Influence of Age, Sex, Technique, and Exercise Program on

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MedicineThe American Journal of Sports

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DOI: 10.1177/0363546508327542

2009 37: 495Am J Sports MedLindsay J. DiStefano, Darin A. Padua, Michael J. DiStefano and Stephen W. MarshallCruciate Ligament Injury Prevention Program in Youth Soccer Players

Influence of Age, Sex, Technique, and Exercise Program on Movement Patterns After an Anterior

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More than 3 million youth are currently registered to takepart in organized soccer within the United States, andparticipation grows approximately 20% every year.1 Whilesoccer is a very popular sport among youth, there is ahigh incidence of injury associated with participation insoccer.16,25,39,57,59 Rupture of the anterior cruciate ligament(ACL) is one of the common lower extremity injuries thatmay occur during soccer. A recent study demonstrated soc-cer to be the most frequent activity to cause ACL injury in

Norway.25 There are an estimated 200 000 anterior andposterior cruciate ligament injuries in the United Stateseach year, with an associated cost of more than $3 bil-lion.32,45 The frequency of ACL injuries appears to increasein soccer athletes around age 11 or 12 with a steady riseuntil age 18.59 Not only are ACL injuries in youth soccercommon and a financial burden, they are also detrimentalto long-term health as radiographic signs of osteoarthritisdevelopment were present in 80% of soccer players lessthan 15 years after an ACL injury, regardless of the treat-ment received.41 While these results were from surgeriesperformed 2 decades ago, it is unknown if any recentimprovements in surgical technique have amelioratedthese long-term effects. These consequences demonstratethe immediate need for effective ACL injury prevention,especially in young soccer athletes.

Influence of Age, Sex, Technique, andExercise Program on Movement PatternsAfter an Anterior Cruciate Ligament InjuryPrevention Program in Youth Soccer PlayersLindsay J. DiStefano,* ATC, MA, Darin A. Padua, ATC, PhD,Michael J. DiStefano, ATC, MA, and Stephen W. Marshall, PhDFrom the University of North Carolina at Chapel Hill, Chapel Hill, North Carolina

Background: Anterior cruciate ligament (ACL) injury prevention programs show promising results with changing movement;however, little information exists regarding whether a program designed for an individual’s movements may be effective or howbaseline movements may affect outcomes.

Hypothesis: A program designed to change specific movements would be more effective than a “one-size-fits-all” program.Greatest improvement would be observed among individuals with the most baseline error. Subjects of different ages and sexesrespond similarly.

Study Design: Randomized controlled trial; Level of evidence, 1.

Methods: One hundred seventy-three youth soccer players from 27 teams were randomly assigned to a generalized or stratifiedprogram. Subjects were videotaped during jump-landing trials before and after the program and were assessed using theLanding Error Scoring System (LESS), which is a valid clinical movement analysis tool. A high LESS score indicates more errors.Generalized players performed the same exercises, while the stratified players performed exercises to correct their initial move-ment errors. Change scores were compared between groups of varying baseline errors, ages, sexes, and programs.

Results: Subjects with the highest baseline LESS score improved the most (95% CI, –3.4 to –2.0). High school subjects (95%CI, –1.7 to –0.98) improved their technique more than pre–high school subjects (95% CI, –1.0 to –0.4). There was no differencebetween the programs or sexes.

Conclusions: Players with the greatest amount of movement errors experienced the most improvement. A program’s effective-ness may be enhanced if this population is targeted.

Keywords: injury prevention; knee injuries; landing; risk factors

495

*Address correspondence to Darin Padua, ATC, PhD, CB#8700, 209Fetzer Gym, Chapel Hill, NC 27599-8700 ([email protected]).

No potential conflict of interest declared.

The American Journal of Sports Medicine, Vol. 37, No. 3DOI: 10.1177/0363546508327542© 2009 American Orthopaedic Society for Sports Medicine



496 DiStefano et al The American Journal of Sports Medicine

Approximately 70% of all ACL injuries are the result ofa noncontact mechanism of injury and usually occur as anindividual is planting, cutting, or jumping.9 Lower extrem-ity movement patterns during these activities play a criti-cal role in injury mechanism because they influence theload and deformational forces on ligaments, meniscus/car-tilage, and bone.5,9,18,43,60 Specific movement patterns com-monly occurring during ACL and lower extremity injuryinclude knee valgus, excessive leg rotation, and decreasedknee flexion.9,37 Knee flexion angle influences ACL stressas vigorous quadriceps contractions at low knee flexionangles (0°-30°) generate anterior tibial shear force thatmay strain the ACL.6,44 Large amounts of ACL loadingoccur during the combined loading state of tibial rotationand knee valgus.5,44 As such, lower extremity rotation (tib-ial rotation) and knee valgus occurring during cutting andjumping maneuvers are viewed to be of large enough mag-nitude to generate extreme loads within the ACL, causingspontaneous rupture of the ligament.2,63,66,67 It is duringthis combined loading state of excessive tibial rotation andknee valgus that researchers have described the ACL to beat greatest risk of injury.5,44

