The Knee Joint || Clinical basis: Epidemiology, risk factors, mechanisms of injury, and prevention of ligament injuries of the knee

Chapter 6

T.E. Hewett, B.T. Zazulak, T. Krosshaug, R. Bahr

Clinical basis: epidemiology,

risk factors, mechanisms of injury,

and prevention of ligament injuries

of the knee

romuscular control about the knee takes place withmaturation in female athletes. In sports, the high-est ACL injury incidences occur during participation in basketball, soccer, and team handball, especially among top-level female athletes that compete in these sports (Table 1). This trend is similar for knee Thinjuries, in general, as well (Table 2). Another sport with a high risk of knee injuries (and ACL injuries) is skiing. However, since the incidence is typically

Introduction

K nee ligament injuries can be devastating to an athlete’s career and pose long-lasting deleteri-ous eff ects in the form of knee osteoarthritis ffff

(1). We will begin with a review of the epidemiology of athletic knee ligament injury in general. Further-more, we will review the mechanisms of injury and relative risk factors for these common athletic inju-ries. Th is chapter will also review the epidemiology, Thmechanism, risk factors, and prevention of knee joint injuries in general; however, it will focus onone of the most serious knee injuries experiencedduring participation in sports: disruption of theanterior cruciate ligament (ACL). Th is injury may Thnot be the most common ligament injury experi-enced in sports; however, it is one of the most seri-ous in terms of absence from sport, pain, disability, and increased risk of development of osteoarthritisabout the knee joint (2). We will therefore discussmethods for prevention of knee ligament injuries,which currently is the only 100% efficacious inter-ffivention for long-term health of the knee joint.

Epidemiology of knee ligament injury in sports

Th e objective of this section is to review the published Thliterature regarding knee ligament injuries in sports,addressing the following questions: what is the inci-dence of knee ligament injury in the athletic popula-tion? How does knee ligament injury incidence vary by gender, age, and sport? Of all knee ligaments, the ACL is the ligament that has been most investigated.Th ere is evidence in the literature that ACL injuries Thvary by gender, by age, and by sport. A sex difference ffffin knee injury risk is apparent in high-risk landing and cutting sports, where female athletes suffer ACLffffinjuries more often than their male counterpartstaking part in the same sports at the same level of competition (3,4). Evidence also suggests that, with growth and development, the incidence of knee liga-ment injury increases in females (5). Decrease in neu-

Table 1 – Risk of ACL injury by sport. The numbers reported are average estimates based on published studies.

Sport Competition incidencea

Training incidencea

Comments

Basketball 0.28–0.40 �0.08–0.16b �

b0.14 �b0.04 �

NBA & WNBA

(6–8)

*NCAA data 2006

(6,7)

Soccer 0.33–2.2 �0.12 �

b0.10 �b0.04 �

*NCAA data 2006

(6,9)

Team

handball

1.3–2.8 �0.23 �

0.03�–

Elite level (1)

Volleyball 0.19 � 0.05 � NCAA data 2006

(Agel J, personal

communication)

Alpine

skiing

4.4� c

4.0 � c

–

–

** per 100,000 skier days (10),

� twice of � in

some studies

Field

Hockey

0.15 � 0.05 � NCAA data 2006

(Agel J, personal

communication)

Ice hockey 0.14 �0.21 �

–

0.02 �NCAA data 2006

(Agel J, personal

communication)

Wrestling 0.70 � 0.06 � NCAA data 2006

(Agel J, personal

communication)

aIncidence is reported for adult, competitive athletes as the number of

injuries per 1000 h of training and competition or per 100,000 skiing days

in alpine skiing.bExplanation for asterisk needed.cExplanation for double asterisk needed.

et al., The Knee Joint © Springer-Verlag France, Paris 2012M. Bonnin

54 The Traumatic Knee

A study of Australian football athletes reported that the risk of sustaining non-contact ACL inju-ries increased during high-evaporation and low-rainfall periods (23). Studies of soccer athletes reported that competing on artificial turf does not fiincrease the risk of knee injuries compared with natural grass (24). In contrast, recent studies of professional American football athletes suggest that the incidence of ACL injuries may be greater on the new artifi cial turf designs in comparison to fithe older turf designs (25). In the sport of Euro-pean team handball, it was shown that the risk of ACL injury was 2.4 times greater when competing on artifi cial flfi oors (with an increased coeffifl cientffiof friction) compared with wooden fl oors (26).flEarlier studies of American football athletes have shown that the use of longer cleat lengths and an associated higher torsional resistance at the foot-turf interface places these athletes at increased risk of suff ering knee injuries (24). Thffff ere is little Thdoubt the shoe-playing surface interface is impor-tant to consider when developing intervention strategies to reduce the incidence rate of seriousknee injuries.

Knee bracingTwo investigations have studied the effect of pro-ffffphylactic knee bracing on American football ath-letes. Th e fiTh rst study found signififi cantly fewer medialficollateral ligament (MCL) injuries occurred in ath-letes who used prophylactic bracing in comparison to those who did not use braces (27); however, the eff ect of prophylactic bracing on ACL injuries could ffffnot be determined because of the small sample size. In contrast, the second study did not find statisti-fically significant difffi erences between braced and ffffunbraced athletes in terms of the risk of sustaining MCL injuries (27). Additional research is needed in this area to establish the eff ect that prophylacticffffbraces have on knee injuries.

reported as the number of injuries per 100,000 skierdays in studies on skiing injuries, it is not possible tocompare these fi gures to those from team sports.fi

Key risk factors for knee injuries

A large number of studies have investigated poten-tial risk factors for severe knee injuries, and a major-ity of this work has focused on ACL injuries. Whenconsidering athletes that compete at the elite orcollegiate level, the risk of suff ering an ACL injury ffffranges between 2 and 8 times greater for women compared to men for similar sports at similar levels of competition, and consequently, there appears tobe a consensus across sports that gender is a risk fac-tor for ACL disruption (21). However, there remainsa signifi cant knowledge gap concerning the risk fac-fitors for serious knee injuries such as an ACL disrup-tion. Th is is because only a few well-designed pro-Thspective studies are available, and most studies have assessed only one factor in isolation. Consideringthat the risk factors for ACL injury are multifacto-rial (e.g., multiple risk factors act in combination to increase an athletes risk of suff ering an ACL injury) ffff(22), this approach is not appropriate to assesshow risk factors act in combination to increase an individuals risk of suff ering a severe knee ligament ffffinjury. Further, a large majority of these studieshave identifi ed sex difffi erences in anatomy as wellffffas hormonal and neuromuscular function; however,they fail to relate these differences to the risk of suf-fffffering a severe knee ligament injury.

