Retention of Movement Pattern Changes After a Lower Extremity Injury Prevention Program Is Affected by Program Duration

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1 AJSM PreView, published on November 7, 2011 as doi: / Retention of Movement Pattern Changes After a Lower Extremity Injury Prevention Program Is Affected by Program Darin A. Padua,* y PhD, ATC, Lindsay J. DiStefano, z PhD, ATC, Stephen W. Marshall, PhD, Anthony I. Beutler, MD, Sarah J. de la Motte, PhD, ATC, and Michael J. DiStefano, y MA, ATC Investigation performed at the University of North Carolina, Chapel Hill Background: Changes in movement patterns have been repeatedly observed immediately after completing a lower extremity injury prevention program. However, it is not known if movement pattern changes are maintained after discontinuing the training program. Hypothesis: The ability to maintain movement pattern changes after training has ceased may be influenced by the program s duration. The authors hypothesized that among individuals who completed either a 3-month or 9-month training program and who demonstrated immediate movement pattern changes, only those who completed the 9-month training program would maintain movement pattern changes after a 3-month period of no longer performing the exercises. Study Design: Cohort study; Level of evidence, 2. Methods: A total of 140 youth soccer athletes from 15 separate teams volunteered to participate. Athletes movement patterns were assessed using the Landing Error Scoring System (LESS) at pretest, posttest, and 3 months after ceasing the program (retention test). Eighty-four of the original 140 participants demonstrated improvements in their LESS scores between pretest and posttest (change in LESS score.0) and were included in the final analyses for this study (n = 84; 20 boys and 64 girls; mean age, years; age range, years). Teams performed 3-month (short-duration group) and 9-month (extendedduration group) injury prevention programs. The exercises performed were identical for both groups. Teams performed the programs as part of their normal warm-up routine. Results: Although both groups improved their total LESS scores from pretest to posttest, only the extended-duration training group retained their improvements 3 months after ceasing the injury prevention program (F 2,137 = 3.38; P =.04). Conclusion: Results suggest that training duration may be an important factor to consider when designing injury prevention programs that facilitate long-term changes in movement control. Keywords: injury prevention; motor learning; ACL; soccer; movement control Prevention of noncontact anterior cruciate ligament (ACL) injuries in young, physically active individuals is important given the associated costs and long-term disability. Recent estimates suggest that approximately $3 billion *Address correspondence to Darin A. Padua, PhD, ATC, University of North Carolina at Chapel Hill, 216 Fetzer Gym CB#8700, Chapel Hill, NC (dpadua@ .unc.edu). y Department of Exercise and Sports Science, Sports Medicine Research Laboratory, University of North Carolina, Chapel Hill, North Carolina. z Department of Kinesiology, University of Connecticut, Storrs, Connecticut. Department of Epidemiology, University of North Carolina, Chapel Hill, North Carolina. Uniformed Services University of the Health Sciences, Bethesda, Maryland. One or more of the authors has declared the following potential conflict of interest or source of funding: Dr Padua received a research grant from the National Academy of Sports Medicine to support this research. The American Journal of Sports Medicine, Vol. XX, No. X DOI: / Ó 2011 The Author(s) annually are spent on medical care associated with ACL injuries in the United States. 9,14 The long-term prognosis after ACL injury is unclear, as research indicates a high rate of early-onset knee osteoarthritis even with optimal surgery and rehabilitation in this population. 13,19,29 Furthermore, while individuals are able to return to sports participation after ACL injury, the ability to maintain participation at high levels is compromised. Perhaps most startling is the high recurrence rate of ACL injury, as nearly 10% of individuals who tear their ACL experience another ACL injury to either the ipsilateral or contralateral side. 25,28,31 The consequences of ACL injury are particularly problematic in the youth soccer population as adolescent female soccer players have a high ACL injury rate. 14 Thus, there is a need to prevent ACL injuries, especially in the youth soccer population who are at high risk. A systematic literature review indicated that there is moderate evidence (level B) to support the use of exercisebased injury prevention programs to reduce the rate of ACL injuries. 21 More recently, a meta-analysis of existing ACL injury prevention studies reported that exercise-based 1

