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1 Introduction

Since the first publications of knee ligament injury prevention, training programs appeared in the sports medicine literature for skiing in 1995 [1] and female high school athletes in 1996 [2]; at least 30 intervention programs have been published that focused on female athletes (Table 21.1). Multiple investigations have been conducted to determine the effectiveness of these programs in reducing anterior cruciate ligament (ACL) injury rates [7, 18, 20, 25, 26, 31, 32, 34, 35, 39], improving knee kinematic and kinetic factors [2, 4, 5, 8,9,10,11,12, 15, 17, 19, 21, 22, 24, 29, 42,43,44,45, 48,49,50,51,52,53,54,55,56,57, 59, 61,62,63, 65, 67,68,69,70,71,72,73,74,75], enhancing strength or other athletic performance indicators [3, 5, 8, 10,11,12,13,14,15, 17, 19, 22,23,24, 29, 38, 42, 45, 48,49,50,51, 53, 54, 56,57,58, 61,62,63, 66, 67, 76, 77], and improving static and dynamic balance [13, 16, 29, 40, 47, 60, 64, 66].

Table 21.1 Summary of published ACL injury prevention programs for female athletes
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There exist differences in opinion regarding the frequency, intensity, duration, and components that should comprise an ACL intervention training program. One issue is if a significant reduction in the injury rate can be accomplished with “warm-up programs” that are relatively short in session duration (10–20 min), but long in total training duration (for instance, one season). This is in contrast to preseason programs that last 6–8 weeks but require 60–90 min of training per session. A second issue is whether ACL intervention training should be modified according to the athlete’s age and sport. Can a program that is successful in adolescent soccer players has similar outcomes in adult handball players? A third issue is whether athletes identified as having a high risk of sustaining a noncontact ACL injury should undergo a different training program than those who are believed to be at a lower risk for this injury. The difficulty with this issue is that there does not exist to date a comprehensive model that predicts ACL injury risk according to all of the potential risk factors: anatomical, environmental, hormonal, neuromuscular, familial, playing surface, equipment, cardiovascular conditioning, and nutrition.

Multiple meta-analyses [78,79,80,81,82,83,84] that assessed the ability of neuromuscular training programs to reduce ACL injury rates concluded that these intervention programs were indeed effective. However, these studies combined data from very different types of training programs and did not answer the major issues discussed above. Systematic reviews (that did not combine data of programs) have reported that, while some programs are effective, others do not significantly reduce the risk of ACL injury and stress the importance of understanding the variation in training protocols and study design in ascertaining the differences in outcomes [7, 85,86,87].

There are important qualifiers in the studies included in this chapter. First, nearly all of the investigations analyzed preplanned tasks in a controlled laboratory setting. The effectiveness of these types of programs in improving potentially deleterious neuromuscular indices under reactive, unplanned athletic conditions is unknown. Secondly, few studies provided effect sizes (ES) in addition to P values when reporting effects of training [10,11,12, 17, 61, 69]. The ES measures the magnitude of the effects of treatment and is especially relevant in studies with small sample sizes [88]. It is probable that some statistically significant findings (P < 0.05) reported in the studies in this chapter may have limited clinical relevance. A third qualifier is that few studies conducted a prospective power calculation of the sample size required to discern a detectable difference (95% CI) in knee and hip kinetic and kinematic factors resulting from the training program [4, 9, 11, 15, 19, 42, 44, 52, 68,69,70]. Finally, the determination of the magnitude of change required to actually reduce the risk of an ACL injury in knee and hip kinetic and kinematic factors remains unknown and is speculative at best.

The goals of this chapter are to review the current available data regarding the outcome of ACL intervention programs on the reduction of noncontact ACL injuries, improvement of neuromuscular deficiencies or at-risk body positions and movements, and enhancement of athletic performance indices in female athletes. This chapter serves to summarize the data, and the reader is referred to other chapters for further detail regarding these outcomes. Only programs that focused on adolescent or adult female athletes are included.

Critical Points

  • More than 30 ACL intervention programs are published.

  • Differences exist regarding frequency, intensity, duration, and components.

