Injuries to the anterior cruciate ligament (ACL) are associated with early development of osteoarthritis (13) and are most common among individuals between the ages of 16 and 18 years (36). However, the frequency of ACL injury increases steadily around ages 11 to 12 (36), and sex differences in ACL injury rates and neuromuscular risk factors for injury appear to emerge after children encounter puberty (32,43). Therefore, there is reason to believe intervening with children at a young age may result in better long-term outcomes for ACL injury prevention. However, it is unclear whether this young age group responds to traditional ACL injury prevention programs that typically involve complex directions and movements and may be beyond their level of comprehension.
Only 2 previous studies have evaluated the potential to change neuromuscular risk factors with an ACL injury prevention program in athletes under 12 years of age (7,11). Using similar programs demonstrated previously to be effective in older populations, Kilding and Tunstall (11) observed improvements in several performance variables (e.g., vertical jump height), whereas Grandstrand et al. (7) failed to see changes after the intervention and hypothesized that the children were unable to perform some of the exercises. Furthermore, Kilding et al. recommended that more variety should be added to the intervention to decrease boredom and improve compliance. These findings suggest children may require age-specific injury prevention programs.
Strength gains in children are primarily attributable to neural adaptations as compared with muscle hypertrophy in adults (8,28). Skeletally immature children are also prone to overuse injuries (6,16), and thus training intensity should be a consideration in programs and include gradual progressions and time for recovery. Unfortunately, several ACL injury prevention programs require longer than 30 minutes of training or involve heavy resistance (10,12,26). Young children also have been shown to require more feedback when learning a task (40) and use attention differently than adults (3) and therefore may benefit from this type of intervention more than their older peers. Addressing these differences between children and adults through the development of an injury prevention program designed specifically for young athletes may improve the ability to change neuromuscular risk factors for ACL injury in a young population.
An inability to maintain proper balance or postural control has been proposed as a neuromuscular risk factor for several lower extremity injuries (1,17,18,35). Fortunately, isolated balance training can improve balance measures in healthy adult individuals (4,20). However, knowledge regarding the effectiveness of balance training during an integrated ACL injury prevention program is scarce despite the fact that balance exercises are a frequent component of these programs. In addition, no one has investigated the effects of any type of training on balance ability in children under 14 years of age.
A common limitation of ACL injury prevention programs is poor compliance (31,39). Hewett et al. (9) demonstrated in a meta-analysis that compliance ranged from 28% to 100% in studies that evaluated injury outcomes after ACL injury prevention programs. Although the primary purpose of these interventions is to modify injury risk factors, positive changes in performance variables (e.g., vertical jump height) likely accompany improvements in strength, balance, and movement. Athletes and coaches may improve compliance with these programs if provided with evidence demonstrating the programs improve athletic performance as well as reduce injury rates. Unfortunately, this information is lacking in much of the previous literature on ACL injury prevention programs.
The purpose of this study was to evaluate the effects of ACL injury prevention programs on balance and vertical jump ability in young children. A second purpose was to determine whether a program designed specifically for a pediatric (PED) population would result in greater balance and vertical jump improvements compared with a traditional (TRAD) program that has been effective with modifying potential injury risk factors in older populations (10,15,25) but with only limited effectiveness with young athletes (2,7) as well as compared with no program at all (CON). We hypothesized that an age-specific intervention consisting of gradual progressions that begin with simple exercises and more instruction and feedback would result in greater improvements in balance ability and vertical jump height and power compared with a traditional, or non-age-specific, program and no program.
