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Female athletes are 4–6 times more likely to suffer an ACL injury than males in comparable sports. A link between landing biomechanics and ACL injury has led to the development of injury prevention focused training protocols. It is often difficult to measure the protocols’ efficacy of different protocols on reduction of ACL injury-related factors.
The purpose of this study was to test the effects of in-season neuromuscular training on a field-based evaluation used to help identify athletes at risk for ACL injuries. The hypothesis was that the ACL injury prevention training program included with an in-season soccer program would demonstrate increased improvement in the Tuck Jump Assessment (TJA) scores at post-season follow-up testing relative to standard in-season soccer training.
Forty-nine female soccer players were tested with TJA before and after participation in either in-season injury prevention training (IN) or standard in-season soccer training (CTRL). Participants were filmed performing the TJA with digital video cameras and scored by two separate raters, each viewing randomized videos. The groups received neuromuscular training synthesized from previous protocols demonstrated to decrease ACL injury. A mixed design (2X2; group by time) repeated measures ANOVA was used to test the interaction and main effects of group (ACL intervention training in-season vs. standard soccer in-season training) and time (pre vs. post-season) on dynamic TJA scores.
There was a significant main effect of time on TJA score (p=0.04) for athletes measured at pre- and post-season. The IN group reduced measured landing and jumping deficits from 5.4 ± 1.6 to 4.9 ± 1.0 points following training. CTRL showed a 14% reduction in TJA deficit points following the soccer season.
The tested hypothesis that the in-season ACL intervention training can be utilized to reduce measured TJA deficits above and beyond a standard in-season soccer protocol was not supported. Future research is warranted to determine if a combination of intensive pre-season and reduced in-season maintenance training is optimal for improvement of dynamic movement biomechanics during the TJA and ultimately preventing ACL injuries.
Adolescent female soccer players are 4–6 times more likely to sustain an anterior cruciate ligament (ACL) injury compared with male soccer players.22, 30 This sex disparity may be caused by decreased neuromuscular control during the execution of sports movements, particularly in landing and pivoting movements. This results in lower limb mechanics which may increase ACL injury risk. 18, 26, 36 Several injury prevention programs have been developed in order to reduce the risk of ACL, as well as, other lower extremity injuries.8, 11, 16, 20, 21, 27, 28, 38, 42–44
One major factor differentiating neuromuscular training programs is the varied programming within the yearly calendar. Performing the training in-season or pre-season may have varying effects on injury risk throughout the competitive season. If pre-season training is omitted, there is potentially a greater injury risk in the early season, while the consequences of not performing in-season training may manifest themselves as the season continues and the potential benefits of the pre-season training becomes negligible.
In-season training is characterized by shorter, less intensive training as systemic fatigue can hinder sports performance.11 The most effective and efficient programs in both types of protocols should include four essential components: balance, biofeedback, strength and plyometric exercises.32 In a meta-analysis of ACL injury prevention training studies, Hewett et al. (2006) concluded that an in-season protocol alone is likely the most cost-effective; however, the decrease in risk of injury may not manifest itself until much later in the season due to the reduced intensity of the training.23 Nevertheless, several studies have tested the influence of in-season training and have had positive results. Gilchrist et al. (2008) indicated that an on-field warm-up program significantly reduced ACL injury rates,11 Pollard et al. (2006) demonstrated that joint kinematics and kinetics were improved after an in-season intervention,25 and Zebis et al. (2008) reported increased EMG activity for the medial hamstrings.14 The authors proposed this increased activity would reduce dynamic knee valgus and the resultant injury risk.31
Prospective measures of high dynamic knee valgus (i.e. knee abduction moment) during landing, predict ACL injury risk in young female athletes.31 In addition, a large scale prospective study found that military cadets who sustained ACL injuries demonstrated knee landing mechanics related to these coronal plane knee deficits.10 Several investigations have demonstrated that female athletes more often exhibit excessive coronal plane load and motion landing mechanics compared to males during landing and pivoting movements.18, 24, 26, 29, 33, 35–37, 39, 40 In validation of theses laboratory findings, females often demonstrate knee landing alignments associated with high knee abduction load at the time of injury.19, 34, 41
The aforementioned studies required the use of expensive, 3D motion capture equipment to evaluate and predict ACL injury risk by tracking kinetics and kinematics. The cost of using 3D motion analysis to measure kinetics and kinematics can be in the range of $1000 per athlete per test.4 These costs easily exceed the budgets of most high school athletic programs. In addition, Myer et al. recently demonstrated that laboratory-based injury risk identification techniques can be successfully applied to clinical practice.3, 4, 6 The current study seeks to expand on this concept through the implementation of a field-based evaluation to test the effects of in-season neuromuscular training. 1, 13 This study implemented a “clinician-friendly” plyometric assessment that requires substantially less equipment and personnel than 3D motion analysis. In this assessment, athletes performed consecutive tuck jumps for ten seconds while the clinician subsequently identifies any of ten possible deficiencies associated with neuromuscular risk factors shown through motion analysis (e.g., “lower extremity valgus at landing”).1, 13 Each athlete’s baseline performance was then compared to their post-training performances. An athlete who was identified with at least six out of the ten risk factors in the tuck jump would theoretically receive high-intensity training options, since neuromuscular interventions best benefit high-risk athletes. 1, 13 The purpose of this study was to identify the effects of an in-season warm-up training program on young female soccer players. The hypothesis tested was that the ACL prevention training program included with an in-season soccer program would demonstrate increased improvement in the Tuck Jump Assessment (TJA) scores at post-season follow-up testing relative to standard in-season soccer training.