All previous research investigating injury preventionexercise interventions has used a “one-size-fits-all” (gener-alized) approach to exercise prescription where all individ-uals performed the same set of exercises without regard tothe individual’s specific needs.† The effectiveness of injuryprevention exercise interventions may be improved bydeveloping individualized interventions and collectingdata to understand the mechanisms for an intervention’ssuccess. None of the research investigating injury preven-tion exercise interventions has incorporated an initialassessment to identify specific deficiencies for an individ-ual. Currently, training programs for elite athletes aredeveloped from an initial assessment and customized tomeet the specific needs of the individual, and as a result,there have been tremendous gains in individual physicalperformance.14,23 Similarly, rehabilitation and recondition-ing programs for individuals recovering from physicaland/or medical disability are based on an initial assess-ment and customized for the patient.52 A similar approachmay need to be taken with injury prevention to maximizethe effectiveness of injury prevention exercise interven-tions in reducing the risk of injury during sport and recre-ational activity. In support of this concept, it has recentlybeen indicated that customized injury prevention exerciseinterventions are the next phase of the research in thisarea of injury prevention.29

Individuals displaying various potentially injuriousmovement patterns may respond differently to aninjury prevention program. Recent research supports thistheory as it has been shown that an individual’s initialbiomechanical profile influences the effectiveness of aninjury prevention program.47 In this study, those individu-als displaying large external knee abduction momentswere classified as “high risk,” and conversely, those withsmall knee abduction moments were classified as “lowrisk.”47 Subjects in the high-risk group were able to reduce

their knee abduction moments greater than those inthe low-risk group.47 These results highlight the potentialimportance for evaluating subjects’ improvement basedon their initial profile. The subjects in the “high-risk”group had greater moments to reduce so it is not surpris-ing that this group improved, but these changes may havebeen obscured if the subjects were not divided into riskcategories before the analysis. These findings also suggestthat an initial movement assessment to identify individu-als with varying levels of movement quality mayimprove the effectiveness of injury prevention programs.Individuals with poor movement technique may respondmore favorably than those with good movement technique,which may suggest the need to place a greater trainingemphasis on those individuals with poor movementtechnique. However, more research is necessary to investi-gate whether individuals with varying levels of movementquality respond differently to an injury preventionprogram.

There is minimal information regarding how differentpopulations respond to an ACL injury prevention program.The rate of ACL injury is 2 to 5 times greater in femalethan male athletes in comparable sports, but due to greaterexposure to sports, the absolute number of ACL injuries inmale athletes is greater than the absolute number infemale athletes.3,4,8,21,24,30,40,46 To our knowledge, female ath-letes are often evaluated before and after an injury preven-tion program, but only 1 study has evaluated how maleathletes respond to an injury prevention program, and nostudies have compared results between sexes.11 Besidesfocusing primarily on female athletes, the majority of pre-vious research on injury prevention programs used eitheradult or high school–aged subjects.11,31,42,50,56 Outcomes ofinjury prevention programs may be enhanced if the pro-grams are implemented to individuals before they reachthe age associated with the highest injury risk. However,minimal research has been conducted with youth athletesless than 14 years of age to evaluate if they are able torespond to an injury prevention program.28

The primary purpose of this study was to comparechanges in landing technique between subjects with differ-ent baseline levels of movement error after completion of aninjury prevention program. A secondary objective of thisstudy was to evaluate if injury prevention programs strati-fied to specific movement errors, compared with a general-ized (“one-size-fits-all”) program, would result in differentimprovements between pretraining and posttraining meas-urements. Our final objective was to compare improve-ments from an injury prevention program between sexesand age groups. We hypothesized high school–aged femalesubjects with poor landing technique would improve themost after completing the stratified program.

MATERIALS AND METHODS

We conducted a cluster-randomized controlled trial to studythe effect of a stratified versus generalized neuromuscularinjury prevention program in changing landing techniquein youth male and female soccer players during the course†References 11, 31, 32, 38, 42, 50, 51, 61, 64.



Vol. 37, No. 3, 2009 Movement Patterns After an ACL Injury Prevention Program 497

of a soccer season. The stratified injury prevention pro-gram consisted of a group of exercises stratified to correctcertain movement errors as well as a set of team exercises.In contrast, the generalized program only involved a largerset of team exercises. We chose to randomize at the teamlevel because randomization of players on the same teamto different injury prevention programs would probablyresult in contamination. This randomization techniquepermits teams to perform a warm-up together, which iscommon practice in the general population and enhancesthe external validity.20

We recruited 27 youth soccer teams from the under-11(10 years old) to the under-18 (17 years old) divisions ofan area soccer association. All teams were in the compet-itive division of the soccer association, and players wereplaced on the teams through a selection process based ontheir skill level before the season. The teams were ran-domly assigned using a coin flip to either a stratified (13teams) or generalized (14 teams) injury prevention pro-gram after stratifying the teams by sex and age group. Allof the teams chose to adopt the injury prevention pro-gram as a replacement for their normal warm-up routine.As a result, all players performed the injury preventionprogram, but movement assessment was only conductedon players who volunteered to participate in the study.Before the season, players and their parents wereinformed about the study and asked for their voluntaryparticipation. University-approved informed assent andconsent forms were completed by players and their par-ents, respectively, if they chose to participate. Table 1 pro-vides detailed demographic information of players whochose to participate in this study.