2 2 Padua et al The American Journal of Sports Medicine injury prevention programs performed as part of a preseason and in-season program were able to decrease injury rates. 33 Exercise-based injury prevention programs are believed to be effective by improving biomechanical and neuromuscular characteristics during functional tasks, such as movement patterns during jumping, landing, and cutting maneuvers. Unfortunately, the effects of exercise-based injury prevention programs may be transient based on reports of injury rates returning to pretraining levels within 1 year of discontinuing an exercise-based injury prevention program. 18 This finding suggests that some types of exercise-based injury prevention programs may not facilitate true learning of new biomechanical and neuromuscular characteristics. Rather, these programs may only facilitate temporary changes in the performance of functional tasks that degrade over time when the exercise program is no longer performed. Learning new motor skills is typically defined as a relatively permanent change in a person s capability to perform a task. 32 Studies of motor skill learning are often based on retention tests, where participants are retested after a time interval of no longer performing the training intervention. Previous research demonstrates significant improvements in movement patterns, such as knee flexion, knee valgus, and hip motion, immediately after completing an exercise-based injury prevention program. 12,16,20,26 Immediate improvements in biomechanical and neuromuscular characteristics after an injury prevention program is completed suggest that function was improved, but this does not indicate that learning new motor skills was achieved. It is possible that learning new motor skills may depend on the duration of the training program. Previous research investigating the effects of injury prevention programs on movement patterns have utilized a 6- to 12-week intervention period. 4,7,8,15-17,26,30 However, we are not aware of any previous research investigating the effects of program duration on changes in movement patterns. Thus, while a traditional program duration of approximately 3 months can successfully improve biomechanical and neuromuscular function, the retention/decay of these new motor skills is not known. If an injury prevention program facilitates only temporary changes in biomechanical and neuromuscular characteristics, then this may explain previous reports of increased injury rates after discontinuing a program. It is also unknown if extended duration training periods may be required to permanently learn new motor skills. Research has not investigated if changes in biomechanical and neuromuscular characteristics are retained over time after discontinuing an exercise-based injury prevention program. Therefore, the purpose of our study was to compare the retention of improvements in movement patterns between a short-duration (3 months) and extended-duration (9 months) injury prevention program. We hypothesized that movement pattern improvements would not be retained 3 months after program completion in the short-duration program but would be retained in the extended-duration program. We assessed retention by a single retest 3 months after completion of each program. MATERIALS AND METHODS Design We used a repeated-measures design to evaluate if the duration of a lower extremity injury prevention program affected the retention of movement technique improvements in youth soccer athletes. Fifteen youth soccer teams agreed to participate in the study. Teams were stratified by age and sex and cluster randomized to 1 of 2 ACL injury prevention programs (generalized intervention or stratified intervention). 5 Seven teams performed the program for 3 months (short-duration group), and 8 teams performed the program for 9 months (extended-duration group). All athletes on participating teams performed the injury prevention programs as part of their normal warm-up routine, but only athletes who volunteered to participate in the research study were assessed for movement technique during 3 test sessions. Participants and researchers were blinded to program assignment at the time of inclusion. The same researchers assessed participants movement technique using the Landing Error Scoring System (LESS) during the 3 test sessions (pretest, posttest, and retention test). The pretest and posttest were conducted within 1 week of the beginning and end of the intervention period, while the retention test occurred 3 months after the intervention period. Teams did not perform the injury prevention programs during the time period between the posttest and retention test. The retention period was selected, as it represented each team s off-season and the players did not have formal interactions with coaches during this time. We surveyed participants at the retention test to verify that no formal training was performed during the retention period. Participants A total of 140 youth soccer athletes from 15 separate teams volunteered to participate. Eighty-four of the original 140 participants demonstrated improvements in their LESS scores between the pretest and posttest (change in LESS score.0) and were included in the final analyses for this study (n = 84; 20 boys and 64 girls; mean age, years; age range, years). Figure 1 summarizes how participants were enrolled to the extended-duration and shortduration groups. All participants were free from any injury or illness that prohibited soccer activity at the time of all 3 test sessions. Participants and their parents completed assent and consent forms, respectively, prior to the initial test session, which were approved by the university s institutional review board. Procedures Data were collected at the start of the team s soccer season. Participants performed 3 trials of a standardized jumplanding task (Figure 2). They began the task standing on a box 30 cm high that was placed a distance of half their body height away from a landing area, which was marked by a rope line. Participants were instructed to jump

