Literature Review of Prevention Programmes and their Effects on Female Soccer
Anterior cruciate ligament (ACL) injuries are up to six times more likely to occur in female soccer players compared to their male counterparts. ACL injuries can cause long-term physical, personal and professional impairments for athletes, as well as a high economic cost. The purpose of this literature review is to analyze the effects of ACL injury prevention programs (IPP) on the reduction of ACL injury rates and on modifying biomechanical risk factors associated with an ACL injury.
A search was conducted using Pubmed and Medline. All articles including “ACL prevention” and “female soccer players” were considered. Only articles that included an intervention and female soccer players were included. This review focuses on ten studies that implement an IPP and examine their effects on ACL injury rates, biomechanical changes, and player performance in female soccer players.
Of the studies included, only two used ACL injury rates as their main outcome measure, Gilchrist et al. and Walden et al. Gilchrist et al. implemented the Santa Monica Prevent Injury Enhance Performance(PEP) Program. This 15-minute warm-up is designed specifically to prevent non-contact ACL injuries and includes five components: a warm-up, stretching, strengthening, plyometrics, and sports-specific agility. Participants included 1435 female soccer players with an average age of 19.88 years, randomized into intervention or control groups. The IPP program was implemented three times weekly for 12 weeks. A total of seven ACL injuries were reported in the intervention groups, two of which were non-contact. In the control group, 18 ACL injuries were reported, ten of which were non-contact. Despite a 41% reduction in ACL injuries in intervention teams, the results did not reach statistical significance until the final six weeks, as no ACL injuries were reported by intervention teams. Although the PEP program aims to reduce non-contact ACL injuries, this study shows it may also have a protective effect against contact ACL injuries. It also indicates the need for ongoing participation over a number of weeks to begin seeing the protective effect.
A similar study by Walden et al. implemented a 15-minute neuromuscular warm-up program twice weekly for seven months(one season). Exercises included one-legged squats, pelvic lifts, two-legged squats, the bench, the lunge, and a jump/landing exercise. The program focused on strengthening leg and core muscles, balance training, landing technique, and proper knee alignment. Subjects included 4564 female soccer players aged between 12 and 17, randomized into intervention or control groups. Overall, seven ACL injuries were reported in the intervention group and 14 in the control group. The findings showed a reduction of 64% in the rates of ACL injuries. However, the absolute rate difference did not reach statistical significance. This may have been due to the small number of ACL injuries overall.
A sub-study was carried out by Hägglund et al. to examine the effect of compliance levels on results found by Walden et al. Participants were stratified into tertiles of compliance(low, intermediate, and high) based on their mean number of weekly training sessions. At the team compliance level, no significant differences in injury rates were seen between the three tertiles or the control groups. At an individual level, the players in the high-compliance tertile had an 88% reduction in ACL injury rate compared with the low-compliance tertile. Players in the low compliance tertile appeared to have had no protective effect at all, showing similar injury as the control.
Four studies in this review focused on methods of changing biomechanical risk factors associated with ACL injuries. Ford et al. compared kinetic versus kinematic-based real-time biofeedback as methods of correcting knee abduction posture and load. This was a randomized crossover study completed by four female soccer players aged 14 to 16. The main outcome measure used was knee abduction moment and angle during a drop vertical jump (DVJ) pre and post-feedback. During kinematic biofeedback sessions, knee abduction/adduction angle was displayed on a screen as participants performed repetitive double-legged squats. Knee abduction/adduction moment was displayed during kinetic biofeedback sessions. Results showed a significant reduction in maximum knee abduction angle after kinetic biofeedback but only showed a trend toward reduction after kinematic biofeedback. There was no long-term follow-up to see if participating in biofeedback sessions lead to maintained improvements. As well as this, with such a small sample size, we cannot estimate the effects that may be seen with a larger population.
Zebis et at. identified reduced electromyography(EMG) preactivity in the semitendinous(ST) muscle along with increased EMG preactivity in the vastus lateralis(VL) muscle during side cutting as a risk factor for ACL damage. In this study, they explored the effects of an IPP that included wobble board and ball exercises, on the reduction of these risk factors. Participants included 18 soccer players and 22 handball players aged 15 to 16 who were split into control or intervention groups. The IPP consisted of three weekly 15-minute sessions of progressively increasing difficulty, for 12 weeks. Participants were tested during a standardized side-cutting maneuver in a 3D biomechanics laboratory. A statistically significant difference was seen in VL-ST EMG preactivity between groups at follow-up. Overall, this study showed an altered pattern of agonist-antagonist muscle preactivity during standing, side cutting, and landing after 12 weeks of participation. Only one knee injury was registered in the intervention group, during the study compared to three knee injuries in the control group.