Female high school varsity and junior varsity soccer players from two Westchester County, New York school districts volunteered to participate in the study. The in-season neuromuscular training group (IN) consisted of fifteen subjects (n=15) and thirty-four subjects from another high school soccer team formed the control group (CTRL). Subject height, mass, age, and descriptions of previous injuries were recorded for each subject (Table 1). Parents or guardians signed informed consent forms approved by the Institutional Review Board, and assent from the child participants were obtained prior to study participation.
Both groups were tested before and after the soccer season. Standard camcorders were mounted on tripods, providing independent sagittal and frontal plane views of the subject. Subjects were shown a video presentation and a live demonstration of correct tuck jump technique. Subjects were then allowed to ask questions and were provided with unlimited practice time. Subjects were instructed to place their feet on markings that were 35 cm apart. Subjects completed consecutive tuck jumps for ten seconds as described.1, 13
Videos were imported into iMovie 6.0.3 (Apple, Cupertino, CA), synchronized, and merged into split-screen using a plug-in (SplitScreen & PiP, StupendousSoftware.com) Two raters, blinded to both the training status and training type, subsequently evaluated for the presence of criteria-based biomechanical deficits shown in videos provided in a random order These criteria have been previously reported as potential underlying contributors to increased risk of knee injury in female athletes (Figure 1).1, 13 To determine an athlete’s pre- and post- season score, their individual score was averaged between the two raters.
CTRL received no intervention and continued their regular in-season routine. This routine was a standard soccer warm up including light jogging and static stretching. IN received a neuromuscular intervention that was developed using previously published protocols and further adapted for this study (Table 2).12, 15 The protocol implemented with the IN group consisted of an abbreviated protocol of five, two-week progressions of six exercises: single-leg anterior progression, single-leg rotary progression, unanticipated hop-to-stabilization, hop-to-stabilization and reach, tuck jump progression, and hamstring strength progression. Progressions were presented to the coaching staff in the form of a training manual (Table 1). The coach was instructed to give continuous verbal feedback to the subjects during and after the intervention and were given the common verbalizations and visualizations “land light as a feather,” “on your toes,” “straight as an arrow,” “shock absorber” and “recoil like a spring” as suggestions.44, 45
A mixed design (2X2; group by time) repeated measures ANOVA was employed to test the interaction and main effects of group (neuromuscular intervention versus standard soccer training) and time (pre-season/training vs in-season/training) on TJA scores. Statistical analyses were conducted in SPSS (SPSS, Version 17.0, Chicago, IL). Statistical significance was established a priori at p≤0.05.
Athlete compliance for the IN group was noted weekly by individual coaches using attendance and exposure reports. Compliance was measured by dividing the total number of sessions attended by the total sessions offered. Compliance in the IN neuromuscular intervention was 95 percent.
For all athletes measured at pre- and post-season, there was a significant main effect of time on the tuck jump assessment score (p = 0.04). Both teams significantly reduced their tuck jump assessment scores. The in-season trained group reduced their measured landing and jumping deficits from 5.4 ± 1.6 points to 4.9 ± 1.0 points following training and the season. In addition, athletes who performed standard soccer in-season training (CTRL) also showed a reduction from 5.8 ± 1.6 to 5.0 ± 1.5 points following a soccer season. There was not a significant interaction of training status (p=0.65) between the IN and CTRL study groups.
Periodized strength and conditioning programs are quickly becoming the standard for high school sports. Due to the proposed positive effects of injury prevention training, this type of training should be included in a comprehensive program. Despite the demonstrated efficacy of injury prevention training for female athletes, compliance has historically been relatively low when the training is focused on injury prevention.38 Combining injury prevention training with the team’s regular pre-season and in-season training may be the optimal way to increase compliance, as it merely alters the routine rather than making wholesale changes to their regimen. However, the current results do not support the use of in-season ACL intervention training to reduce measured TJA deficits above and beyond a standard in-season soccer protocol. There may be a uncovered dose-response relationship associated with neuromuscular training which would improve the efficacy of this training protocol.5 The inclusion of injury prevention training during the pre-season may be a necessary component to achieve positive effects on risk factors associated with injury and may be an important limitation to the program presented in the current study.9
In addition, the importance of pre-season training was further highlighted by the work of Ghilcrist and colleagues.11 Based on this data, it appears that pre-season training, the often neglected training element, is responsible for the athletes’ safety during the first half of the competitive season. By instituting a thorough pre-season training regimen, the athletes could potentially see the benefits as soon as their competitive season begins. A positive dose-response relationship has been suggested in which six to eight weeks of training is critical for inducing positive changes in injury prevention.5, 9 However, the volume of injury prevention training during the regular season may be too low to illicit desired effects, as a greater amount of time is dedicated to performance.