Experimental Approach to the Problem
A cluster-randomized controlled trial was used to evaluate changes in balance ability and vertical jump height and power before and after an intervention period. Six teams from the Under-10 (9 yr old) and Under-11 (10 yr old) age levels were recruited from a local soccer association that agreed to participate in this study. After an initial testing session (pretest), the 6 teams were stratified by sex and cluster randomized into 1 of 3 injury prevention programs: PED, TRAD, or CON. The TRAD program was modified from previous ACL injury prevention programs that have been shown to be effective with participants in high school or college (10,15,25). The PED program was a modified version of the TRAD program that accounted for differences between children and adults with regard to motor learning, cognitive and physical development, and responses to exercise. Teams assigned to the CON program did not perform an injury prevention program during the intervention period. As a result, 1 boys' and 1 girls' team were assigned to each program. A seventh team, which was a boys' team, was added because of a small roster size with the boys' CON team and was consequently assigned to the CON program to ensure sufficient sample size. All players on these teams performed their respective injury prevention program as part of their normal warm-up, but only players who volunteered to participate in the study completed the 2 testing sessions (pretest and post-test). Figure 1 presents a summary of the study design.
Sixty-six youth soccer players (males: n = 37 age = 10 ± 1 yr, mass = 34.16 ± 5.36 kg, height = 143.07 ± 6.27 cm; females: n = 28 age = 10 ± 1 yr, mass = 33.82 ± 5.37 kg, height = 141.02 ± 6.59 cm) from 7 teams volunteered to participate in this study (Table 1). All participants were free from any injury or illness that prohibited soccer activity at the time of initial testing. Before the first testing session, all parents and players read and completed informed consent and assent forms, respectively, which were approved by the university's institutional review board.
All participants attended 2 identical testing sessions in a research laboratory. The intervention period began within 1 week after the first session (pretest), and the second session (post-test) occurred within 1 week after completion of the intervention period. All participants wore their own running shoes. The same shoes were worn for both testing sessions. Participants performed a maximal vertical jump test and a dynamic balance assessment in a randomized order. All testing occurred on participants' dominant limbs, which were the legs preferred to kick a ball for maximal distance.
Ground reaction force data were collected from a nonconductive force plate (model #4060-NC Bertec Corporation, Columbus, OH, USA). Time-to-stabilization (TTS) test data were collected with a sampling frequency of 180 Hz (33). Data were collected during the TTS test for 10 seconds after initial ground contact, which occurred when the vertical ground reaction force exceeded 10 N. Data during a maximum vertical jump were sampled at 1,000 Hz and used to calculate power and vertical jump height. All ground reaction force data were collected through Motion Monitor software (Innovative Sports Training Inc, Chicago, IL, USA).
Maximal Vertical Jump Test
Maximal vertical jump height and power were assessed during a double leg countermovement maximal vertical jump test, which has demonstrated good intersession reliability (22). Participants began with their feet shoulder width apart while standing with their dominant foot on a force plate (Bertec Corporation, Columbus, OH, USA). An overhead goal was used to encourage maximal performance (5). The participants were instructed to jump for maximal vertical height and try to touch the overhead goal. They performed 2 practice trials, and the overhead goal was placed slightly above the participant's highest practice jump. Thirty seconds of rest were allowed between each of 3 trials.
Participants performed a dynamic balance assessment using TTS measures, which have been used previously to evaluate training effects (34). Participants stood on a 30 cm high box placed half of their body height away from a force plate with their hands on their hips. Participants jumped forward from the box with their nondominant foot and landed with their dominant foot in the center of the force plate while keeping their hands on their hips and their nondominant foot off of the ground. Participants were instructed to balance as quickly as possible without putting their nondominant foot down. Participants practiced the task until they indicated they felt comfortable with the task and performed it correctly. Three trials were performed but were repeated if the participant was unable to maintain this single limb landing position with their hands on their hips or if a subsequent hop occurred after landing. Participants repeated 4 trials during the pretest and 1 trial during the post-test on average. The number of repeated trials did not differ between groups (p = 0.94).