External risk factors

SurfaceTh e shoe-surface interaction has been studied Thas an ACL injury risk factor in different sports. ffff

Table 2 – Risk of knee injury by sport, age, and sex. The numbers reported are average estimates based on published studies.

Sport Age group Incidence in femalesa Incidence in malesa ReferencesBasketball 14–18 years

Collegiate

Adult

0.71 AH

0.37 AE

4.4 AE

0.31 AH

0.25 AE

2.5 AE

(11)

(12)

(7)

Soccer Collegiate

Collegiate

7–50+

0.40 AE

1.6 AE

0.13 AH

0.33 AE

1.3 AE

0.094 AH

(13)

(3)

(14)

NA (15)Field hockey Collegiate 0.30 AE

Lacrosse Collegiate 0.20 AE 0.20 AE (16,17)

Gymnastics Collegiate 0.53 AE NA (18)

Volleyball Collegiate 0.22 AE NA (19)

Football Collegiate NA 1.58 AE (20)

a Incidence is reported as the number of injuries per 1000 athlete hours (AH) of training and competition or 1000 athlete exposures (AE). NA: data not

available.

Clinical basis: epidemiology, risk factors, mechanisms of injury, and prevention of ligament injuries of the knee 55

studies have not found such a relationship (35,36). Th e data on BMI appear to be inconsistent, making Thit diffi cult to establish reliable conclusions.ffi

GeneticsTwo case-control studies reported that familial tendency is a risk factor for ACL injury (37,38). Athletes who suff er an ACL tear are twice as likely ffffto have a relative with an ACL tear compared with age-, sex-, and sport-matched controls.

RaceIn a recent 4-year cohort study, it was found that white European American athletes were 6.6 times more likely to suffer an ACL tear compared withffffother ethnic groups (39).

Knee alignmentTh e Q angle of the knee has been studied as a pos-Thsible explanation for the gender difference in ACL ffffinjury rates, with the rationale that high Q angles may be associated with excessive valgus loading of the knee. Th ese studies consistently report higher ThQ angles in females (40–42). A case-control study reported that the mean Q angles of athletes sus-taining knee injuries were signifi cantly largerfithan the mean Q angles for athletes who were not injured (14° vs. 10°) (43). In contrast, others have reported that the risk of suff ering a knee injury ffffwas not related to anatomical alignment differ-ffffences such as Q angles (44). One study reported that pelvic width to thigh length ratio, and not Q angle, predicts dynamic valgus angulation aboutthe knee during the single leg squat (45). It has also been shown that a long femur to tibia ratio may be a risk factor for ACL injuries in competitivealpine skiers (46).

Intercondylar notch widthOne of the most studied factors in relation to ACL injury is the femoral intercondylar notch width. Several investigators have hypothesized that a nar-row intercondylar notch or notch width index (e.g., the ratio of the width of the femoral notch to the width of femoral condyles when observed via x-ray in a coronal plane view) may predispose athletes to an increased risk of ACL injury (47–50). One cause could be that a narrower femoral notch is associ-ated with a smaller, weaker ACL. Another possi-bility is that impingement of the ACL against the femoral intercondylar notch may be more predom-inant when the notch is narrow. Th is may induceThmicro tears of the ligament during participation in athletics that subsequently progress to macro tears that weaken the ligament and predispose it to an increased risk of a complete tear. Research is

Ski bindingsAlthough alpine ski bindings have been devel-oped to eff ectively protect skiers from tibia and ffffankle fractures, the present-day alpine ski binding designs are inadequate for preventing ACL disrup-tions, even when the bindings function as designedand are properly adjusted (28). In a large prospec-tive study of ACL-deficient professional alpine ski-fiers, the risk of sustaining a subsequent knee injury (e.g., injury to other structures about the kneesuch as articular cartilage and the menisci) was 6.4times greater for non-braced skiers in comparisonto braced skiers (29).

Competition vs. trainingParticipating in a game appears to be a strong risk factor for knee injury. Studies of European team handball athletes have shown that the relativerisk of sustaining an ACL injury is approximately 30 times greater during competition than duringtraining (30). As can be seen from Table 1, the dif-ference in injury risk between training and compe-tition is a consistent fi nding across sports. At this fipoint in time, the explanation for this large differ-ffffence has not been determined, but the most likely explanation is that the intensity of play is much higher during competition and that during train-ing much time is spent on basic training activitieswith a low risk of injury.

Internal risk factors

Previous injuryRecent studies have suggested that having a pre-vious injury may be a risk factor for subsequentinjury – either a rupture of the ACL graft, an ACL rupture to the contralateral knee, or another type of acute or overuse knee injury (31–33).

AgeAlthough there appears to be a consensus that the risk of suff ering an ACL injury increases for fffffemale athletes during their growth spurt, there are no investigations that have included age as a potential risk factor in the analysis of serious kneeinjuries in skeletally mature athletes. Similarly, no investigations of the eff ect of age on the likelihood ffffof suff ering a knee injury in skeletally immature ffffathletes exist.

Body compositionAn increased body mass index (BMI) has been found to be associated with an increased risk of suff ering ACL injuries in female cadets attendingffffthe US Military Academy (34). However, other


Th e use of an athlete’s self-report of menstrual his-Thtory is inadequate to determine the phase of cycle at the time of injury (60), and fi ndings from suchfistudies reveal confl icting results (21,60). Review flof studies that have used hormone measurements reveals that the risk of suff ering a non-contact ACLffffinjury is greater during the pre-ovulatory phase of the menstrual cycle in comparison to the post-ovu-latory phase (61).