    • Effectiveness of warm-up programs

    • Modified according to age

    • Identification of at-risk athletes

  • Chapter summarizes data on ability of programs to:

    • Reduce ACL noncontact injury rate

    • Improve neuromuscular deficiencies

    • Improve athletic performance indicators

2 Reducing the Incidence of Noncontact ACL Injuries

Ten intervention studies to date have reported noncontact ACL injury rates in female athletes according to athlete exposures (AE; Table 21.2) [7, 18, 20, 25, 26, 31, 32, 34, 35, 39]. Several other studies that reported data on intervention programs either did not provide ACL injury rates according to AE or did not indicate if the ACL injuries were noncontact in nature and are not included [1, 27, 30, 37, 89,90,91,92]. In addition, injury intervention studies that focused only on male athletes are not included in this review [88, 93, 94, 101].

Table 21.2 Effect of intervention programs on ACL noncontact injury rates in female athletes according to athlete exposures
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Three programs—Sportsmetrics, Prevent Injury and Enhance Performance (PEP) program, and Knee Injury Prevention Program (KIPP)—statistically reduced the noncontact ACL injury rate [7, 20, 34]. Sportsmetrics was conducted in 700 high school female athletes before the start of the athletic season (see Chap. 17). The results from the trained athletes and 1120 control athletes demonstrated ACL injury rates of 0.03 and 0.21 per 1000 AE, respectively (P = 0.03). The PEP program was conducted in 1885 high school female soccer players over the course of one season (see also Chap. 20, Table 20.3) [20]. A significant reduction was reported in the noncontact ACL injury incident rate between the trained and 3818 control players (0.09 and 0.49 per 1000 AE, respectively, P < 0.0001). KIPP training was conducted in 485 high school female basketball and soccer players before practices over the course of one season (see also Chap. 20, Table 20.4) [34]. A significant decrease was found in the noncontact injury incident rate between the trained and 370 control players (0.10 and 0.48 per 1000 AE, respectively, P = 0.04).

Table 21.3 Effect of ACL intervention training on landing forces
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Table 21.4 Effect of ACL intervention training on knee and hip moments
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Several studies [18, 25, 26, 30,31,32, 35, 39] reported that other intervention programs failed to have an effect on reducing ACL injury rates. Issues pertaining to poor compliance with training and a small number of noncontact ACL injuries were commonly cited as the reasons for the outcomes of these investigations. In general, ACL intervention programs published to date have had multiple methodological problems which preclude definitive answers regarding which programs are effective and which are ineffective. The lack of randomization and control, limited statistical power due to small number of exposures and ACL injuries, failure to determine ACL injury incidence according to AE, poor compliance with training, poor documentation of contact versus noncontact ACL injuries, and changes in study protocols over the course of investigations were found. Even with these acknowledged problems, recent conference and committee statements [95,96,97] have concluded that neuromuscular retraining can reduce the incidence of noncontact ACL injuries in female athletes. The International Olympic Committee current concepts statement [98] is shown below:

  1. 1.

    The program should include strength and power exercises, neuromuscular training, plyometrics and agility exercises.

  2. 2.

    Design as a regular warm-up program to increase adherence.

  3. 3.

    Focus should be on performance of the hip-knee-foot line and “kissing knees” should be avoided (excessive valgus strain).

  4. 4.

    Maintenance and compliance of prevention program before, during and after the sports participation season are essential to minimize injuries.

  5. 5.

    The drop vertical jump test should be used to identify players at risk.

  6. 6.

    The program must be well received by coaches and players to be successful.

  7. 7.

    Evaluation of success or failure of a prevention program requires large numbers of athletes and injuries.

The American Academy of Pediatrics issued the following three-part policy statement in 2014 regarding ACL injury prevention training [96]:

  • Neuromuscular training appears to reduce the risk of injury in adolescent female athletes by 72%. Prevention training that incorporates plyometric and strengthening exercises, combined with feedback to athletes on proper technique, appears to be most effective.