Implementation of Injury Prevention Programs
The 4 intervention teams performed their respective program as part of their normal practice warm-up during the 9-week intervention period, whereas teams assigned to the CON programs completed a warm-up designated by their coaches. The principal investigator or a research assistant taught the teams the program within 1 week of completing the pretest session, supervised the program implementation at every practice to provide feedback and technique instruction, and monitored compliance. Proper technique was continually stressed to all of the participants while they performed the exercises. The teams assigned to the CON program were supervised once every other week to ensure contamination of the programs did not occur.
Traditional ACL Injury Prevention Program
The TRAD program consisted of static flexibility, balance, strengthening, agility, and plyometric exercises on both limbs. Participants ran forward a distance of 10 m after completing each exercise to make it a dynamic warm-up. The speed of this run gradually increased as the participants progressed through the warm-up. All exercises were performed on both legs, and the program required approximately 12 to 14 minutes to complete. Table 2 provides a detailed description of the traditional ACL injury prevention program.
The static flexibility exercises involved stretching of the gastrocnemius, adductor, hip flexor, and quadriceps muscles. Participants also performed 3 balance exercises. The first balance exercise was a double limb jump with a 180° twist in the air followed by a double limb landing and stabilized hold for 1 second (“180° jump”). The second balance exercise required participants to maintain a single limb stance with their knee slightly flexed as they threw a soccer ball back and forth with a teammate (“single limb ball toss”). The third balance exercise involved a hop forward from 1 limb to a single limb landing and balance (“forward hop to balance”).
Strengthening, agility, and plyometric exercises composed the remainder of the TRAD ACL injury prevention program. One strengthening exercise targeted the core musculature (“hip bridge”), whereas the remaining 2 strengthening exercises focused on the muscles of the lower extremity, specifically the quadriceps and hamstrings (“walking lunges,” “single leg squat”). The agility exercises required lateral movement ("sideways shuffle), dynamic direction changes (“z-cuts”), and forward propulsion (“bounding”). Finally, 4 plyometric exercises were incorporated into the TRAD program. Two plyometric exercises required primarily either horizontal or vertical motion, coordination, and strength (“broad jump,” “squat jumps”), whereas the remaining 2 plyometric exercises focused on changing either sagittal or frontal plane directions while hopping back and forth over a line (“forward line hops,” “sideways line hops”).
Pediatric ACL Injury Prevention Program
The PED program consisted of 3 phases. The first phase was performed twice per week, and the final 2 phases were performed 3 times per week. The 3 progressive phases in the PED program were further delineated. The first week of each phase was an introductory phase with time spent emphasizing proper form, verbal and visual feedback, and scaled down repetitions of each exercise. The remaining weeks of each phase included the addition of 1 or 2 exercises and added movement between all exercises. Table 2 provides a list of the pediatric ACL injury prevention program. All 3 phases required 12 to 14 minutes to complete.
Similar to the TRAD program, the PED program incorporated several of the exercises into a dynamic warm-up protocol. The 2 programs were very similar during the first phase by requiring participants to run at progressively increasing speeds after the exercise movement. However, participants completing the PED program performed a “timing” run after the exercise movements instead of the speed forward run during the second phase. The “timing” run involved 2 participants finishing the exercise movements at the same time, running at a diagonal, and crossing in front of or behind the opposite player. This movement required the participants to control their body and use visual information about another moving player to direct their motion to avoid a collision. During the third phase, a sidestep cut was performed at the end of the diagonal run.
Instead of static flexibility exercises, the PED program consisted of dynamic flexibility exercises. The dynamic flexibility exercises targeted similar muscles as the static flexibility exercises, such as the gastrocnemius, hamstrings, hip flexor, and gluteal muscles. The PED program consisted of some of the same balance exercises as the TRAD program, such as the “single limb ball toss,” the “180 degree jump to balance”, and the “forward hop to balance.”. However, the “single limb ball toss” and the “180° jump to balance” were each only completed during 1 3-week phase. The “forward hop to balance” exercise progressed during the second and third phases to include movement in the frontal and transverse plane (“sideways hop to balance,” “twisting hop to balance”). Finally, the PED program also performed a single limb balance exercise while a partner pushed the other partner in different directions (“single limb balance with perturbations”) during the last phase.