Patella tendon-tibia shaft angleOne of the most investigated hypotheses in recentyears is whether a quadriceps contraction per-formed with the knee near extension can create a force of suffi cient magnitude on the patellar ten-ffidon such that it produces an anterior translation of the tibia relative to the femur and ruptures the ACL (63,64). Th e patella tendon-tibia shaft angleTh(PTTSA) is the sagittal plane angle between the tibia (ankle to knee joint) and the line of action of the patellar tendon. When the knee is near full extension, an increased PTTSA produces a larger magnitude of anterior-directed force on the tibia and increases the loads transmitted to the ACL. It has been shown that females have greater PTTSA throughout the range of knee flexion compared to flmales, and that this angle is greater when the knee is close to full knee extension. However, no risk factor studies have included PTTSA in the analy-ses. Th e same is true for the slope of the tibial Thplateau in the sagittal plane. An increased slope of the plateau will generate an increased anteri-or-directed force on the tibia when compression forces, such as that produced during landing orplant and cut maneuvers, are produced across the knee (65).

Landing mechanics Another explanation for the gender difference in ffffACL injury rate is that females may land with the knees in a more extended position compared with men, creating a higher PTTSA and possibly gen-erating an increased anterior-directed shear force on the tibia (66,67). Although several studies have investigated this knee fl exion hypothesis, there flis no consensus in the literature. In a recent pro-spective risk factor study of 205 female athletes in soccer, basketball, and volleyball, where knee flex-flion angles produced during jump landings were assessed, no significant difffi erences were found ffffbetween injured and uninjured subjects (31). TheThsame study showed that valgus motion and valgusmoments predicted ACL injuries with a sensitivity of 78% and a specifi city of 73%. Also, leg domi-finance was studied, but this factor was not found tobe associated with risk of suff ering an ACL injury. ffffIt should be mentioned that only nine ACL injuries

needed to delineate the role of notch impingementas an ACL injury risk factor. One review study (51) and two prospective cohort studies (48,50), as well as several other lesser quality studies (52,53), havefound that athletes with a decreased femoral notchwidth are at increased risk of suff ering an ACL ffffinjury. Th ere are, however, also several studies that Thdo not show such an association (47,49,54), andas a consequence, it remains unclear how notch width geometry, or notch width index, is related to increased risk of suff ering an ACL injury.ffff

ACL propertiesThe size and material properties of the ACL are fac-Thtors that may infl uence the risk of this ligament fltearing. It has been shown that ACLs, in females are smaller than in males when normalized forbody weight (47). In addition, it has been reportedthat female ligaments have lower strain and strain energy density at failure as well as 22.5% lower modulus of elasticity (55). However, at the current point in time, no risk factor studies have consid-ered these variables. On the other hand, there are studies that have established increased anterior knee laxity as a risk factor for females (56), and this may be a result of differences in the size and mate-ffffrial properties of the ACL. It is important for usto point out that these studies are relatively small,and such factors can only explain a relatively smallproportion of the variance in ACL injury risk. Gen-eralized joint and ligament laxity have also been proposed as risk factors for knee ligament injuries; however, most of these studies have evaluated theeff ect on these variables on all lower extremity ffffinjuries as a group and not knee ligament injuriesin isolation. A few exceptions exist, and these stud-ies suggest that increased generalized joint laxity and increased hyperextension of the knee may beassociated with increased risk of suff ering an ACLffffinjury (34,57).

Foot pronationIt has been suggested that foot pronation may lead to anterior tibial translation, and subsequently sprain the ACL; however, the results are conflict-fling. Th ree case-control studies found a relation-Thship between foot pronation/navicular drop and ACL injuries, whereas one found no such relation-ship (56,58,59).

Hormone levelsTo determine how diff erent phases of the men-ffffstrual cycle aff ect ACL injury risk, it appears neces-ffffsary to accurately describe the hormone milieu withserum or urine-based measures of hormone con-centrations and then use these data to accurately identify the phase of the cycle when injury occurs.


injury risk factors, and consequently, it is not pos-sible to determine the relative importance of thesefactors at this point in time.It should be noted that factors such as anatomic alignment and ligament properties are not easily modifi able, and therefore, direct intervention on fisuch factors may be difficult. Nevertheless, if we ffican identify individuals at risk (e.g., athletes who anatomically are prone to ACL injury), it may be possible to initiate individualized injury preven-tion measures. It has been shown that combining two or more risk factors such as notch width andBMI results in a dramatic increase in the risk of ACL injury (relative risk 26.2) compared to having only one of the two factors in isolation (relative risks of 3.5 and 4.0, respectively) (34).

were included in this study, so new studies should be conducted to confirm these fifi ndings.fi

Other neuromuscular factorsSeveral neuromuscular measures have been pro-posed as possible risk factors for ACL injury. Quadriceps dominance, ligament dominance, limbdominance, muscle reaction time, time to peak force, muscle stiff ness, muscle strength, poor neu-ffffromuscular control of the trunk, and fatigue are allfactors that may have an influence on an athlete’sflrisk of suff ering an ACL injury (22,68). It has been ffffshown that females exhibit greater quadriceps dominance and that they have less muscle stiffnessffffand muscle strength compared to males. However, none of these factors have been studied as ACL

Table 3 – Internal and external factors for knee ligament sprains in diff erent sports. The numbers reported are average estimates basedon the studies available.

Risk factor Relative risk Evidence CommentsExternal risk factors

Surface 2–2.4×* + Few studies. Greater risk of injury when competing on higher friction

fl oors, but only for women.

Meteorological conditions 1.9–2.8× + Only one study. Non-contact ACL injuries more frequent during high

evaporation and low-rainfall periods.

Footwear NA + Only two studies. Shoes with longer cleats produced signifi cantly

greater ACL injury rates.

Game vs. Practice 29.9× +(+) Higher ACL injury rate during competition.

Internal risk factors

Gender 2–8× ++ Risk of ACL injury is greater in women compared to men when

participating in the same sport at the same level of competition.

Previous injury 3.1–11.3× + Risk for new knee injuries in general as well as new ACL injuries, both in

the reconstructed and the contralateral knee.