3 Vol. XX, No. X, XXXX Retention of Movement Pattern Changes After Training 3 Improved (n=33) Short (n=11) Generalized Intervention (n=56) Short (n=33) 15 youth soccer teams (140 participants) No Change (n=23) Extended (n=22) Improved (n=51) Short (n=22) Stratified Intervention (n=84) forward from both limbs, land just past the line, and jump for maximum vertical height immediately after landing. Participants were given practice trials until they indicated they were comfortable with the task and performed it correctly. Trials were excluded and repeated if the participants jumped vertically from the box or if they failed to jump for maximum height upon landing. Two digital video cameras (Sony DCR-HC30, Park Ridge, New Jersey) were placed 10 ft in front of, and to the right of, the participants to capture frontal and sagittal plane images and recorded all jump-landing trials. ACL Injury Prevention Program Implementation No Change (n=33) Extended (n=29) Extended (n=51) Figure 1. Flow chart of enrollment of participants into the extended- and short-duration training groups. Teams performed a 10- to 15-minute ACL injury prevention program prior to each practice (approximately 3-4 times per week) during the intervention period. The intervention period was 3 months for the short-duration group (n = 33) and 9 months for the extended-duration group (n = 51). The teams in the current study are from a separate population of youth soccer athletes as those reported in DiStefano et al. 5 However, the 2 intervention programs used in the DiStefano et al study (stratified and generalized) were identical to those programs utilized in the current study. 5 DiStefano et al reported no difference in movement improvements, as measured by the LESS, between the 2 programs. 5 Our intervention groups (short duration and extended duration) consequently consist of participants who completed both generalized and stratified programs. Figure 1 demonstrates how many participants from the 2 intervention groups in the current study completed the 2 injury prevention programs (stratified and generalized). The inclusion/exclusion criteria were the same for all participants, regardless of which program they were randomly assigned. The difference between the 2 injury prevention programs (generalized and stratified) was in the specific exercises performed. DiStefano et al provide a detailed description of both ACL injury prevention programs. 5 All participants on teams assigned to the generalized intervention program performed the same static flexibility, balance, strengthening, plyometric, and agility exercises. However, participants on teams assigned to the stratified intervention program performed exercises based on their movement during a double leg squat, which was also recorded during the pretest. These participants were divided into subgroups, which consisted of toe out, medial knee displacement (MKD), or neutral alignment, based on their technique during the double leg squat. The 3 subgroups completed a specific set of flexibility and strengthening exercises. After the subgroup exercises, the 3 groups came back together as a full team to perform a set of team exercises. The team exercises involved strengthening, balance, plyometric, and agility exercises. Research assistants taught all of the teams their respective program and visited each team at least once a week during the intervention period to monitor compliance and correct exercise technique. The research assistants responsible for program implementation and compliance monitoring were not involved with movement technique assessment. Participants were instructed to think about their movement and rely on a specific set of cues when performing all of the exercises. These cues included keep your toes pointing forward, keep your knees over your toes, and land as soft as possible. The coaches and team captains were instructed to progressively increase the number of repetitions during the intervention period. Data Reduction The digital video of the jump-landing tasks was imported into video-editing software (Windows Media Player, Microsoft, Redmond, Washington) after data collection. Two research assistants graded the videos using the LESS. Research assistants were blinded to program assignment and testing session. We have previously demonstrated the LESS to be a valid and reliable (interrater reliability [ICC 2,1 ] = 0.84; standard error of the mean [SEM], 0.71) clinical movement analysis tool that evaluates specific jump-landing characteristics. 23 When scoring the LESS, the researcher looks for obvious movement errors. Thus, a 1-point change in the total LESS score can be associated with moderate to large differences in certain biomechanical variables. 23 A higher LESS score indicates a greater number of landing errors and consequently poor jumplanding technique. The average LESS score from the 3 trials at each testing session was used for data analyses. Statistical Analyses We performed a mixed-model analysis of covariance (ANCOVA) to evaluate the effects of time (3 levels: pretest, posttest, and retention test) and group status (2 levels:

4 4 Padua et al The American Journal of Sports Medicine Figure 2. The standardized jump-landing task consists of 2 segments: (1) participant jumps down from the box and lands on the ground, and (2) participant immediately jumps vertically upward as high as possible. Reprinted with permission from Padua et al. 23 TABLE 1 Landing Error Scoring System Scores (Unadjusted Values for Sex, Age, and Program Type) for the Extended- and Short- Training Groups at Pretest, Posttest, and Retention Test Group Time Mean 6 Standard Deviation 95% Confidence Interval Extended Pretest duration Posttest Retention test Short Pretest duration Posttest Retention test short duration and extended duration) after adjusting for sex (2 levels: male and female), age (11-17 years), and program (2 levels: stratified and generalized). We used a Tukey honestly significant difference post hoc test when necessary to evaluate significant differences between groups and time points. SPSS 16.0 (SPSS Inc, an IBM Company, Chicago, Illinois) with an a priori level of significance of.05 was used for all analyses. LESS Extended Short Pre-test Post-test Retention-test Time Figure 3. Significant group-by-time interaction for Landing Error Scoring System (LESS) scores (values adjusted for sex, age, and program type). Both the extended- and short-duration groups had significantly decreased LESS scores from pretest to posttest (*). The extended-duration group s LESS scores at retention test were improved compared with pretest but not different from posttest (y). The short-duration group s LESS scores at retention test were significantly increased from posttest and no longer different compared with pretest (z). * RESULTS We observed a significant time-by-group interaction (F 2,137 = 3.38; P =.04) (Figure 3). Post hoc testing revealed that both groups (short duration and extended duration) significantly improved their LESS scores between pretest and posttest. The extended-duration group s LESS scores at retention test were also better (lower) than at pretest. There was no difference between the retention test and the posttest for the extended-duration group. However, the short-duration group s LESS scores were better (lower) during posttest but had reverted to pretest levels at the retention test point. There was no difference between the 2 groups at pretest and posttest, but the extended-duration group demonstrated significantly improved (lower) LESS scores at the retention test compared to the short-duration group (Table 1). DISCUSSION There is a moderate level of evidence to support the use of injury prevention programs to reduce the risk of ACL and other lower extremity injuries. 10,21,33 Unfortunately, the effects of these programs may be transient. Previous research indicates that injury rates were decreased when performing an injury prevention program 3 times weekly during a 5- to 7-week training period during the preseason and then once a week during the competitive season. 18 However, injury rates returned to original (preinjury prevention program) levels after the programs were no longer performed. 18 The goal of most injury prevention programs is to improve movement quality. Thus, the finding of elevated injury rates following completion of injury prevention training calls into question the ability of injury

5 Vol. XX, No. X, XXXX Retention of Movement Pattern Changes After Training 5 prevention programs to facilitate retention of changes in movement quality. Our findings indicate that retention of changes in movement quality is influenced by the duration of training. Most importantly, our findings demonstrate that individuals who completed an extended-duration injury prevention program (9 months) successfully retained changes in overall movement quality, as measured by the total LESS score, following a 3-month detraining period of not performing the prescribed exercises. However, the short-duration training group (3 months) did not retain their improvements in movement quality and regressed back to their pretraining levels of movement quality after the 3-month detraining period. To our knowledge, this is the first study to examine the influence of training duration on retention of movement pattern changes during an injury prevention program. Our findings indicate that extended-duration training periods may be required to facilitate long-term retention of movement control. These findings have important ramifications on the design and the practical implementation of injury prevention programs. Motor learning theory indicates that learning a new skill (eg, movement pattern) should be accompanied by relatively permanent changes in the performance of the task. 32 The lack of retention observed suggests that learning new movement patterns did not occur in the shortduration training group, even though the group did demonstrate initial improvement in movement quality immediately following completion of the training program. In contrast, the extended-duration group s LESS scores at the retention test were not different from posttest scores and maintained a significant improvement from pretest scores. This may indicate that the extended-duration training effectively facilitated true learning of new movement patterns during the jump-landing task. We are