Movement Assessment

Subjects had no symptoms of injury limiting soccer partic-ipation at the time of movement assessment and wereevaluated at the beginning (pretest) and the end (posttest)of their season. Teams between the ages of 10 and 13 years(under-11 to under-14 team divisions) participated in a9-month season (August to May), while the older agegroups, between the ages of 14 and 17 years (under-15 tounder-18 team divisions), only performed the programs for4 months (females: August to December; males: January to

May) due to conflicts with their high school soccer seasons.Both movement assessments (pretest and posttest)occurred before the start of a soccer practice so fatigue didnot influence the results.

Subjects performed 3 trials of a standardized jump-land-ing task during both movement assessments. Subjectsjumped forward from a 30-cm high box a distance of half oftheir body height, which was marked using a line placed onthe ground according to each subject’s height. Upon landingon both feet, subjects were instructed to jump vertically formaximal height and land in approximately the same loca-tion. All subjects received the same oral instructions beforethe jump-landing task, which included “jump forward fromthe box with both feet so that you land with both feet justafter the line” and “as soon as you land, jump up for maxi-mum height and land back down.” A research assistantdemonstrated the maneuver once for each team to mini-mize coaching effects. The subjects were instructed to per-form the task as naturally as possible and not to beinfluenced by the demonstration. Subjects were permittedto practice the jump-landing task until they felt comfort-able with the task and performed it correctly. Trials wereexcluded and repeated if subjects jumped vertically fromthe box instead of forward or if the subjects did not jump formaximal height on landing. Two video cameras (Sony DCR-HC30, Park Ridge, NJ) were positioned 10 ft in front of andto the right of the subject to record all 3 jump-landing tasktrials. The camera positioning allowed capture of bothfrontal and sagittal plane images.

Movement technique was assessed during the jump-landing task before and after the season through theLanding Error Scoring System (LESS), which will bediscussed further in the Data Reduction/Analysis section.In addition to the 3 jump-landing task trials, subjects alsocompleted a double-legged squat task during the pretestsession. Subjects in the stratified injury prevention pro-gram performed a specific set of exercises based ontheir movement patterns during the double-legged squattask (described further in Stratified Intervention). Thedouble-legged squat task required subjects to stand withtheir feet shoulder-width apart, toes pointing straightahead, hands straight up in the air, while they slowlysquatted toward the ground as far as comfortably possibleand returned to the starting position.

TABLE 1Subject Demographicsa

Sex Group Age (years ± SD) Height (cm ± SD) Mass (kg ± SD)

Male General Program 13 ± 2 164 ± 13 51 ± 14MKD 12 ± 1 153 ± 17 41 ± 14TO 14 ± 2 171 ± 12 60 ± 13NA 13 ± 1 173 ± 5 56 ± 8

Female General Program 14 ± 2 162 ± 8 51 ± 10MKD 13 ± 2 160 ± 9 50 ± 9TO 13 ± 2 159 ± 7 49 ± 10NA 13 ± 2 155 ± 8 47 ± 10

aSD, standard deviation; MKD, medial knee displacement; TO, toe out; NA, neutral alignment.




Implementation of Injury Prevention Programs

Research assistants trained the teams on their programat the beginning of a practice within 1 week of the pretestsession. Coaches received training materials explainingtheir team’s respective injury prevention program,attended a brief lesson on their team’s program, and weretaught the exercises by the research staff before thispractice. All teams completed their respective programwithin 10 to 15 minutes at the start of every soccer train-ing session, approximately 3 to 4 times per week,throughout the season. Proper technique was emphasizedto the subjects for all of the exercises. Subjects received aconstant set of instructions and oral cues to rememberwhile performing the exercises. These instructions andoral cues consisted of “keep your toes pointed straightahead,” “keep your knees over your toes,” and “land softlyon your toes while bending your knees.” Coaches wereinstructed to increase the number of exercise repetitionsas the players progressed with the program.