Liebert established and implemented an IPP to improve core strength and proprioception. The IPP consisted of a 15-minute warm-up including 5 exercises (Common lung matrix, single-leg hops, single-leg deadlifts, soccer ball toss, and shuttle runs). This study included 31 high school female soccer players. Participants were tested pre and post-season using the “Y balance” test, single-leg hop and hold, triple crossover hop, superman, and shuttle run test. Results showed an overall improvement in core strength and proprioception, as well as hip internal rotation. Coaches also indicated a reduction in knee injuries during the implementation of the IPP but statistics of injuries for previous seasons had not been collected. The age of the players and duration of the program was not included by the author in this study. Therefore we cannot conclude the timeframe in which this program must be implemented to see effects. Also, we cannot examine how these results may have been influenced by age as ACL injury risk in females can differ between age groups. Finally, there was no control used in this study. Therefore we cannot conclude if these improvements are solely due to the implementation of this program or were influenced by other factors such as level of training and gameplay during the season.
The final study that examined altering biomechanical risk factors was by Pollard, Sigward, and Powers. They implemented the same IPP as Gilchrist et al., the PEP program, and examined its effects on modifications of hip and knee mechanics during a drop-Landing task. Regular feedback about lower limb movement patterns was given throughout with an emphasis on reducing knee valgus. Participants included 30 female soccer players aged 11 to 17, who completed the program twice weekly for 12 weeks. A biomechanical testing session was carried out on each subject before and after participation. Kinematics and ground reaction forces were collected during a drop-landing task using reflective markers and appropriate equipment. The authors concluded that participation in the PEP program decreased reliance on knee extensor muscles and improved the use of hip extensor muscles during a drop landing task. Results showed decreased knee extensor moments and increased energy absorption at the hip after 12 weeks.
Two studies that appeared in the initial search examined how the level of these biomechanical changes can be influenced by factors such as age, gender, and movement patterns. Distefano et al. compared 173 male and female soccer players aged 11 to 16, who were randomized into generalized or stratified groups. Stratified players were further divided into three groups(medial knee displacement, toe-out or normal) based on their movement patterns during a double-legged squat. Specific exercises were given to correct these movements. Generalized players were given stretching and strengthening exercises for lower limb muscle groups to be completed three to four times weekly. Participants aged 10 to 13 years participated for nine months. Their older counterparts only participated for four months due to conflicts with their seasons. Performance was measured during a drop-landing task using the Landing Error Scoring Scale(LESS). Older participants showed better improvements in fewer scores than younger participants, despite participating for a shorter duration. Significant increases were seen in participants with a “poorer” landing technique at baseline. No significant differences were seen between the sexes or intervention groups. This study indicates that individualized IPPs are no more successful in altering movement patterns than generalized programs. The main findings indicate that athletes with “poorer” movement patterns at baseline have more potential to benefit from IPPs than those who do not.
A more recent study by Thompson-Kolesar et al. looked specifically at investigating if the FIFA Medical and research center 11+ IPP is more effective in younger versus older athletes. The F-MARC11+ is a 25-minute IPP that focuses on core stabilization, eccentric training of thigh muscles, proprioceptive training, dynamic stabilization, and plyometrics with straight leg alignment. The study recruited 51 adolescent athletes (aged 10-12) and 43 adolescent athletes (aged 14-18) who were divided into intervention and control groups. The program was implemented twice weekly for seven to eight weeks. Athletes were tested during cutting and jump/landing tasks before and after participation. Preadolescent intervention athletes showed more significant improvements in initial contact knee angle and peak knee valgus moment versus adolescent intervention athletes and preadolescent controls. For all activities in both age groups, changes in the initial contact and peak knee valgus angles were not significantly different between the intervention athletes and the control athletes. Results showed that preadolescent female football players displayed riskier movement patterns at baseline. Contrary to the results found by Distefano et al. (2009), younger athletes had greater improvements following participation in an IPP.