  • Pediatricians and orthopedic surgeons should direct patients at highest risk of ACL injuries (e.g., adolescent female athletes, patients with previous ACL injury, generalized ligamentous laxity, or family history of ACL injury) to appropriate resources to reduce their injury risk (http://www.aap.org/cosmf). Such discussions also should be appropriately documented in the patient’s medical record.

  • Pediatricians and orthopedic surgeons who work with schools and sports organizations are encouraged to educate athletes, parents, coaches, and sports administrators about the benefits of neuromuscular training in reducing ACL injuries and direct them to appropriate resources (http://www.aap.org/cosmf).

The ability of neuromuscular retraining programs to reduce the incidence of noncontact ACL injuries in female athletes is most likely due to the increased awareness of injury situations and changes in neuromuscular indices that improve balance, strength, and coordination; provide for safer landing, pivoting, and cutting techniques; increase joint stabilization; and enhance muscular preactivation and reactive patterns to be discussed next.

Critical Points

  • Ten studies reported ACL injury rates according to athlete exposures in females:

    • Three significantly reduced ACL noncontact injury rates: Sportsmetrics, PEP, and KIPP.

    • Others failed to reduce ACL noncontact injury rates:

      • Poor compliance with training

      • Too few ACL injuries, limited statistical power

      • Lack of randomization

      • Changes intervention protocols over time

3 Changes in Knee and Hip Kinetics and Kinematics

A wide variety of studies have been published to date which analyzed the effectiveness of knee injury prevention programs in changing kinematic or kinetic factors in female athletes [2, 4, 9,10,11,12, 15, 17, 19, 21, 24, 42,43,44,45, 48,49,50,51,52,53,54,55, 59, 61, 62, 68,69,70].

3.1 Landing Forces

Statistically significant decreases in landing forces from a vertical jump [2], step-land [24], single-leg hop [42], rebound jump-land [68], and stop-jump [50] have been reported following neuromuscular training (Table 21.3). A mean reduction of 456 N (103 pounds, 46.72 kg) during a vertical jump was reported after Sportsmetrics training [2] in female high school volleyball players (Fig. 21.1). Another study [50] reported a mean reduction of 22% during a stop-jump task after training in recreational female athletes 18–30 years of age. One study reported decreases in impact forces on a single-leg hop in a group of patients who completed a balance training program; however, a group that completed a plyometrics training program demonstrated increases in impact forces [42]. None of these studies reported ES.

Fig. 21.1
figure 1

Vertical jump test on force plate

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In contrast, no reduction in landing forces were reported in several other investigations. These included tests involving a unilateral step-down or forward lunge [17], a vertical jump [15, 48], a drop-jump and vertical stop-jump [49], three stop-jump tasks [51], and a side-step pivot [61]. In five of these six studies, the populations under investigation were collegiate or recreational athletes ≥18 years old. Factors believed to affect the ability of ACL intervention programs to alter landing forces include age (young versus adult), athletic experience (competitive versus recreational), type of instruction, and exercise protocol [15].

3.2 Knee and Hip Moments

Statistically significant decreases in potentially deleterious moments have been noted during planned tasks such as a vertical jump or drop-jump by several investigations after ACL intervention training (Table 21.4) [2, 43, 45, 48,49,50, 63, 65, 70]. The Sportsmetrics training program produced significant decreases in knee abduction and adduction moments on a vertical jump [2]. A similar training program resulted in significant decreases in knee internal valgus (abduction) moments of 28% and internal varus (adduction) moments of 38% on a drop-jump test [43]. A program performed in collegiate soccer and basketball players produced mixed results in terms of reduction of potentially harmful moments [49]. Statistically significant decreases in knee external rotation moments and knee flexion moments were found on a drop-jump test. However, there were no effects on hip external rotation, abduction, or flexion moments or knee valgus moments on this test. ES were not reported in any of these studies.

One study compared the effects of a 4-week core stability program with a plyometric program in knee and hip moments on a drop-jump in a small group of high school athletes [63]. In the plyometric group, significant decreases and moderate ES were noted in knee flexion and knee valgus moments. There were no changes in hip flexion or internal rotation moments. In the core stability group, significant decreases and large ES were noted in hip flexion and hip internal rotation moments; however, no changes were reported in knee moments. Another study found that either plyometric training or plyometric training combined with perturbation techniques significantly decreased knee flexion and internal rotation moments on a reactive lateral jump, with large ES noted [65]. There were no effects of either training protocol in reducing knee moments on a reactive cutting task.