Stratified Intervention

Subjects on teams in the stratified program were furtherassigned to 1 of 3 groups (medial knee displacement, toe out,normal) based on their individual movement assessmentduring the double-legged squat task. Using the videotapes ofthe double-legged squat task recorded during the pretest

session, a single individual assessed the frontal plane imageof the double-legged squat task to assign stratified groupsubjects to 1 of the 3 stratified exercise categories. The sameindividual assessed all of the double-legged squat tasks. Onlysubjects on teams randomized to the stratified program wereassessed with the double-legged squat because their exer-cises were dependent on their group assignment in contrastto subjects on teams assigned to the generalized program.Subjects who demonstrated excessive medial knee displace-ment, which was defined as the patella moving medial to thegreat toe at any point during the squat descent, wereassigned to the medial knee displacement (MKD) exercisegroup (Figure 1A). Subjects whose feet rotated outward dur-ing any point of the squat descent were assigned to the toe-out (TO) exercise group (Figure 1B). Participants whodisplayed both medial knee displacement and toe out wereassigned to the TO group. The neutral alignment (NA) exer-cise group consisted of subjects who did not demonstrateeither medial knee displacement or toe out during thedouble-leg squat task (Figure 1C).

A detailed description of the stratified injury preventionprogram is provided in Appendix 1 (available online athttp://ajs.sagepub.com/supplemental/). The stratified pro-gram was developed from previous ACL injury preventionprograms but expanded to include multiplanar exercises,as well as exercises designed to target the specific move-ment patterns (MKD, TO, or NA).22,32,42,49-51 At the begin-ning of each practice, teams in the stratified groups first

Figure 1. A, Medial knee displacement during double-legged squat task. B, Toe out during double-legged squat task. C, Neutralalignment during double-legged squat task.




divided into their 3 separate subgroups, MKD, TO, or NA,and completed the stratified exercises. Both the MKD andTO stratified exercises consisted of a static stretching exer-cise to lengthen the muscle thought to be influencing theirmovement pattern, as well as an exercise to activate themuscle that may be able to correct their movement pat-tern. To address medial knee displacement, the MKDgroup strengthened and stretched the hip abductor andadductor muscles, respectively. The TO group performedexercises to strengthen and stretch the lower leg muscles.The NA group performed exercises to improve their coreand lower extremity stability and strength. After the groupexercises, all of the individual groups came together toperform a set of team exercises.

Generalized Intervention

A detailed description of the generalized injury preventionprogram is provided in Appendix 2 (available online athttp://ajs.sagepub.com/supplemental/). The athletes onteams randomized to the generalized intervention wereassigned a set of exercises to strengthen and stretch sev-eral lower extremity muscle groups and were providedwith training in good postural alignment during physicalactivity. This intervention is modified from previous ACLinjury prevention programs that have been shown to beeffective with reducing injury rates and modifying neuro-muscular injury risk factors.22,32,42,49-51 The generalizedintervention maintained the overall types of exercisesincluded in these previous studies but abbreviated the pro-gram to make it more feasible as a warm-up.

Compliance Monitoring

Research assistants visited each team at least once a weekduring the season to monitor compliance and correct exer-cise technique. Coaches and players also recorded programcompliance and informed the research assistant about theteam’s participation during the previous week. Compliancerecords were completed after every visit by the researchassistant and used to produce a team compliance scoreafter completion of the season. Four possible scores (1, 2, 3,or 4) could be received for the team compliance score, andthe score was related to what percentage (100% = 4, 75% =3, 50% = 2, 25% = 1) of the time the team was performingthe exercises correctly with no encouragement needed bythe research assistant. For example, a score of 4 indicatedthe team achieved 100% compliance according to coaches,players, and research assistants’ observations. In contrast,teams with 25% compliance, or a score of 1, only performedthe programs when the research assistants were present.Observations that rated compliance between these per-centages were rounded down as a conservative estimate.

Data Reduction/Analysis

LESS. All jump-landing and squat trials were trans-ferred from standard videotapes to a videoediting softwarepackage (Windows Media Player, Microsoft, Redmond,

Washington) after data collection was complete. All jump-landing trials were scored using the LESS by 1 of 2 differ-ent researchers who were blinded to group assignment andtesting session. The LESS is a standardized clinical move-ment assessment tool for identifying improper movementpatterns during the jump-landing tasks. The LESS uses abinary system (0,1) to evaluate landing technique based on9 jump-landing characteristics: knee flexion angle, kneevalgus angle, trunk flexion angle, ankle plantarflexionangle, foot position, stance width, foot contact (heel or toefirst), overall joint motion, and overall impression oflanding “quality.” A higher LESS score indicates a greaternumber of landing errors committed and thus poor jump-landing technique.

The LESS has been shown to be a reliable (intrarater:intraclass correlation coefficient [ICC]2,k = .90, standarderror of the mean [SEM] = 1.08; interrater: ICC2,1 = .83,SEM = 1.50) tool to assess landing errors in large popula-tions of subjects efficiently.10,55 In addition to reliability,predictive and concurrent validity of the LESS has alsobeen established.10,54,55 Female subjects have consistentlybeen shown to demonstrate different movement patterns,such as reduced knee flexion and greater knee valgus, com-pared with male subjects during sports maneuvers, suchas the jump-landing task.13,17,29,35 The LESS was able todifferentiate between sexes in both collegiate and youthathletes as LESS scores were significantly higher infemale athletes.10,55,62 The LESS and an electromagneticmotion analysis tool yield comparable conclusions aboutspecific landing errors, such as knee flexion, knee valgustorque, and vertical ground-reaction forces.10,55 Predictivevalidity of the LESS has been established in prospectiveresearch that has been conducted comparing LESS scoresbetween ACL-injured and noninjured individuals.54 Theresults revealed that ACL-injured subjects demonstratedless knee flexion motion and less flexion in all lowerextremity joints compared with the noninjured subjects.