The PED program began primarily with strengthening exercises and minimal plyometric exercises and transitioned by gradually changing these proportions. As a result, the final phase included only 1 strengthening exercise and several plyometric exercises. Strengthening exercises for the PED program included lunges in 3 planes (“forward/sideways/transverse lunge”), a squat progression (“double limb squat,” “single limb squat”), progressive core exercises (“hip bridge,” “human arrow,” “side plank”), and lower-leg strengthening exercises (“toe walk,” “double/single heel raises”). The plyometric exercises emphasized rapid changes of direction with double to single leg progressions (“forward/sideways line hops”), vertical jumps (“squat jumps,” “tuck jumps”), and consecutive jumps for distance (“broad jumps”). Finally, the PED program incorporated several agility exercises (“side shuffle,” “z-cuts,” “high knee run,” “skipping,” “quick cuts”).
Data from the vertical jump test and the TTS test were exported into a customized software program (MatLab version 7; MathWorks, Natick, MA, USA) for reduction. The time spent in the air, which was determined as the time between toe-off (vertical ground reaction force (VGRF) < 10 N) and initial contact (VGRF > 10N), was used to calculate vertical height with the following formulas (g represents constant acceleration caused by gravity):
Height = 0.5(g(t/2)2) (11)
Power was computed using the following equation:
Power (W) = 61.9 × jump height (cm) + 36.0 × mass (kg) − 1822 (21)
The 3 trials were averaged for all analyses.
The TTS data were reduced using a method described by Ross et al. (35). Anterior-posterior (A/P) and medial-lateral (M/L) ground reaction forces between the eighth and ninth seconds of single limb stance after ground contact were normalized to body weight. These values were used to determine a mean ± SD range of variation value for each component across all 3 trials. The A/P and M/L components of the ground reaction force were rectified and fit with a decay curve (unbounded third-order polynomial). The TTS for each component was identified as the point when the decay curve fell below a specified threshold (mean range of variation +3 SD). The average TTS values from the 3 trials for each component were used for analyses.
Eight possible covariates were evaluated for a significant relationship with the treatment effects for all dependent variables. These covariates included variables regarding anthropometrics (pretest body mass index [BMI], change in BMI between testing sessions, % predicted mature height), demographic information (sex, age in months), memory and learning ability (Learning and Total scores from the Brief Visuospatial Memory Test-Revised [BVMT]), and the initial value of each dependent variable. Body mass index was calculated from height and weight measurements taken before both testing sessions. The percent predicted mature height was determined from the children and their parent's height using equations validated by Malina et al. (14). The BVMT is a cognitive test that evaluates memory and recall of 6 shapes. Change scores were calculated for all dependent variables by subtracting the pretest value from the post-test value.
Separate analyses of covariance were conducted for each dependent variable, and covariates were included in the model if they had a statistically significant effect on the change score. Significant group effects were evaluated with a Bonferroni post hoc correction. We also calculated test-retest reliability values for all of the dependent variables from the CON group data between pretest and post-test. These values and SEM values are reported in Table 3. All data analyses were performed using SPSS version 16.0 (SPSS, Inc., Chicago, IL, USA) with an a priori alpha level of 0.05.
All subjects in the intervention groups (TRAD and PED programs) completed at least 80% of all training sessions and both testing sessions, and no one sustained any injuries requiring lost time from activity during the intervention period. One control subject did not complete the post-test session because of scheduling conflicts. The TTS data from 1 testing session was not usable for 2 intervention subjects. No statistically significant differences between groups existed for height (p = 0.12) or weight (p = 0.52) at the time of the pretest. Complete means and measures of variability are provided in Table 3.