Age NA - No studies were found that included age as a potential risk factor for

ACL injury

BMI 3.5× + Few studies. Higher risk of ACL injury for women with high BMI.

Confl icting results.

Familial tendency 2.0×* + Only two studies. Athletes with an ACL tear are 2 times more likely to

have a relative with an ACL tear.

Race 6.6×* + Only one study. White European American players more susceptible to

ACL tears compared with other ethnic groups.

Q-angle NA + Higher Q-angle in females. Confl icting results.

Leg length NA + Long femur relative to tibia may be a risk factor in skiers. Wide pelvis

relative to femur predicts dynamic valgus in one-legged squats

Intercondylar notch width index 3.7–6.0× + Small notch width index may lead to increased risk of suff ering an ACL injury.

Several positive studies, but also some studies with confl icting results

Ligament cross-sectional area NA - No risk factor studies. Relative larger cross-sectional area in males after

adjusting for body weight

Ligament material properties NA - No risk factor studies. Female ligaments have lower strain and strain

energy density at failure, as well as lower modulus of elasticity

compared to males

Anterior knee laxity 2.7× + Only two studies. The larger, prospective study found a 2.7 fold

increased risk for ACL injury in females with A-P knee laxity values

greater than 1 SD of the mean. A-P knee laxity had no eff ect on risk of

ACL injury in males.


Risk factor Relative risk Evidence CommentsGeneral joint laxity 2.8× + Few studies. Females exhibit greater joint laxity than males. Skiers with

increased hyperextension of the knee are in signifi cantly increased risk

of suff ering ACL injury.

Patella tendon – tibia shaft

angle (PTTSA)

NA - No risk factor studies. PTTSA is greater in females compared to males

Foot pronation/navicular drop NA + Three studies found a relationship, whereas one study found no

relationship.

Phase of menstrual cycle 3.2×* + Studies that have accurately measured phase of cycle with serum or

urine based assays of estradiole and progesterone have observed a

greater proportion of ACL injuries in the preovulatory phase of the

menstrual cycle in comparison to the post-ovulatory phase. One

study of recreational alpine skiers found the odds ratio of suff ering an

ACL disruption was 3.2 times greater during the preovulatory phase

compared to the postovulatory phase of the menstrual cycle.

Knee fl exion during landing NA - Only one risk factor study that showed no relationship. Confl icting

results among studies looking at gender diff erences.

Valgus motion and valgus

moment during landing

NA + Only one risk factor study. Valgus moments have a sensitivity of 78%

and a specifi city of 73% for predicting ACL injury status.

Leg dominance during landing NA - Only one risk factor study, but no signifi cant eff ect of this variable on

ACL injury risk. Females have greater side-to-side (leg dominance)

diff erences in knee loads

Quadriceps dominance NA - No risk factor studies. Females exhibit greater quadriceps dominance

than males.

Muscle stiff ness NA - No risk factor studies. Females have lower muscle stiff ness than males.

Muscle strength NA - No risk factor studies. Females have lower muscle strength than males.

Muscle reaction time NA - No risk factor studies.

Time to peak force NA - No risk factor studies.

Fatigue NA - No risk factor studies. No gender diff erences. Fatigue may alter the

neuromuscular control of knee biomechanics

aRelative risk indicates the increased risk of injury to an individual with this risk factor relative to an individual who does not have this characteristic. A

relative risk of 1.2× means that the risk of injury is 20% higher for an individual with this characteristic.bEvidence indicates the level of scientifi c evidence for this factor being a risk factor for ligament sprains: ++, convincing evidence from high-quality

studies with consistent results; +, evidence from lesser quality studies or mixed results; ?, expert opinion without scientifi c evidence.

*Odds ratio and not relative risk.

NA, not available.

Injury mechanisms for knee injuries

Much attention has been given to treatment of serious knee injuries, ACL and MCL disruptions in particular; however, very little is known about the risk factors that predispose an athlete to theseligament tears and the injury mechanisms that produce these debilitating injuries. During mostOlympic sports, ACL injuries are commonly non-contact in nature and can occur during plant andcut maneuvers (Fig. 1), but a high proportion of these injuries also occurs during landings (Fig. 2). In basketball, landings are the most frequently reported mechanism for ACL injuries (69).It is important to realize that while an athlete may appear to land on both feet with body weight equally distributed between legs, at the pointof impact the entire ground reaction force may

be supported by the leg that suff ers an injury.ffffPrevious studies have suggested that the knee is near extension at the time of ligament injury (i.e., less than 30°) (26,57); however, recent stud-ies that have included a validation of the visual inspection method indicate that the knee is likely injured when it is in a position of greater flexion fl(69,70). Although the majority of ACL injuries are non-contact by definition, the movement pat-fiterns often involve perturbation by an opponent, e.g., body contact prior to the injury (69). Even if a few details are known, we still lack vital knowl-edge about the injury mechanisms. A complete biomechanical description should quantify whole body and knee kinematics, loading directions and magnitudes, and the rate of application of exter-nal and internal forces about the lower extremity (71).


Previous studies on knee ligament injury mecha-nisms produced during alpine skiing have mainly used approaches such as athlete interview and visual inspection of injuries captured on video. Th ese approaches have methodological limitations, Thand there is need for improvements of the existinginjury mechanism descriptions (75).

Hypotheses for non-contact ACL injuries

Although gross biomechanical information about serious knee injuries exists, detailed biomechani-cal information (i.e., joint loading) is not known. For injuries that are caused by a direct blow to the knee, which is the case for many MCL injuries, the loading patterns are more obvious. However, for non-contact ACL injuries, diff erent hypotheses are ffffheavily debated in the scientifi c community.fiStudies have shown that external tibial rotation combined with valgus rotation with the knee in an extended or partially flexed position initiates ACL flstrain as the ligament contacts and then impinges against the medial aspect of the lateral femoral condyle (Fig. 4) (76).