LESS variables. We calculated an average total LESSscore by taking the mean of the total LESS scores from the3 jump-landing trials to analyze overall landing techniqueduring both testing sessions. Next, we calculated a changescore for the average total LESS score (posttest-pretest),causing a negative change score to indicate the subjectimproved after completion of the program. We created 2independent variables to allow us to evaluate changes inLESS score between ages (age group) and levels of baselinemovement error (baseline movement error). To determineif age influenced the program’s effectiveness, we comparedchanges in LESS scores between pre–high school (ages10-13 years) and high school (ages 14-17 years) age sub-jects. This age grouping accounted for the fact that thepre–high school group performed the program for a longerduration (9 months) than the high school–age group(4 months) because the high school–age group teams didnot practice regularly during the high school soccer season.

To examine the influence of an individual’s baselinemovement patterns, we formed groups based on quartilesfrom the pretest LESS scores. Subjects whose LESS scoreswere in the first quartile (LESS range, 0-3.67 points)




demonstrated “excellent” landing technique. Those in thesecond quartile (LESS range, 3.68-5.0 points) were con-sidered to have “good” landing technique. The subjects inthe third quartile (LESS range, 5.1-6.3 points) were con-sidered to display “moderate” landing technique.Individuals in the fourth quartile had the highest scores(LESS range, 6.33-11.0 points) and demonstrated “poor”landing technique. We then compared changes in LESSscores between groups.

Statistical Analysis

We used a generalized linear model, with the team compli-ance score as a covariate to compare the change scorebetween sex, age group, baseline movement error, and pro-gram. Only main effects were included in the model as ourprimary research questions involved determining if pro-gram type mattered, if girls or boys were more sensitive tothe program, if age influenced the ability to change, and ifbaseline scores affected improvement. Complete separa-tion of confidence intervals was used to evaluate pairwisecomparisons for post hoc testing.

Individual items on the LESS were analyzed to deter-mine which specific movement patterns changed or failedto change after subjects performed the injury preventionprograms. First, we produced a composite score for indi-vidual landing characteristics. Subjects were scored withan “error” if the subject demonstrated the specific landingcharacteristic error during 2 or more of the 3 trials; other-wise, that individual item was coded as “no error.” Next, wedevised a coding scheme to identify subjects who improvedtheir landing error versus those who did not improve aftercompleting the program. Subjects who demonstrated aspecific landing error at pretest, but not at posttest, werescored with a binary response of “improve.” Subjects whodisplayed the landing error at pretest but still possessedthe error at posttest received a “no improvement” score.Only a small percentage of subjects demonstrated an errorat posttest but no error at pretest. For the 6 main variablesexamined for improvement, 3 variables had between 5% to10% of subjects demonstrate the error at posttest but notat pretest, and the other 3 variables had this occur in 10%to 20% of subjects. Because of this small percentage andthat our main focus was on evaluating the programs’ effec-tiveness with improving a variable between pretest andposttest, we chose to not discuss these subjects. We didevaluate whether there were any program, sex, or agegroup differences between subjects this applied to butfailed to see any significant differences.

We used a binomial proportion test to evaluate the nullhypothesis that a specific LESS error would not improveafter completing the program (P = .00). These individualitem analyses were only performed on specific LESS errorsif at least 10 subjects demonstrated the error at pretest.For all statistical analyses, we used SPSS 15.0 (SPSS Inc,Chicago, IL), an a priori level of significance of .05, andgeneralized estimating equations to account for the clus-tered random assignment.

RESULTS

Forty percent of the players (90 male subjects: age, 13 ± 2years; height, 166 ± 13 cm; mass, 54 ± 14 kg; and 83 femalesubjects: age, 13 ± 2 years; height, 160 ± 8 cm; mass, 50 ±10 kg) from the 27 recruited teams volunteered for thestudy and completed both testing sessions (Figure 2).Detailed subject demographics based on sex, program, andtraining group assignment are provided in Table 1. Allteams remained in the analysis regardless of compliancescore because we intended to treat all of them. The averagecompliance score was 3.13, indicating that the teams aver-aged more than 75% compliance. No statistically signifi-cant differences were observed between groups for height,weight, age, or pretest LESS score.

Our findings indicate that the level of baseline move-ment error affected the subjects’ abilities to improve aftercompleting one of the injury prevention programs. A sig-nificant difference in change scores was observed betweenlevels of baseline movement error (Wald = 80.22, P < .0001)(Figure 3). By comparing confidence intervals, it is clearlyevident that the “poor” landing technique group improvedtheir LESS score greater than any other group. Also, the“good” and “moderate” baseline movement error groupsimproved more than the “excellent” group.