Pretest score and sex significantly influenced the ability to improve A/P TTS. The adjusted group change scores (sex = 1.43, pretest A/P score = 2.71 s) revealed a significant group main effect (p = 0.003, effect size = 0.17). Post hoc testing demonstrated A/P TTS improved to a greater extent in the TRAD group than in the CON group (p < 0.001, effect size = 0.82) (Figure 2). However, the PED program was not significantly different from the CON program or the TRAD program. We did not observe any difference between the 3 groups (p = 0.56, effect size = 0.02) for M/L TTS.
Pretest score also influenced the ability to improve vertical jump height. Using this value as a covariate, we found significant differences in vertical jump height change scores among the 3 groups (p = 0.04, effect size = 0.10). The TRAD program resulted in greater improvements in vertical jump height ability compared with the CON program (p = 0.15, effect size = 0.71), but the PED program was not different from the CON group or the TRAD program (Figure 3). We did not detect any statistically significant difference between groups in power (p = 0.07, effect size = 0.08), but there were nonsignificant shifts in the means toward the direction of our original hypotheses for both intervention programs. In summary, the TRAD program improved A/P TTS and vertical jump height to a greater extent than the CON program, but the PED program was not different from CON program.
The results of this study are important to the future of injury prevention because they demonstrate that children as young as 9 or 10 years of age can effectively improve their balance ability and performance on a maximal vertical jump after completing a traditional injury prevention program. The program was brief, requiring only 10 to 15 minutes of time, 3 days per week for 9 weeks, and easily substituted as a team warm-up activity. Contrary with our original hypotheses, the TRAD program resulted in positive changes, whereas the PED program, which was designed specifically for the young participants, did not cause any improvements.
Poor balance ability is associated with an increased risk of falls and sustaining several lower-extremity injuries or conditions (1,18,35). In addition, improving balance through training exercises reduces the rate of ankle sprains (19,42) and overall lower extremity injury rates (27). Padua (29) recently demonstrated that the addition of balance training exercises to either plyometric or resistance exercises in an ACL injury prevention program influences the ability to change lower-extremity biomechanics. Therefore, it is reasonable to believe that improving balance ability may reduce the risk of lower-extremity injury in children.
Several studies have demonstrated enhanced balance ability after isolated balance training exercise programs (4,20). Despite this evidence and the frequent incorporation of balance exercises within injury prevention programs, only a few studies have actually assessed the influence of ACL injury prevention programs on balance (23,30). In agreement with our findings, Myer et al. (23) and Paterno et al. (30) reported improvements in balance in high school females. Myer et al. observed improved M/L postural stability without concurrent changes in A/P stability. Similar to our results, Paterno et al. demonstrated A/P stability improvements but no improvements in the M/L direction. The reason M/L stability did not change along with A/P stability is not completely understood. We hypothesize that the balance test demanded A/P stability but may not have been difficult in the frontal plane, as evidenced by faster stabilization times in the M/L direction compared with the A/P direction during the pretest. Therefore, there was more room for improvement in the A/P direction. Our results agree with these previous studies and further demonstrate that balance exercises as part of an ACL injury prevention program can successfully improve dynamic balance ability. Our findings also reveal that an intense balance training program is not necessary to observe improvements because balance exercises were only a small component of the injury prevention programs. Despite similar findings, our investigation is the only one that compared the results with a control group, required significantly less time than either of the 2 previous studies, used a different population, and did not include balance exercises on an unstable surface. Therefore, the results of our study should support the use of simple balance exercises to improve balance ability in a youth population.