3.3 Knee and Hip Flexion Angles

Increases in knee flexion angles on landing from various tasks following ACL intervention training have been demonstrated in several studies, although the data vary in regard to the magnitude of change and whether the improvements occurred at foot strike or during the deepest point of the land, indicated as maximum or peak knee flexion (Table 21.5) [19, 42, 44, 45, 48,49,50, 55, 62, 67,68,69,70]. Slight average increases in knee flexion at foot strike of 5.2° [49] and 5.8° [44] during a drop-jump test were reported in two studies, and a mean increase of 4.9° on a single-leg drop landing was found in another study [62]. One study reported a mean increase of 6.2° in knee flexion (ES 0.83) on a two-legged landing after completion of a standard neuromuscular training program [69]. However, there was no improvement in a single-leg landing task. Several other studies failed to observe an improvement in knee flexion at either foot strike or the maximum point of the landing during a variety of tasks [2, 21, 51, 53, 56, 57, 59, 61, 63]. One study found a concerning significant decrease in knee flexion after completion of either a plyometric or a core stability program [63]. The plyometric group had a mean decrease of 18.5 ± 3.6° (ES 1.79), and the core group had a mean decrease of 16.3 ± 3.4° (ES 1.88).

Table 21.5 Effect of ACL intervention training on knee and hip flexion angles
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Several studies [21, 44, 48,49,50, 56, 57, 68,69,70] have reported statistically significant improvements in either hip flexion, abduction, adduction, external rotation, or internal rotation (Table 21.5). However, several others [2, 51, 53, 63] failed to find improvements in hip angles after intervention training.

One study of 30 athletes reported a mean increase of 8.2° in hip flexion (ES 0.52) on a two-legged landing after completion of a standard neuromuscular training program [69]. However, there was no improvement in a single-leg landing task. This was the only study located that included ES in the analysis of hip flexion angles. Increases in initial and peak hip flexion during a vertical jump were described in one study following plyometric training [48]. However, there were no improvements for initial or peak hip abduction or adduction angles. Another investigation reported significant increases in maximum hip flexion and abduction angles on a stop-jump task [50]. Significant decreases in mean hip internal rotation and increases in mean hip abduction were noted during a drop-jump test following the PEP training program [21]. One study described significant decreases in hip flexion at foot strike and in maximal hip external rotation on a stop-jump task [49]. However, no significant differences were observed in hip kinematics on a drop-jump test.

3.4 Lower Limb Alignment

Multiple investigations have determined the effects of ACL intervention training on lower limb alignment during various jumping tasks (Table 21.6). The assessment involved either measuring the distance between the hips, knees, and ankles from a single-plane video analysis which provides a general indicator of overall lower limb alignment (absolute knee separation distance and normalized knee separation distance values, Fig. 21.2) or measuring varus-valgus angles in multiple planes.

Table 21.6 Effect of ACL intervention training on lower limb alignment
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Fig. 21.2
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Drop-jump video test. Three photographs are produced from the prelanding, landing, and take-off phases. The centimeters of distance between the hips, knees, and ankles are calculated along with normalized knee and ankle separation distances (according to the hip separation distance) using commercially available software (Cincinnati SportsMedicine Research and Education Foundation, Cincinnati, OH). Shown is the test result of a 16-year-old female subject before beginning the Sportsmetrics neuromuscular training program depicting poor knee separation distance and an obvious overall lower limb valgus alignment

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Statistically significant improvements in knee separation distance following Sportsmetrics training have been noted by multiple studies [4, 8, 10,11,12]. The largest group followed (912 trained high school athletes) improved the absolute knee separation distance from 20 ± 8 cm to 27 ± 8 cm (P < 0.0001, ES 0.87) and the normalized knee separation distance from 47 ± 19% to 65 ± 18% (P < 0.0001, ES 0.97) [8]. Another group of high school female athletes improved knee separation distance a mean of 3.25 cm after completing the PEP program [19]. However, two other investigations failed to find significant improvements in lower limb alignment after participating in the PEP program [21, 22].