The ligament impingement theory

Th e ligament impingement theory has also been Thsuggested based on observations from video analy-sis studies (26). Although it remains unknown how

Skiing injury mechanisms

Knee ligament injury mechanisms in traditional alpine skiing have been investigated for some time,and various injury mechanisms have been pro-posed, both for ACL injuries and other ligament injuries (72,73). Some knee injury mechanismsare equipment related, such as when the back por-tion of the ski boot acts to produce a “boot-inducedanterior drawer” (e.g., an anterior-directed force on the tibia that tears the ACL) or when the edgeof the ski is caught in the snow.Some injury mechanisms are associated with cer-tain circumstances, such as backward fall, with theweight of the skier on the inner edge of the tail of the ski resulting in a sharp uncontrolled inward twist of the lower leg (Fig. 3). This scenario is Thtermed the “phantom foot mechanism” and is con-sidered the most common ACL injury mechanism in alpine skiing (74).

Fig. 1 – Frame sequence of a plant and cut team handball injury showingthe athlete at initial ground contact (A), at 40 ms (B), and at 100 ms (C),respectively.

Fig. 3 – Drawing of the body position in the “phantom foot” injury mecha-nism. The weight of the skier is on the inner edge of the tail of the ski result-ing in a sharp uncontrolled inward twist of the lower leg. From [74].

Fig. 2 – Landing injury in basketball. The injured player is seen in whiteshorts in the middle of the images at initial ground contact (A); 33 ms after initial contact, corresponding to the approximate estimated time of rupture (B); and 133 ms after initial contact (C).


Quadriceps loading

Several cadaver studies have shown that quadri-ceps loading strains the ACL when the knee is near extension. It has been hypothesized that a vigor-ous quadriceps contraction when landing on an extended knee can produce high ACL strain values (63,64). In this theory, contraction of the quadri-ceps muscle group and subsequent engagement of the patellofemoral joint produces a load on the patellar tendon, which has an anterior-directedangulation relative to the tibia when the knee is near extension, and this generates a force on the tibia with an anterior-directed component. As the knee is moved from an extended to a fl exed posi-fltion, the orientation of the patellar tendon relative to the tibia moves from an anterior to a posterior direction as does the corresponding direction of theforce produced by the quadriceps extensor mecha-nism. Understanding this biomechanical relation-ship, studies have investigated if the gender differ-ffffence in ACL injury incidence can be explained by the fact that females appear to land with their knee and hip in a more extended position. The fiTh nd-fiings from these studies confl ict. In actual injury flsituations, females are found to be at signifi cantly figreater knee and hip fl exion angles at initial con-fltact with the playing surface and at the assumedpoint of injury in comparison to males (69). ThisThfi nding suggests that landing on straighter knees fimay not be an important reason for the observed diff erence in ACL injury incidence rates between ffffmales and females. Although a novel cadaver study demonstrated that the quadriceps-induced ante-rior drawer mechanism is capable of producing ACL rupture (63), this approach was criticized for not including ground reaction forces, which may act posteriorly on the tibia and help restrain ante-rior translation of the tibia. Three mathematicalThmodel simulation studies of landing and plant andcut maneuvers, which included the eff ect of groundffffreaction and hamstrings forces, all concluded thatthe anterior-directed shear force that acts on the tibia cannot generate the suffi cient magnitude to ffirupture the ACL, even in extreme cases where ham-strings forces are non-existent (65,79,80). Still, the results of these studies were quite different,ffffpossibly due to the fact that realistic modeling and simulation of the knee joint is challenging. In con-trast to the fi ndings from mathematical modelingfistudies, a recently published cadaver study dem-onstrated that anterior tibial translation and ACL strain proportional to the applied quadriceps force were generated when the ground contact forces were also included. However, since the loading was far from injury level, it is not possible to make fi rm conclusions regarding the quadriceps-induced fianterior drawer mechanism from this study. Fur-

the ligament impinges against the medial aspect of the lateral femoral condyle and strains the ACL,there is evidence that valgus loading of the tibia rel-ative to the femur is likely an important aspect of the loads applied to the knee during an ACL injury (77). A recent prospective risk factor study showedthat increased valgus loading when landing from ajump was associated with increased risk of suffer-ffffing an ACL injury amongst soccer, basketball, andvolleyball athletes (31). Several studies also reportthat MCL injuries are frequently seen in combi-nation with non-contact ACL injuries, indicating that valgus loading was present (78). Interestingly, laboratory- based motion analysis studies have dem-onstrated that females develop larger magnitudes of valgus and external torques about the knee when landing from a jump in comparison to males (79), suggesting that knee valgus loading may explain, at least in part, the larger incidence rate of ACL inju-ries seen in females compared to males taking part in the same sport at the same level of competition.A recent video-based analysis study of actual ACL injury situations reported a large number of valguscollapses about the knee for female athletes (69). Th is evidence suggests that valgus loading playsThan important role in many of the non-contact ACL injury situations, at least amongst female athletes. However, there are likely to be other forces andtorques applied to the knee during injury, since ithas been shown that pure valgus loading will rup-ture the MCL fi rst, then the ACL, and second, only fia limited number of MCL ruptures are found in conjunction with non-contact ACL injuries.

Fig. 4 – A sidestep cutting maneuver may lead to valgus and external tibial rotation. The solid arrow indicates a possible impingement of the ACLagainst the intercondylar notch.


athletes. For the optimal design of knee interven-tion programs, we can learn much from a system-atic analysis of the common components of the published interventions, successful and unsuccess-ful, designed to reduce knee injury risk in athletes. Analyzing the common components of the most eff ective and least effffff ective programs is useful for ffffthe development of eff ective intervention proto-ffffcols. Hewett et al. performed a systematic review of intervention studies designed to reduce kneeinjury risk in female athletes and revealed that several programs appear to reduce ACL injury risk (82). In contrast, other studies have failed to show an eff ect of neuromuscular training on the reduc-fffftion of knee injury rates or establish that they can alter lower extremity biomechanics (83,84). Most of what is known has come from studies of femaleathletes, as they are at increased risk of knee injury. Th e mechanism of ACL injury may diffTh er between fffffemales and males, particularly with respect tothe dynamic positioning and control of the knee, as females demonstrate greater valgus collapse of the lower extremity, primarily in the coronal plane. However, female athletes may serve as a working model for any athlete at increased risk of suff eringffffa knee injury. In this section, we will describe many of the prevention methods and neuromuscular training programs and provide broad instructions and guidelines for the exercises. In addition, we will review as many components of the successful, and unsuccessful, programs as possible within the spe-cifi c purview of knee injury prevention in athletes.fiTh ere appears to be a measurable effTh ect of neuro-ffffmuscular training interventions on the reduction of severe knee and ACL injuries. A comprehensivereview of the literature revealed that five of six fistudies demonstrated that neuromuscular train-ing reduced lower extremity injury risk, four of six studies found that neuromuscular training reducedserious knee injury risk, and three of six reported decreased knee and ACL injury risk (82). Below we summarize the components of the most successful programs. Plyometric training and biomechanical analysis of landing, cutting, and jumping tech-niques were common components of the studies that were eff ective at reducing the risk of knee and ffffACL injury in athletes.