To our knowledge, no one has evaluated balance training in a young population consisting of 9- and 10-year-old athletes. Grandstrand et al. (7) investigated the effects of an integrated injury prevention program in children between 9 to 11 years of age on landing technique and reported no improvements. DiStefano et al. (2) compared responses to an injury prevention program between 2 age groups and found that the older population, which included high school aged children, improved their landing technique to a greater extent than the younger population of pre-high school aged athletes. Both of these studies hypothesized that children under 12 years old may require specialized training and basic exercises to improve movement. Although Kilding and Tunstall (11) observed positive improvements in performance measures with this population, the authors recommended that programs be modified and varied for this young population. Therefore, we developed and studied the effects of a PED program that incorporated additional exercise progressions, more continuous feedback, more variety, and reduced initial frequency. We hypothesized that the PED program would result in greater changes compared with the TRAD program; however, the TRAD program was the only program to cause balance improvements compared with the CON group. Despite these findings, the PED program does appear to cause improvements that may be clinically significant as suggested by the within-group strong effect size and the medium effect size when the results are compared with the CON program.
We believe these results occurred because the programs differed with regard to the type of balance training exercises. The TRAD program required 30 seconds of continuous single limb balance 3 times per week for 9 weeks and included the forward hop to balance task for the entire duration of the program. In contrast, the PED program consisted of 30 continuous seconds of static single limb balance for only 3 weeks and included progressive versions of the forward hop to balance exercise by incorporating the frontal and transverse plane. The combination of more time spent in a static single limb balance position (TRAD program = 1,800 s, PED program = 270 s) and performing the assessment task appears to lead to greater balance improvements. The forward hop to balance assessment appears to be unintentionally designed to illustrate improvements from the TRAD program. The principle of exercise specificity states that human bodies will respond to the demands placed upon it, which means that training a task should result in improvements on that specific task. The TRAD program practiced the forward hop to balance exercise repetitively for 9 weeks and improved in ability to stabilize during that task. Although this finding is not surprising, it is important to show that young children can respond to training. However, our ability to conclude the PED program did not cause clinically significant changes in balance ability that may lead to reduced injury risk is limited. Future research should assess a different type of dynamic balance ability to see whether improvements transfer to additional tasks.
Although several interventions have been successful in changing ACL injury rates risk factors, others have not found any significant changes (31,39). These unsuccessful interventions cited poor compliance as a limiting factor. To evaluate the efficacy of ACL injury prevention programs and promote widespread dissemination, athletes, parents, and coaches must support the use of these programs. There is reason to believe support may be enhanced if these integrated programs can improve athletic performance measures in addition to risk factors associated with injury (9). Villarreal et al. (41) demonstrated in a recent meta-analysis that plyometric training results in strong effects on vertical jump height (effect size = 0.84), with greater than 7% improvements. Our results agree with the findings of this meta-analysis because we observed similar effect sizes with the TRAD program and slightly lower effect sizes for the PED program. The TRAD program improved vertical jump height by 7%, whereas the PED program sustained 5% improvements.
Although the studies included in the meta-analysis discussed previously mainly included studies that used isolated plyometric training, a few studies have evaluated vertical jump changes after an integrated training program with mixed results. Our results agree with Myer et al. (23), who observed improvements in vertical jump height ability after high school female athletes completed an ACL injury prevention program. Similarly, Kilding and Tunstall (11) reported increases in power during a vertical jump test in males between the ages of 9 to 11 years after completing an ACL injury prevention program. In contrast, Steffen et al. (38) did not observe performance changes in adolescent females and hypothesized the intensity of their 15-minute program may have been insufficient. Villarreal et al. (41) supported this hypothesis by concluding that the training protocol intensity influences the ability to change vertical jump ability. We hypothesize this intensity factor may have contributed to our findings that the TRAD program was the only program to sustain significant improvements in vertical jump height compared with the CON program. The TRAD program included 9 weeks of plyometric exercises, whereas the PED program slowly progressed the strengthening exercises to include plyometric exercises. The rationale for the PED program's gradual incorporation of plyometrics was that young or physically immature individuals are at a greater risk for overuse injury compared with older individuals (6,37). Therefore, we designed the PED program to gradually build strength and proper technique before the children began strenuous jumping activities. Although the TRAD program achieved greater improvements in vertical jump height, it is possible the PED program moved with better technique. Future research should simultaneously evaluate jump height or other performance measures with lower extremity biomechanics. Similar to the balance data, the principle of exercise specificity provides explanation for these findings because the TRAD program practiced maximum vertical jumps for 9 weeks and gained improvements in vertical jump height.
