estimates, we also used a meta-regression analysis to explore possible sources of heterogeneity.

Sugimoto et al [27] also showed that the greatest ACL injury reduction occurred in the highest adherence group (RR = 0.27 [95% CI: 0.07–0.80]). Yet this research contained only 6 studies (including one that self-reported an adherence rate of 100%) and was restricted to studies of female athletes. There has been no meta-analysis of the effect of adherence to an IPP on preventing LE injuries. That we did not find a dose-response relationship and that there was significant heterogeneity among studies were not unexpected, as the IPPs were designed to target ACL injury prevention and may not be protective for non-ACL knee, hip, or ankle injuries. The results of the meta-regression indicated that the a priori factors that we determined could be possibly associated with injury rates (study design, age, sex, or direct observation) did not explain the heterogeneity for LE injuries. A post hoc analysis, comparing articles on soccer only versus other sports and coach-led versus AT-led in the meta-regression, did not show any differences in injury reduction. Furthermore, the meta-regression showed no effect of sex on reduction of ACL or LE injuries. Thus, it is likely that injury reduction is similar for both male and female athletes. However, only 4 studies included male athletes, and this small number precludes assessment of a dose-response relationship.

Previous research has shown that to optimize the benefits of IPPs for athletes, coaches, and ATs, these programs should be integrated into regular training sessions [23,32]. Furthermore, another meta-analysis reported that programs with sessions of longer duration (more than 20 minutes each) resulted in greater ACL injury reduction among female athletes compared with those with shorter duration [28]. That meta-analysis demonstrated that an adherence rate of greater than 75% provided the most significant protective effect. Thus, a critical first step in the implementation of IPPs is to educate coaches and athletes on the dose-dependent effects, with the goal of improving uptake [23]. Some studies found that the use of IPPs decreases throughout the season due to factors including infrequent practice days, numerous competitions, and lack of time. There is a need to further evaluate motivational barriers to ensuring high adherence throughout the entire season [4,22,24,26].

In conclusion, poor adherence rates may explain why the rate of ACL injuries has not decreased with the advent of IPPs. The results of this meta-analysis demonstrate that regular use of IPPs has resulted in decreases in the rate of ACL and LE injuries in athletes. Educating coaches and athletes on the dose-dependent reduction of ACL injury with IPP may increase awareness and promote routine incorporation into warm-up sessions to reduce the risk of this devastating knee injury.

Supplemental Material

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Footnotes

Acknowledgements

The authors thank Nick Cepeda for his assistance with data collection.

Declaration of Conflicting Interests

The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Andrew D. Pearle, MD, reports relationships with Stryker, DePuy, and Smith & Nephew, and ownership interest in Engage Surgical. Robert G. Marx, MD, reports relationships with Springer, Demos Health, MEND Nutrition Inc, and Journal of Bone and Joint Surgery. The other authors declare no potential conflicts of interest.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was aided by the 2018–2019 Clinical Outcomes Grant from the International Society of Arthroscopy, Knee Surgery, and Orthopaedics Sports Medicine and the Orthopaedic Research and Education Foundation.

Human/Animal Rights

All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2013.

Informed Consent

Informed consent was not required for this review article.

Level of Evidence

Level IV, systematic review and meta-analysis of level I–IV studies

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Disclosure forms provided by the authors are available with the online version of this article as .

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ORCID iDs

Robert G. Marx

Bridget Jivanelli

Supplemental material

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