Only a few investigations reported improvements in knee valgus angles following training [54, 59], while several studies [21, 48,49,50,51,52, 55, 62, 68,69,70] found no training effects.

Critical Points

  • Multiple studies have assessed changes in kinematic or kinetic factors in female athletes after training:

    • Landing forces (mixed results):

      • May be affected by age, athletic experience, type of instruction, and exercise protocol

    • Moments:

      • Majority studies decreased knee and hip moments

    • Knee flexion angles:

      • Increased in several studies but varied in amount of change and when improvements occurred during tasks

    • Hip flexion angles:

      • Mixed results

    • Lower limb alignment:

      • Improved overall lower limb alignment in coronal plane in several studies on drop-jump test after Sportsmetrics training, but no change in knee valgus angles in multiple studies upon completion of other programs

    • Several methodological problems found:

      • Nearly all investigations analyzed preplanned tasks in a controlled laboratory setting.

      • Few provided effect sizes or conducted prospective power analyses to determine adequate sample size.

    • The determination of the magnitude of change required to actually reduce the risk of an ACL injury in knee and hip kinetic and kinematic factors remains unknown and is speculative at best.

4 Alterations in Lower Extremity Strength and Muscle Activation Patterns

A frequent finding among the studies included in this chapter was a statistically significant increase in lower extremity isokinetic (Fig. 21.3) or isometric strength (Table 21.7). Improvements in the strength of the hamstrings [2, 8, 9, 14, 17, 19, 22, 42, 50, 51, 57, 61, 62], quadriceps [8, 22, 48, 50, 51, 57, 99], gluteus maximum and medius [50, 51], hip abductors [19, 56, 70], and hip extensors [70] and in the hamstrings to quadriceps ratio [2, 8, 9, 14, 17, 19, 42] have been reported after 6–9 weeks of training. Only a few studies reported no improvements in muscle strength after training [13, 29, 77].

Fig. 21.3
figure 3

Isokinetic knee flexion-extension strength test on Biodex isokinetic dynamometer (Biodex Corporation, Shirley, NY)

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Table 21.7 Effect of ACL intervention training on lower extremity strength, muscle activation patterns
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In addition, several studies [5, 16, 48, 52, 53, 58, 61, 62, 67] reported changes in electromyographic muscle activation patterns after ACL intervention training that appear to demonstrate an earlier onset of hamstrings activity, along with a reduction in quadriceps activity during drop-jump, vertical jump, and side-cut activities. These alterations in muscle activation patterns are believed to be important in the prevention of ACL ruptures.

Critical Points

  • Improved lower extremity muscle strength:

    • Hamstrings: ten studies

    • Quadriceps: five studies

    • Hamstrings to quadriceps ratio: six studies

  • Improved hip muscle strength: two studies

  • Muscle activation patterns:

    • Alterations in eight studies: earlier onset hamstrings activity, reduced quadriceps activity

5 Effect on Balance

Several studies have demonstrated improved balance following ACL intervention training programs (Table 21.8). The Star Excursion Balance Test has been used the most frequently to determine dynamic balance, with improvements found in reach distances in several studies [13, 40, 60, 64, 66].

Table 21.8 Effect of ACL intervention training on balance
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One study [47] involving high school athletes reported improvements in single-leg total stability and anteroposterior stability on the Biodex Stability System (Biodex Corporation, Shirley, NY, Fig. 21.4) after training. There was no improvement in medial-lateral stability in these subjects. Another investigation [60] found significant improvements in the Balance Error Scoring System, which is comprised of six different 20-s balance tests in different stances and on different surfaces. A group of 27 athletes who completed the training program had fewer errors than that recorded before training and also compared with a control group. These subjects also improved scores on the Star Excursion Balance Test in distances successfully reached with a single leg in anteromedial, medial, posterior, and lateral directions.

Fig. 21.4
figure 4

Single-leg balance test on Biodex Stability System (Biodex Corporation, Shirley, NY)

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Critical Point

  • Improvements in Star Excursion Balance test reach distances found in several studies.