Plyometrics is an important component for reduction of ACL injury risk in athletes

Th e evidence for including a plyometric component Thas a portion of a knee and ACL injury prevention program is relatively strong. Th e systematic review Thof the literature reported by Hewett et al. found that reduced ACL injury risk occurred in those inter-ventions that included plyometrics as part of the

ther investigations are required to delineate therole of quadriceps-induced anterior drawer loading of the tibia in producing ACL injuries.

Internal rotation

Th ere are several factors that suggest internal rota-Thtion of the tibia relative to the femur on a relatively extended knee could be a potential mechanism of non-contact ACL injuries. First, cadaver and humanstudies have shown that the ACL is strained when torques are applied to internally rotate the tibia rel-ative to the femur (81). Second, internal rotation isfrequently reported to be the mechanism of injury in athlete interview studies (30), in video analysis(26), as well as suggested from the associated clini-cal fi ndings. Motion analysis studies of side step cut-fiting maneuvers usually show a dominance of inter-nal tibial rotation during the stance phase. However, males are reported to exhibit this pattern to a largerdegree than females, indicating that other loading scenarios are likely associated with many female non-contact ACL injuries. Hyperextension has alsobeen proposed as a possible mechanism, but this seems unlikely considering that such injuries havenot been reported in any video analysis studies.

Conclusions

Th ere is a need to develop better research meth-Thods to investigate the mechanisms of serious kneeinjuries, particularly ACL injuries. Mathematical simulation models have the potential to include all important aspects of an injury (e.g., ground reac-tion forces, tibiofemoral contact mechanics, and neuromuscular control patterns) in a computer environment, thus avoiding any hazard to athletes. However, considerable challenges exist in generat-ing valid models that replicate joint biomechanicsat the time of injury. Therefore, such model-based Thanalysis should be accompanied by video analysis as well as clinical studies looking at the associated joint damage (arthroscopy, radiology, CT, MRI), cadaver testing, motion analysis of similar “close-to-injury situations,” and in-vivo studies whereligament strain and forces are measured.

Preventing knee injuries among athletes

Developing “optimal“ ” neuromuscular training llinterventions for decreasing ACL injury

Neuromuscular training appears to be effective at ffffreducing knee injury risk, particularly for female


Training programs that incorporate plyometrics result in safe levels of varus or valgus stress about the knee, and may increase “muscle-dominant” neu-romuscular control patterns and reduce “ligament-dominant” neuromuscular control patterns.Studies by Hewett et al. (85,86), Myklebust et al. (1) and Mandelbaum et al. (87) all incorpo-rated high-intensity plyometric movements into the design of their intervention programs(Figs. 5–9). Th e studies by Heidt Th et al. (88) andSoderman et al. (89), did not reduce ACL injury risk. Th is can be explained, at least in part, by Ththe fact that the studies by Soderman et al. (89)

training program, while those that did not include plyometrics did not reduce knee or ACL injury risk (85,86). Th e focus of plyometrics should be on Thproper landing, cutting, and jumping techniques and body mechanics during these movements.

Fig. 6 – Wall jump: The athlete stands erect with her arms semi-extended overhead. This vertical jump requires minimal knee fl exion. The gastrocne-mius muscles should create the vertical height. The arms should extend fullyat the top of the jump. Use this jump as a warm-up and coaching exercise asthis relatively low intensity movement can reveal abnormal knee motion inathletes with poor side-to-side knee control.

Fig. 5 – Athletic position: The athletic position is a functionally stableposition with the knees comfortably fl exed, shoulders back, eyes up, feetapproximately shoulder-width apart, the body mass balanced over the balls of the feet. The knees should be over the balls of the feet and chest should be over the knees. This is the athlete-ready position and is the starting andfi nishing position for most of the training exercises. During some of the exercises the fi nishing position is exaggerated with deeper knee fl exion in order to emphasize the correction of certain biomechanical defi ciencies.

Fig. 7 – Tuck jump: The athlete starts in the athletic position with her feet shoulder-width apart. She initiates the jump with a slight crouch downward while she extends her arms behind her. She then swings her arms forward as she simultaneously jumps straight up and pulls her knees up as high as possible. At thehighest point of the jump the athlete is in the air with her thighs parallel to the ground. When landing the athlete should immediately begin the next tuck jump. Encourage the athlete to land softly, using a toe to mid-foot rocker landing. The athlete should not continue this jump if they cannot control the high landing forceyor if they utilize a knock-knee landing.


the studies that did not incorporate high-inten-sity plyometrics did not report reductions in ACL injury risk. Hence, the plyometric component of a pre-season intervention program appears to reduce serious ligamentous injuries, specifi cally fiACL injuries.Plyometrics may be used as combined analysis and training tools, with verbal or visual feedback, for control of body motion, both during deceleration and acceleration, and knee loading, especially withrespect to the reduction of abduction (or “valgus”) torque about the knee. For example, Hewett et al.have shown that plyometric jumps force control of knee abduction torque (Fig. 7) (86). The other exer-Thcises that were included in the interventions that reported decreased ACL injury risk were lateral jumps over barriers (this forces trainees to stabilizetheir trunk in the coronal plane while moving both lower extremities side to side), landing and balanc-ing on compliant surfaces and perturbed single-leg balancing, and hop and holds for extended periods (Figs. 12 and 13).