In addition to baseline level of movement error, ageappeared to influence the potential for improvement as we

EnrolledSubjectsn = 310

PretrainingAssessment

n = 237

RandomAssignment

StratifiedIntervention

n = 155

GeneralizedIntervention

n = 155

PosttrainingAssessment

n = 173

MKD Groupn = 48

TO Groupn = 67

NA Groupn = 26

In-SeasonCustomized

Training

In-SeasonCustomized

Training

In-SeasonCustomized

Training

In-SeasonGeneralized

Training

Figure 2. Flowchart of study procedures. MKD, medial kneedisplacement; TO, toe out; NA, neutral alignment.




observed a significant difference in change scores betweenthe 2 age groups (Wald = 7.27, P = .007) (Figure 4). Thehigh school–age group improved their LESS total score toa greater extent than the pre–high school age group. Nosignificant differences in change scores were observedbetween programs (Wald = 0.65, P = .42) (Table 2) or sexes(Wald = 0.87, P = .35) (Table 3). We did not include testingsession as an independent variable so we were not able toperform statistical analysis to evaluate a time main effect.However, we did observe an overall negative LESS totalchange score (posttest-pretest = –0.66 ± 1.9), suggestingthat all subjects, regardless of intervention group,appeared to improve their landing patterns after comple-tion of the injury prevention program.

The individual item analysis yielded significantimprovements for all of the variables evaluated (P < .0001):knee flexion at initial contact, trunk flexion at initial con-tact, foot external rotation, narrow stance width, knee flex-ion displacement, knee valgus displacement, and trunkflexion displacement. Our objective with this analysis wasto determine if there were specific LESS items that were

responsible for changes in the total LESS score. Therefore,we performed the same binomial proportion test as beforebut changed the null hypothesis to evaluate if there weredifferences in proportions between subjects who improvedthe item and those who did not (P = .5 instead of P = .0).When we performed this analysis, we observed significantimprovements in the following variables: knee flexion atinitial contact (P = .009, 65% improvement), trunk flexionat initial contact (P = .005, 70% improvement), and kneeflexion displacement (P = .001, 93% improvement) (Table 4).Contrary to these results, the majority of subjects did notimprove foot external rotation (P < .0001, 23% improve-ment), narrow stance width (P < .0001, 29% improvement),or knee valgus displacement (P < .0001, 21% improvement)(Table 5).

DISCUSSION

The most important finding is that subjects who possessed thelargest number of landing errors at baseline achievedthe greatest improvements after completion of the injury

95% CI Δ Score

0

1

2

3

4

5

6

7

8

9

10

Poor(–3.4 to –2.0)

Moderate(–1.5 to –0.6)

Good(–0.8 to 0.4)

Excellent(0.08 to 0.9)

LE

SS

To

tal S

core

Pretraining

Posttraining*

††

Figure 3. Landing Error Scoring System (LESS) scores(mean ± SD) grouped by landing technique quartiles for pre-training and posttraining and 95% confidence interval (CI) ofchange scores. *Significantly greater change in LESS scorethan all other groups (P < .05). †Significantly greater changein LESS score than excellent landing group (P < .05).

95% CIΔ Score

0

1

2

3

4

5

6

7

8

Pre–High School(–1.0 to –0.4)

High School(–1.7 to –.98)

LE

SS

To

tal S

core

PretrainingPosttraining

*

Figure 4. Landing Error Scoring System (LESS) scores(mean ± SD) for pre–high school and high school–age groupsand 95% confidence interval (CI) of change scores.*Significantly greater change in LESS score compared withpre–high school age group (P < .05).

TABLE 2Program Average Total Landing Error Scoring System(LESS) Scores (mean ± SD) Before and After the InjuryPrevention Program and Change Scores (mean ± SE)a

Change 95% CI of Program Pretest Posttest Score Change Score

General 5.13 ± 2.0 4.15 ± 1.77 –1.04 ± 0.24 –1.50 to –0.57Stratified 4.99 ± 1.80 4.50 ± 1.84 –0.81 ± 0.18 –1.15 to –0.46

aSD, standard deviation; SE, standard error; CI, confidence interval.

TABLE 3Sex Average Total Landing Error Scoring System

(LESS) Scores (mean ± SD) Before and After the InjuryPrevention Program and Change Scores (mean ± SE)a

Change 95% CI ofSex Pretest Posttest Score Change Score

Male 4.43 ± 1.7 3.92 ± 1.79 –1.08 ± 0.26 –1.58 to –0.57Female 5.75 ± 1.9 4.76 ± 1.74 –0.76 ± 0.19 –1.14 to –0.39

aSD, standard deviation; SE, standard error; CI, confidence interval.




prevention program. Subjects in the “poor” baseline move-ment error group averaged more than 6 points on theLESS before beginning the injury prevention program.This group demonstrated 3 times as much improvement byimproving 2.5 points, which is 2 points higher than theoverall mean change of 0.66 points. The “moderate” and“good” groups also experienced significant improvement intheir landing technique. In contrast to these groups, the“excellent” group, or subjects with the smallest amount oferror, did not appear to improve greatly or actuallydigressed slightly in their landing ability.