Further complicating the matter are the restrictions that most states have on contact time between players and coaches. The dates at which sports teams can begin practicing are set by each state to ensure student-athletes’ safety, but these regulations also put a premium on the time when coaches are allowed to interact with their players. Sadly, injury prevention training often is first to get cut. 38 By combining injury prevention training with both the pre-season and in-season training, the critical mass for improvements may be easier to achieve.
Vescovi et al. emphasized another key point in the rationale for the use of both pre-season and in-season training. Their study showed that in-season injury prevention training alone was not enough to illicit any performance gains in a similar population. 7 This speaks to both the dose-response relationship as well as the need to incorporate performance goals for the training to improve compliance. Our Tuck Jump Assessment can be used as both an injury prevention tool but also as a measurement of athletic performance.
Compliance in any intervention is crucial. In this study, the compliance rate for the in-season intervention was 95 percent. Factors influencing such high compliance could include the motivational nature of this particular intervention, duration of the intervention, pre-season versus in-season programs, athlete/trainer ratio, and type of instructor (coach or athletic trainer). Future research should attempt to tease out these differences. Bien (2010) recently noted that increased compliance is more likely to occur in warm-up programs, rather than in interventions that do not precede an athletic practice. Results of a recent meta-analysis indicated a potential dose-response relationship between neuromuscular training compliance and reduction of ACL incidence rates. 5 High attendance and completion rates of prescribed neuromuscular training sessions appear to be an important component for preventing ACL injuries in young female athletes. Additions to the protocol aiming to increase athletic performance measures (such as vertical jump, sprint speed, or strength) could improve compliance in future populations.
In a similar report, Brent and colleagues evaluated the effects of pre-season only and in-season neuromuscular training only to a standard soccer training season using the field-based evaluation employed in the current study.9 These authors reported that the pre-season ACL intervention resulted in a 1.4 (95% CI 0.6 to 2.2) point reduction compared to this in-season neuromuscular or standard soccer training. In the current study the IN group did not significantly reduce the number of mean flaws per video in comparison to control. Thus the in-season neuromuscular intervention and regular soccer warm-ups have similar effects on tuck jump assessment score suggesting that participation in the sport itself could have some effect on tuck jump biomechanics (e.g., a subject’s progressive fitness level could translate into improved performance on the tuck jump assessment). However, to achieve greater reductions in deficits, and potentially improve the efficacy of reduced injury risk at the onset of the competitive season, a preseason additive neuromuscular training protocol may be warranted. 9, 11, 44
The time difference between the pre- and post-testing for IN and CTRL was only five weeks due to a relatively short soccer season. In a meta-analysis of studies with neuromuscular interventions, Hewett et al. concluded that in order for the program to be effective, it must have a minimum duration of six weeks,31 while Bien et al. concluded a minimum of eight weeks was necessary for injury prevention.2 Further studies should investigate whether starting an intervention earlier (in the preseason) and continuing it throughout the season, would lead to significant improvements compared to “typical” soccer training.
The tuck jump assessment allows a coach or clinician to evaluate an athlete’s risk of injury without the use of expensive equipment. By using the TJA throughout the yearly training cycle, overall deficits can be targeted (Figure 2) and progress can be monitored (Figure 1) which may allow the neuromuscular training to be more thoroughly directed. Targeted training as suggested has been shown to improve the efficacy of similar training programs implemented during pre-season and may benefit ACL injury prevention programs that are implemented during17 the season as well. The results also indicate a potential dose-response to the neuromuscular training.5, 9 Preliminary results indicate that combining injury prevention training with the team’s traditional sport training throughout the competitive season has added benefits in terms of reducing risk of ACL injury based on improvements to the athlete’s tuck jump assessment score. Future research is warranted to determine the relationship of reduced deficits gained from utilization of the presented techniques with actual reduction of injury in athletes treated with targeted training.
Athletes measured pre- and post-training significantly decreased their mean total scores on the tuck jump assessment after the intervention period and/or the season. However, subjects in the current study who received the in-season proprioceptive training did not reduce their deficits in the TJA above and beyond a standard soccer season training protocol. There may be a dose-response relationship to the neuromuscular training targeted to prevent ACL injury. Future research is warranted to determine if pre-season combined with in-season maintenance training is optimal in improving biomechanics and reducing ACL injury risk. Both pre-season and in-season neuromuscular training may be best utilized to help minimize the risk factors associated with ACL injuries, especially in the early season. Coaches can evaluate the training progress of their athletes using a field-based Tuck Jump Assessment with a significantly lower cost than previous methods.
The authors would like to acknowledge funding support from National Institutes of Health Grants R01-AR049735 and R01-AR055563.
The authors would like to acknowledge funding support from National Institutes of Health Grant R01-AR049735, R01-AR055563 and R01-AR056259.
The Cincinnati Children’s Hospital Medical Center and Byram Hill High School Institutional Review Boards approved this study.
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