Movement biomechanics, technique, and educationcomponents: coronal plane is key

Th e evidence in support of including movement Thbiomechanics; landing, cutting, and jumping tech-nique; and education components of the effective ffffinterventions is also relatively strong. Olsen et al.(90) have reported that in sports performed on court and turf surfaces, most ACL injuries occur by non-contact mechanisms during landing andlateral pivoting. The biomechanics of these land-Thing and cutting movements can be improved with

and Heidt et al. (88) had little chance of estab-lishing if their intervention could reduce the risk of injury because the sample size was likely too small, and from this perspective, these should be considered preliminary studies. With this caveat, the studies that incorporated high-intensity ply-ometrics reported reduced ACL injury risk, while

Fig. 8 – Broad jump and hold: The athlete prepares for this jump in theathletic position with her arms extended behind her at the shoulder. Shebegins by swinging her arms forward and jumping horizontally and verti-cally at approximately a 45° angle to achieve maximum horizontal distance.The athlete must stick the landing with her knees fl exed to approximately90° in an exaggerated athletic position. The athlete may not be able to stickthe landing during a maximum eff ort jump in the early phases. In this situ-ation, have the athlete perform a submaximal broad jump in which she can stick the landing with her toes straight ahead and no inward motion of herknees. As her technique improves, encourage her to add distance to her jumps, but not at the expense of perfect technique.

Fig. 9 – One hundred and eighty-degree jump: The starting position for this jump is standing erect with feet shoulder width apart. She initiates this two-footed jump with a direct vertical motion combined with a 180° rotation in mid air, keeping her arms away from her sides to help maintain balance. When she lands sheimmediately reverses this jump into the opposite direction. She repeats until perfect technique fails. The goal of this jump is to achieve maximum height with a full 180° rotation. Encourage the athlete to maintain exact foot position on the fl oor, by jumping and landing in the same footprint.


Fig. 10 – Squat jump: The athlete begins in the athletic position with her feet fl at on the mat pointing straight ahead. The athlete drops into deep knee, hip andankle fl exion, touches the fl oor (or mat) as close to her heels as possible, then takes off into a maximum vertical jump. The athlete then jumps straight up verti-cally and reaches as high as possible. On landing she immediately returns to starting position and repeats the initial jump. Repeat for the allotted time or until hertechnique begins to deteriorate. Teach the athlete to jump straight up vertically, reaching as high overhead as possible. Encourage her to land in the same spot on the fl oor, and maintain upright posture when regaining the deep squat position. Do not allow the athlete to bend forward at the waist to reach the fl oor. The athlete should keep her eyes up, feet and knees pointed straight ahead, and have their arms to the outside of their legs.

neuromuscular training. Neuromuscular trainingcan increase coronal and sagittal plane control of the lower extremity. For example, during a squat jump (Fig. 10), a two-footed plyometric activity,post-training results show that lower extremity valgus alignment can be reduced at the knee and hip. Conversely, during a single-leg task such as a hop and hold maneuver (Figs. 12–16), the most signifi cant changes may occur in the sagittal planefiof the knee.Th ere is strong evidence in support of landing, cut-Thting, and jumping technique training and its effectffffon reducing ACL injury risk. Hewett et al. reported that technique and phase-oriented training, that corrected jump and landing techniques in ath-letes reduced ACL injuries in a female interventiongroup compared to female controls and resulted in injury levels similar to a male control group (85). Myklebust et al. reported that the incidence of ACL injury in elite handball was reduced with training designed to improve awareness of lower extrem-ity alignment and knee control during cutting and landing activities (1). Th e studies by Hewett Th et al.(85), Mandelbaum et al. (87), and Myklebust et al.(1), which successfully reduced ACL injury risk, all incorporated landing technique analysis andfeedback during training into their intervention

programs. Of the non-effective studies in our lit-fffferature review, none incorporated landing/cutting technique, and while only the Wedderkopp study (91) found a decrease in traumatic lower extremity injuries, none of these interventions reduced ACL injury risk. Methods for altering biomechanical technique include those of Hewett et al. (85,86),which utilized a trainer to provide feedback and awareness to an athlete during training, and of Myklebust et al. (1), which utilized partner train-ing to provide the critical feedback regarding lower extremity alignment, particularly valgus (inward) positioning of the knee.Johnson et al. (92) reported that education and public awareness of the high occurrence and mechanisms of ACL injury can decrease injuries in alpine skiers by greater than 50%. A reduction of ACL injuries in ski instructors was achieved by using “guided learning” techniques that educated skiers to avoid “high-risk” skiing positions, such as the skier positioned with a majority of the weight on the downhill ski and their hips below their knees. Th is approach is supported by Pra-Thpavessis et al. (93), who reported that verbal or visual or biofeedback regarding technique may decrease reaction forces at the knee and reduce ACL injuries.


Fig. 13 – X-hops: The athlete begins faces a quadrant pattern stands, on a single limb with their support knee slightly bent. She hops diagonally, landsin the opposite quadrant, maintains forward stance and holds the deepknee fl exion landing for three seconds. She then hops laterally into the side quadrant and again holds the landing. Next she hops diagonally backwardand holds the jump. Finally, she hops laterally into the initial quadrant andholds the landing. She repeats this pattern for the required number of sets. Encourage the athlete to maintain balance during each landing, keepingher eyes up and visual focus away from their feet.

Fig. 14 – Single leg balance: The balance drills are performed on a balance device that provides an unstable surface. The athlete begins on the devicewith a two-leg stance with feet shoulder width apart, in athletic position.As the athlete improves the training drills can incorporate ball catches andsingle leg balance drills. Encourage the athlete to maintain deep knee fl ex-ion when performing all balance drills.

Fig. 11 – Broad jump to vertical jump: The athlete performs three succes-sive broad jumps, and immediately progresses into a maximum eff ort verti-cal jump. The three consecutive broad jumps should be performed as quickly as possible and attain maximal horizontal distance. The third broad jumpshould be used as a preparatory jump that will allow horizontal momentum to be quickly and effi ciently transferred into vertical power. Encourage theffiathlete to provide minimal braking on the third and fi nal broad jump toensure that maximum energy is transferred to the vertical jump. Coach theathlete to go directly vertical on the fourth jump and not move horizontally.Utilize full arm extension to achieve maximum vertical height.