Villarreal et al. (41) also found that men have a greater ability to sustain improvements than women. This sex effect may explain the contrasting findings of the studies that used integrated injury prevention programs because Kilding and Tunstall (11) studied only males, whereas Steffen et al. (38) included only females in their respective studies. Unfortunately, a limitation of the current study is insufficient power to examine a program and sex interaction to evaluate whether sex affected our results. Future research should further evaluate the ability of young females compared with males to improve performance measures after an integrated injury prevention program.
The baseline values of the balance and vertical jump data influenced whether a positive change occurred. Our results showed that individuals with greater capacity for improvement sustained the greatest changes, which agrees with Myer et al. (24) and DiStefano et al. (2). Both of these studies observed that baseline values of lower-extremity biomechanics affected the ability to modify these variables. These findings suggest injury prevention programs may be the most beneficial for individuals who have the most room for improvement, and future research may benefit from targeting these individuals. In addition, future research should account for this factor in analyses to prevent significant findings from being obscured by individuals who have a limited capacity for improvement.
Application of this study's results is limited by its cluster-randomized design. We used this design versus a randomized controlled design to maximize external validity. The injury prevention programs were designed to be included as a team warm-up. Implementation of the respective programs and preventing treatment contamination would have been difficult if athletes on the same team performed a different set of exercises or none at all. Furthermore, practical implementation of the respective programs would likely involve teams performing the same, not different, activities as a warm-up. Because of the small number of clusters (teams), we were unable to directly account for the cluster-randomization effect in the statistical analyses, and thus our standard errors are potentially underestimated. However, the use of change scores and covariates addresses the possible influence of pretest group differences at the team level.
The results of this study demonstrate that a young population of athletes can improve their dynamic balance ability and maximum vertical jump height by performing a traditional injury prevention program. Although improvements were only observed with the TRAD program, we believe these findings are attributable to exercise specificity because the TRAD program consisted of exercises that were very similar to the tasks assessed. However, it is possible the PED program caused improvements in different types of tasks because this program stressed the same type of exercises as the TRAD program but with further progressions. The TRAD program was also performed 3 times more than the PED program because the PED program had a reduced initial level of frequency to provide the children with more time to adapt to the program. This greater frequency may have affected the results, and future research should evaluate what duration of injury prevention programs result in change. Progressive and variable exercises may enhance participants' enjoyment of the injury prevention program and lead to improved compliance. Therefore, future research should explore slight modifications of the PED program. The balance improvements obtained by the young children may lead to reduced lower extremity injuries, such as ACL injuries, because balance has been proposed as a risk factor for several lower-extremity injuries. Furthermore, the improvements observed in vertical jump height may result in improved compliance and further widespread implementation of injury prevention programs leading to decreased injuries from sport participation.
Children as young as 10 or 11 years of age can improve their balance and vertical jump ability if they practice these tasks specifically, suggesting that injury prevention programs for this age group should be designed with the concept of exercise specificity in mind. Injury prevention programs performed as a 10 to 12 minutes of dynamic warm-up appear to be able to change performance measures, which should be stressed to athletes and coaches to improve compliance with these programs because improved compliance with injury prevention programs may result in reduced injury rates.
The authors acknowledge the Injury Prevention Research Center (IPRC) at the University of North Carolina at Chapel Hill and the National Academy of Sports Medicine for their financial support of this project. They thank Scott Ross, Lauren Stephenson, Johna Register-Mihalik, Jeanne Graf, Danielle Canonge, Elizabeth Dameron, and Katie Kaiser for their time and help with this project. They also acknowledge the athletes, parents, and coaches from the Triangle United Soccer Association for their support and cooperation.
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