Even though the “excellent” group did not improve theirlanding technique, the finding that the 3 other baselinemovement error groups improved is significant becausenumerous landing errors theoretically correlate with agreater risk of injury. Consequently, the subjects in these 3groups appear to be the subjects where improvement maybe the most critical to reduce injury risk. Similar to ouranalysis using baseline movement error, Myer et al47 usedpretest knee abduction moments during a drop-verticaljump to divide subjects into “high-risk” or “low-risk” groupsand compared the effects of a neuromuscular training pro-gram between these groups. In agreement with our currentfindings, the authors concluded that subjects in the “high-risk” group, or those with greater hip abduction moments,responded more favorably to the training program thansubjects in the “low-risk” group.

These findings suggest that neuromuscular training pro-grams are most effective with athletes who display poormovement techniques before beginning the program. Tomaximize the effectiveness of future injury prevention pro-grams, it may be necessary to use clinical screening tools toidentify individuals with high-risk biomechanical profilesand target injury prevention programs toward these indi-viduals. Comparing training effects between individual bio-mechanical characteristics also allows greater differencesto be observed between subjects and reveals that subjectswith greater potential risk factors respond more favorably

to an intervention program. The improvements observed inboth of these studies may have been diminished if the sub-jects had not been compared according to their degree ofmovement error or external abduction moment. Authors offuture studies aimed at changing movement patterns orrisk factors should consider these findings when designingtheir analysis to ensure their results account for differencesin improvement potential based on initial assessment.

On the basis of our previous observation that subjectswith higher LESS scores at baseline demonstrated thegreatest improvement, we hypothesized the pre–highschool age group, who possessed more landing errors thanthe high school–age group, would either improve equally orbetter than subjects in the high school–age group.62

Contrary to this hypothesis, the high school–age groupsustained greater improvements from the injury preven-tion program compared with their younger counterparts.This finding is also surprising because the younger playersperformed the injury prevention program for 4 monthslonger than the older players. No other study has com-pared duration length between programs so it is unknownwhether this result is due to differences in the amount oftraining or the age difference between the groups. In hind-sight, assessing the younger age group at the end of boththe fall and spring seasons would have provided moreinsight into this finding and should be a consideration forfuture studies.

Intervening during early adolescence has been suggestedto help reduce ACL injuries,30,65 but little is known about thepotential for this population to respond to an injury preven-tion program. Grandstrand et al28 is the only other study, toour knowledge, to incorporate athletes as young as 10 yearsof age into an injury prevention program and the authorsdid not observe any improvements in injury risk factors. Theauthors concluded that the program may have been too dif-ficult for the young players to complete. In contrast, simpleverbal cues instructing young children to bend their kneesand land softly resulted in reduced landing forces.58 These

TABLE 4Variables That Improveda

Landing Error No. With Error at Pretest Improved, no. (%) Not Improved, no. (%) Level of Significance

Knee flexion at initial contact 85 55 (65) 30 (35) P = .009Trunk flexion at initial contact 53 39 (70) 17 (30) P = .005Knee flexion displacement 15 14 (93) 1 (7) P = .001

aTest proportion = 0.50.

TABLE 5Variables That Did Not Improvea

Landing Error No. With Error at Pretest Improved, no. (%) Not Improved, no. (%) Level of Significance

Foot external rotation 70 15 (23) 55 (77) P < .0001Narrow stance width 113 33 (29) 80 (71) P < .0001Knee valgus displacement 123 24 (21) 89 (79) P < .0001

aTest proportion = 0.50.




conflicting findings suggest extensive ACL injury preven-tion programs may not be appropriate for young prepubes-cent athletes or may suggest the need for a specializedinjury prevention program that matches these athletes’stage of motor and growth development. Although our highschool–age group improved to a greater extent than thepre–high school age group, improvements were still presentin the younger subjects. Their posttest LESS scores becamesimilar to the older subjects’ pretest scores, illustrating thatthe younger players are improving to some degree, andfuture research with this age group is encouraged.