A study by Paudua et al. looked at the same stratified and generalized IPPs as Distefano et al. but examined how the duration of the program influenced the retention of movement pattern changes. Participants included 140 male and female soccer players aged 12 to 16. Teams were divided into stratified and generalized interventions. They were further divided into short-duration and extended-duration groups that participated in the interventions for three and nine months respectively. Subjects were tested before, immediately after, and three months after completion of the program using the LESS. Both groups improved fewer scores from pre to post-participation testing sessions. The extended-duration group maintained improvements three months post completion, whereas those who only participated for three months had reverted to pre-participation results.
The final study in this review was carried out by Vescovi and VanHeest and focused on the effects of the PEP program on player performance rather than injury prevention. The PEP program was completed three times weekly for 12 weeks by female soccer players aged 13 to 18 years. Players were assessed at baseline, six and 12 weeks using a linear sprint, countermovement jump (CMJ), and two agility tests as outcome measures. Included in the results were 15 intervention athletes and 16 control athletes that completed all three testing sessions. The main findings showed small improvements in linear sprint times that were limited to the first 6 weeks, no change in CMJ height after 6 or 12 weeks, and a 2–4% decline in agility performance in both groups. Overall this study indicates that the PEP program failed to improve performance in any of the outcome measures used.
Female soccer players exhibit a higher number of risk factors than their male counterparts. Hence, single-component IPPs may not provide the ideal therapeutic dosage to significantly impact ACL injury rates in female soccer players. Distefano et al. highlighted that single component training programs were no more effective in reducing ACL injury risk factors than generalized programs. The present literature review indicates that multi-component interventions including plyometrics, strengthening, stretching, proprioception, and core strengthening appear to be successful in modifying risk factors associated with ACL injury (decreasing landing forces, correcting knee valgus/varus moments, and increasing effective muscle activation). Both Gilchrist et al. and Walden et al. showed direct reductions in ACL injuries in female athletes that completed a multi-component IPP, versus those who did not.
There is a lack of research into the ideal duration of these programs. The majority of studies included in this review implemented a 12-week program, two or three times weekly. Gilchrist et al. found that the PEP program was more successful at reducing injury rates in the last six weeks of the 12-week program. This may be due to the increased athlete exposure to the program and a greater opportunity for neuromuscular re-education to occur. Padua et al. indicated that improvement in biomechanics after participation in short-term IPPs may be transient. This suggests that training duration may be an important factor to consider in an IPP. Incorporating longer duration programs may be necessary to achieve long-term changes in movement patterns.
Numerous studies have indicated the importance of feedback to minimize more risk-related movement patterns in athletes. The only study in this review that explored this was Ford et al., which compared kinetic and kinematic biofeedback. Results showed higher reductions in maximum knee abduction angle after kinetic versus kinematic biofeedback. However, the methods used in this study may not be widely available to soccer teams. Therefore, both kinetic and kinematic feedback may not be a realistic way to correct movement patterns associated with an ACL injury. More accessible forms for feedback, such as verbal feedback from a coach, may be a helpful adjunct to a multi-component IPP, ineffectively altering biomechanical movement patterns.
From the two studies that investigated the influence of age on the success of IPPs, we are left with conflicting results. Preadolescent subjects responded less favorably to the IPP implemented by Distefano et al. when compared to their adolescent counterparts. Younger participants in this study performed the intervention for five months longer but were not assessed after four months of participation, as were the older athletes. Assessing the younger athletes of this study at the same time as the older subjects would have provided more insight into this finding and should be a consideration for future studies. Preadolescent athletes showed greater improvements from baseline in the study by Thompson-Kolesar et al. This is most likely due to poorer baseline movements often seen in younger athletes.
Overall it is clear that a multi-component IPP is effective in reducing biomechanical risk factors associated with an ACL injury. Exact timeframes for implementation have not been established, however, long-term programs appear to lead to maintained biomechanical changes. The studies in this literature review do not provide enough evidence to indicate the ideal age to begin implementation, but there are no apparent contraindications to taking part in an IPP in preadolescent years. Finally, as indicated by Hägglund et al., an individual’s compliance to an IPP is crucial to see desired protective effects.