unable to directly compare our findings with previous research, as we could not identify previous studies investigating if changes in movement patterns after an ACL injury prevention program were retained after a prolonged detraining period. Related research has investigated the effects of verbal instruction, a common component of injury prevention programs, on the long-term retention of changes in vertical ground-reaction force when landing from a jump. 27 Prapavessis et al compared the effects of instruction and no instruction on vertical ground-reaction force reduction in children during a jump-landing task. 27 The findings of Prapavessis et al 27 indicate that providing oral instructions to land more softly significantly reduced vertical ground-reaction forces at the end of each of the 3 training sessions performed over a 5- day training period. However, in Prapavessis et al s study, 27 the vertical ground-reaction force values returned to baseline levels following a 3-month detraining period, indicating there was no retention in jump-landing biomechanics after a prolonged detraining period. These findings, combined with ours, suggest that individuals are able to successfully modify their jump-landing biomechanics within a relatively short time period. However, immediate changes in biomechanical and neuromuscular factors following an ACL injury prevention program do not necessarily translate to long-term retention of the acute biomechanical and neuromuscular alterations in movement control. The concept of overlearning may best explain the retention of movement quality changes observed in the extended-duration training group. 6 Overlearning was first described through the pioneering work of Ebbinghaus in 1913 and is considered a fundamental property in the relationship between training duration and retention. 6 Overlearning is simply described as the continued practice/ training of a given set of tasks or exercises after reaching a high level of performance at the task or exercise. 6 Retention is believed to occur by allowing the trainees to repeat their performance of a given task and confirm the correctness of their response. Assuming similar rates of improvement, the extended-duration group performed an additional 6 months of training after having improved their movement quality during the jump-landing task. Thus, this additional training time likely allowed for continued practice beyond the point of initial improvement and provided further feedback on the correctness of the extended-duration group s movement patterns, leading to retention over time. The importance of overlearning in facilitating retention is a well-established principle in motor learning and psychology literature. In a meta-analysis of overlearning studies, Driskell et al reported that retention of new skills is enhanced when learning or practice is continued for a period of time beyond the initial mastery of the new skill. 6 A recent line of research has investigated the effects of overlearning on the retention of motor pattern adaptations related to fall prevention during gait in an elderly population. 1-3,24 These studies agree with our research in that retention of motor adaptations associated with fall prevention was greater when participants underwent a more intensive training period that allowed for overlearning to occur. Bhatt et al also demonstrated that retention of motor adaptations associated with fall prevention was present up to 4 months after the participants completed the training sessions. 3 Based on our findings and those of Bhatt et al, 3 the incorporation of overlearning principles during movement retraining programs may be a critical factor in developing retention of new movement patterns to reduce the risk of future injury. Another important finding was that the amount of change in movement quality was not influenced by the training duration, even though training duration did affect retention. Specifically, the overall improvement in movement quality was not different between the short- and extended-duration training groups immediately after completing the programs. This was evidenced as LESS scores were significantly improved in both groups from pretest to posttest; however, there was no difference between the groups pretest and posttest LESS scores. We believe the amount of improvement following completion of both programs is clinically important given the large effect sizes between pretest and posttest scores for the short-duration (Cohen d = 1.3) and extended-duration (Cohen d = 1.0) groups. It is of interest to note that while the extendedduration training group performed the same exercises 3