Sex differences exist with both ACL injury rates andmovement patterns. Female athletes consistently performcommon sport maneuvers with reduced trunk, hip, andknee flexion, as well as greater hip and knee transverseand frontal plane motion compared with their male coun-terparts.‡ We also observed differences between sexes asmale subjects began the injury prevention program withfewer landing errors than the female subjects. Similar tothe age group analysis, we hypothesized that female sub-jects would sustain greater improvements compared withthe male subjects. However, male and female subjectsresponded similarly to the injury prevention program. Wedid observe that after training female athletes’ landingtechnique is comparable to males’ landing technique beforecompleting the injury prevention program. It is unknownif these improvements equate to reductions in femaleinjury rates. To our knowledge, no other study has com-pared changes due to a neuromuscular training programbetween sexes; in fact, no other study has even investi-gated whether male athletes alter neuromuscular risk fac-tors after a prevention program. While female athletes areat a greater risk of injuring their ACL in sports, such assoccer and basketball, more male athletes actually injuretheir ACLs each year than do female athletes.45 Futureresearch needs to evaluate male responses to injury pre-vention programs to effectively reduce ACL injuries.

The secondary objective of this study was to compareneuromuscular risk factor improvements between a gener-alized and stratified injury prevention program. Ourhypothesis was that a stratified approach would result ingreater improvements; however, our results do not supportthis hypothesis as there was no difference between pro-grams. We cannot conclude that the improvementsobserved by both programs are not due to maturationalone without having the ability to compare change scoresto a control group. However, all of the improvementsdetected between the baseline movement error groups, agegroups, and sexes lead us to believe that these changes aredue to the injury prevention program. One hypothesis wehave for failing to see differences between the 2 programsis that, in hindsight, there were too many similaritiesbetween the programs and the stratified program was notindividualized enough. Other studies comparing 2 pro-grams against one another have evaluated interventionswith predominantly one type of exercise, such as plyomet-ric or dynamic stability, versus a program with a different

principal exercise type.48 While the 2 programs included inthis study consisted of several different exercises, both pro-grams were multifaceted and included similar amounts ofbalance, plyometric, strengthening, and agility exercises.Although the subjects performed different exercises tar-geted at the same purpose, it is possible the exercisesloaded the body similarly, resulting in comparable results.

We performed the individual item analysis to observe ifcertain variables were responsible for changes in the totalLESS score. On the basis of these results, it appears thevariables knee flexion at initial contact, knee flexion dis-placement, and trunk flexion at initial contact are able tobe successfully improved with the use of a neuromusculartraining program. These findings may lead to a reductionin ACL injuries as knee flexion has frequently been citedas a risk factor for injury.7,12,23 Unfortunately, only a smallpercentage of subjects who demonstrated excessive footexternal rotation and knee valgus displacement, as well asnarrow stance width, were able to improve this error afterthe training program. These results suggest plane ofmotion may influence the capability for improvement.Variables that improved the greatest were all motions inthe sagittal plane, while the majority of the variables thatdid not change were motions in the frontal and transverseplanes. Future research should evaluate whether theincorporation of certain exercises affects modification ofrisk factors in specific planes of motion.

In general, injury prevention programs have shownpromising results in decreasing overall injury rates, ACLinjury rates, and modifying neuromuscular risk factors. Arecent systematic review indicated there is moderate evi-dence to illustrate that injury prevention programs incor-porating balance, plyometric, strengthening, and agilityexercises can decrease ACL injury rates.53 It was noted inthis systematic review that very few studies used a ran-domized controlled trial design, thus weakening the levelof evidence supporting the effectiveness of current injuryprevention programs. Future research incorporating ran-domized controlled trial designs is needed to strengthenthe evidence surrounding ACL injury prevention pro-grams. Another limitation of previous research is theinclusion of programs that require more than 30 minutesto complete. To best ensure maximal public health effectand adoption by the community at large, injury preven-tion programs should be easy to implement and time effi-cient. Although a limitation of this study is the lack of apure control group, both injury prevention programsimplemented were randomly assigned to teams, requiredonly 10 to 15 minutes to complete, were effectively super-vised by team coaches, were efficiently incorporated intodaily practice routines, and successfully altered move-ment patterns.

CONCLUSION

These findings suggest that multifaceted exercise inter-ventions incorporated as part of a normal warm-up routinesuccessfully modified movement technique; however, addi-tional research is needed that incorporates biomechanical‡References 15, 17, 19, 26, 27, 29, 33-36.




analyses to better understand the influence of exerciseinterventions on biomechanical factors associated withinjury risk. Such research may be particularly importantin high school–age soccer athletes who appear to be atgreatest risk and most amenable to changes in movementpatterns. Players with the greatest amount of movementerrors, and potentially greatest amount of injury risk, sus-tained the greatest improvement after the program. Injuryprevention programs may be more efficient if they arespecifically targeted for these athletes. In addition, thisstudy demonstrated that no further benefits are observedfrom an injury prevention program designed for specificmovement patterns compared with a traditional “one-size-fits-all” program; however, the 2 programs compared inthis study may have been too similar, leading to these sim-ilar results. The results of this study indicate great poten-tial for ACL injury prevention programs.

ACKNOWLEDGMENT

Funding support for this project was provided through aresearch grant by the National Academy of SportsMedicine. We acknowledge Micheal A. Clark, DPT, MS,PES, CES, for his contributions toward the exercise pro-gram design and development. We also acknowledgeTriangle United Soccer Association for its support andcooperation with this project.

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