6 Enhancing Athletic Performance

There have been multiple studies which documented changes in athletic performance indicators following ACL injury prevention training in female athletes (Table 21.9) [5, 8, 10,11,12, 45, 49, 53, 54, 56, 57, 66]. Vertical jump height is one of the most common indices tested, with mixed results reported. Improvements have been noted in several studies, with mean published post-train increases ranging from 1.2 to 4 cm; however, most of the studies reported small ES [8, 10, 11, 45, 49, 53, 66]. Several other studies [12, 15, 19, 22,23,24, 32, 38, 77] found no significant increases in jump height after training.

Table 21.9 Effect of ACL intervention training on athletic performance
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Statistically significant increases in the distance hopped during various single-leg hop tests have been reported after training [8, 45, 49, 54, 56, 57]. In a study of 280 high school athletes, a mean increase in the triple crossover hop test of 33 ± 54 cm (P < 0.0001, ES 0.47) was found following Sportsmetrics training [8]. In a group of 18 recreational adult athletes, improvements in the triple hop test (mean, 43 cm, P < 0.001, ES not provided) were reported after 8 weeks of plyometric training [56]. Elite adult basketball players improved the distance on the triple hop test by a mean of 110–111 cm (P = 0.001, ES not provided) after 4 weeks of plyometric training [54].

Sprint times have been assessed in several investigations before and after training, with conflicting results reported. In a group of 221 high school athletes, the agility T-test time improved from 12.10 ± 1.01 s to 11.51 ± 0.83 s (P < 0.0001, ES 0.64) after Sportsmetrics training [8]. Similar findings were reported in 62 high school soccer players [12]. One study reported improvements in 10-m and 20-m sprints (P < 0.05, ES 1.2). However, several studies reported no improvements in sprint speed after training [5, 11, 23, 32, 38, 77].

Estimated VO2max has been measured following Sportsmetrics training using the multistage fitness test [100]. One study [10] involving 34 female high school volleyball players reported a mean improvement following training from 39.4 ± 4.8 to 41.4 ± 4.0 mL/kg/min (P < 0.001, ES 0.45). A second study [11] of 57 female high school basketball players reported a mean improvement from 34.6 ± 4.5 to 39.5 ± 5.7 mL/kg/min (P < 0.0001, ES 0.43). A third investigation [12] of female high school soccer players found a mean improvement from 37.9 ± 4.5 to 410.1 ± 4.7 mL/kg/min (P < 0.0001, ES 0.23).

Critical Points

  • Vertical jump height: mixed results

  • Single-leg hop: distance hopped consistently improved

  • Sprint tests: mixed results

  • Agility tests: consistently improved

  • Estimated VO2max: improved after Sportsmetrics training

Conclusions

Few ACL intervention training programs have undergone rigorous investigation regarding their effectiveness in reducing injury rates, improving potentially deleterious lower limb kinematic and kinetic factors, and enhancing athletic performance indicators. Only three programs significantly reduced the incidence of noncontact ACL injuries (Sportsmetrics, PEP, and KIPP). At the time of writing, only one investigation on the effectiveness of KIPP program (on reducing landing impact forces) had been published; no other analyses of this program in terms of kinematic or kinetic factors were available. The PEP program, studied in four investigations [15, 19, 21, 22], showed little effect in improving the knee valgus moment, knee valgus angle, vertical jump height, sprint time, and agility. However, this program did result in increased knee flexion and hip abduction, decreased hip internal rotation, and improvements in the strength of the quadriceps and hamstrings. The Sportsmetrics program has been analyzed in several investigations, both within the authors’ center [2,3,4, 6,7,8,9,10,11,12, 16] and at independent institutions [5, 13,14,15, 17]. The majority of studies have shown improvements in lower limb alignment, hamstrings strength, hamstrings to quadriceps ratio, vertical jump height, single-leg hop test distances, speed, and estimated maximal aerobic capacity. Future investigations should prospectively determine adequate sample size using power analyses, report ES in addition to P values, and analyze unplanned, reactive tests in laboratory studies.