6 6 Padua et al The American Journal of Sports Medicine times longer than the short-duration group (9 months vs 3 months), they did not achieve added benefits in their overall level of improvement. We conclude from these findings that the duration of training does not appear to affect the magnitude of improvement in movement quality. Previous research investigating motor control changes during a reaching task also indicates that the duration of training was not associated with the amount of initial improvement in motor control. 11 However, the duration of training was an important factor influencing the successful retention of motor control changes. 11 These findings agree with our results that the short- and extendedduration groups achieved identical improvements in movement quality, yet only the extended-duration group retained these changes after a 3-month detraining period. Joiner and Smith describe retention of motor control adaptations to be largely influenced by a slow learning process that occurs through repeated practice of a given task after achieving initial proficiency. 11 The slow learning process is believed to represent the mechanism by which overlearning occurs. Ultimately, the dynamics of the slow learning process may determine the overall capacity for long-term retention of motor learning. 11 These findings suggest that maximizing the long-term benefit of an injury prevention program may not come from maximizing the amount of overall improvement in movement control but rather by maximizing the amount of learning achieved during the slow learning process so that overlearning may occur. 11 Future research is needed to better understand how to maximize learning during the slow process when performing an ACL injury prevention program. These findings may have important clinical implications regarding the successful implementation of ACL injury prevention programs. Several studies have demonstrated a significant decrease in knee injuries over the course of a season when incorporating an exercise-based injury prevention program. 21,33 However, the benefits of an exercise-based injury prevention program appear to be transient. This was demonstrated by Myklebust and Bahr 18 who reported injury rates to return to pretraining levels within 1 year after teams discontinued their injury prevention programs. This suggests that the benefits of these previous programs may not have been retained over time, similar to what was observed in the short-duration training group. These findings suggest that individuals who have completed an injury prevention program less than 9 months in duration may require supplemental training following periods of not performing the exercises. Thus, a booster type of exercise program may be required in these cases. Future research investigating the effects of a booster type of exercise is needed to determine if such programs can minimize the lack of retention in short-duration training programs. Limitations Several limitations should be considered when interpreting the findings of this study. First, the LESS is a clinical assessment of movement patterns, and we did not measure 3-dimensional lower extremity biomechanics in this study. However, the LESS has been shown to have good concurrent validity, as significant differences in lower extremity biomechanics do exist between individuals who have high (poor movement quality) and low (excellent movement quality) LESS scores. 23 In addition, preseason LESS scores were shown to be higher in those who go on to experience a noncontact ACL injury compared to those who do not sustain injury. 22 Thus, while the LESS is not a direct measurement of lower extremity biomechanics, it appears to be a valid clinical assessment of movement quality. Another potential limitation of this study was the age group of participants we studied (mean, 14 years; range, years). It is not clear if the duration of training will have similar effects in younger or older populations. It should also be noted that the extended-duration training group was followed for 6 months longer than the short-duration training group because of differences in training periods. The additional 6 months may introduce maturation factors that could have influenced these findings; however, we do not believe that 6 months of additional maturation time would have significant effects on these findings. Another limitation is that we only assessed retention at a single time point (3 months posttraining). We do not know if retention of new movement patterns would remain beyond 3 months. Future research should consider investigating different age groups and retention time periods. Finally, we had no control over any training programs that participants participated in outside of the team setting. Anecdotally, however, use of other injury prevention training programs other than at team practices did not occur. CONCLUSION These findings indicate that 3-month and 9-month injury prevention programs facilitate similar improvements in movement technique. However, only the 9-month intervention program demonstrated retention of overall movement technique improvements (total LESS score). This suggests that the training duration may be an important factor to consider in designing and implementing injury prevention programs. It may be necessary to incorporate longer duration programs to ultimately achieve long-term changes in movement control. This may be critical to improving the effectiveness of current injury prevention programs and ensuring that individuals develop permanent changes in movement control. ACKNOWLEDGMENT We acknowledge Micheal A. Clark, DPT, MS, PES, CES, for his contributions toward the exercise program design and development. We also acknowledge Triangle United Soccer Association and Montgomery Soccer Inc for their support and cooperation with this project. REFERENCES 1. Bhatt T, Pai YC. Long-term retention of gait stability improvements. J Neurophysiol. 2005;94(3):

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