Fig. 12 – Hop and hold: The starting position for this jump is a semi-crouched position on a single leg. Her arms should be fully extended behind her at the shoulder. She initiates the jump by swinging the arms forwardwhile simultaneously extending at the hip and knee. The jump should carrythe athlete up at an approximately 45° angle and attain maximum distance for a single-leg landing. Athletes are instructed to lands on the jumpingleg with deep knee fl exion (to 90°). The landing should be held for a mini-mum of three seconds. Coach this jump with care to protect the athlete from injury. Start her with a submaximal eff ort on the single leg broad jump so she can experience the level of diffi culty. Continue to increase the distanceffiof the broad hop as the athlete improves her ability to stick and hold thefi nal landing. Have the athlete keep her visual focus away from her feet, to help prevent too much forward lean at the waist.


A core component? Evidence for the effects of “coreffffstability” training

Th ere is not clear evidence whether “core stabil-Thity” exercises should be incorporated into an intervention to reduce knee ligament injuries. Itis not clearly defi ned what “core stability” exer-ficises actually represent and what their effectsffffare on the muscles that stabilize the trunk, hip and pelvis. However, Zazulak et al. (68,96) dem-onstrated that measures of “core” or trunk pro-prioception and displacement predicted risk of ACL injury in collegiate athletes with high sen-sitivity and specifi city. Interestingly, this efffi ectffffwas observed in female, but not male, collegiate athletes. Th is may indicate the need for including Thtrunk perturbation and strengthening in optimal interventional training programs. The fiTh ndingsfisupport the integration of proprioceptive stability training in ACL injury interventions, at least for females. Krosshaug et al. (69) suggested that pre-ventive programs to enhance knee control should focus on avoiding valgus motion about this joint and include distractions resembling those seen in match situations.

Strength training eff ects on knee injury riskffff

Resistance training alone has not been shown to reduce ACL injuries. However, inferential evidencesuggests that resistance training may reduce injury based on benefi cial adaptations that occur infi

Single-leg balancing component and ACL injury risk

Th ough single-leg balance training alone may not be Theff ective for decreasing ACL injury rates in femaleffffathletes, as the small studies that incorporated single-leg balancing alone did not report decreased knee or ACL injury risk in female athletes, it may be an important component of neuromuscular train-ing designed to decrease non-contact knee and ACL injury. Th e studies by Hewett Th et al. (85,86) and Man-delbaum et al. (87) incorporated single-leg stability training, primarily utilizing hold positions from a decelerated landing. Single-leg stability can be gainedwith balance training on unstable surfaces. Mykle-bust et al. (1) utilized partner training on Airex mats. Again, however, the intervention programs that usedbalance training in isolation were not effective in ffffreducing knee injuries in females. Wedderkopp et al.(91) reported a reduction in all soccer-related injuries,although not knee or ACL injuries. Soderman et al.(89) were not eff ective in reducing injuries in female ffffsoccer players. Wedderkopp et al. (91) and Soder-man et al. (88) focused on balance training, primarily utilizing unstable wobble boards. Therefore, balanceThtraining alone may not be as eff ective for ACL injury ffffprevention as when it is combined with other typesof training. Interestingly, Caraffa ffff et al. (94) studiedmale soccer players and showed a significant efffi ect of ffffbalance board exercises on reducing ACL injury and reported that balance training may be more effec-fffftive in males than females. Alternatively, Beynnon et al. (95), in a prospective study, reported that femaleathletes who suffered fiffff rst-time ankle injuries hadfigreater body sway than female athletes who did not go on to injury, while male athletes who went on to ankle injury did not demonstrate increased trunk sway compared to uninjured males.

Fig. 15 – Bounding: The athlete begins this jump by bounding in place. Once she attains proper rhythm and form encourage her to maintain thevertical component of the bound while adding some horizontal distanceto each jump. The progression of jumps progresses the athlete across thetraining area. When coaching this jump, encourage the athlete to maintain maximum bounding height. Fig. 16 - Hop and hold.


ing sessions should be performed more than one time per week, preferably at least two and up to fi ve times per week. Thfi e total pre-season training Thduration of the intervention program should be a minimum of 6, preferably 8 or more, weeks in length. Pfeifferffff et al. (83) reported that 20 min of “in-season” exercise 2 days per week was not suf-fi cient to decrease ACL injury risk in high school fiage female basketball players. Gilchrist et al. (99)also reported that an “in-season” program, with no “pre-season” component, was only effective in the fffflast half of the season.Th e most effTh ective programs are progressive in nature. ffffExercises should progress to techniques that initiate perturbations that force the athlete to decelerate and control the body in order to successfully perform the landing, cutting, and jumping techniques. The inter-Thvention should preferably be phasic in nature. Three Thexercise phases, such as technique, power, and per-formance phases, are often utilized to facilitate pro-gressions designed to improve the athletes’ability tocontrol body motion during dynamic activities. All exercises in each phase should be progressed to exer-cise techniques that incorporate perturbations that force the athlete to decelerate and control the body in multiple planes of motion, particularly the coronal plane, in order to successfully perform each technique with optimal form and safety level.

Conclusions

Th ere is good evidence that neuromuscular train-Thing decreases knee and ACL injury incidence in ath-letes. Plyometrics in combination with biomechan-ics and technique (e.g., jumping/landing) training appear to induce neuromuscular changes that reduce ACL injury risk. Increased lower extrem-ity muscle recruitment and strength likely have a direct eff ect on the loading of the ACL during activ-ffffities that involve cutting and landing. Although ACL injuries likely occur too quickly (less than 70 ms) for refl exive muscular activation (greater than fl100 ms), athletes can adopt preparatory muscle recruitment and movement patterns that reduce the incidence of injuries caused by unexpected perturbations. Th e studies discussed above pro-Thvide strong, but not unequivocal, evidence that neuromuscular intervention training and educa-tion programs are likely to be an effective solution ffffto the problem of gender inequity in ACL injury.Selective combination of neuromuscular training components may provide additive effects, furtherffffreducing the risk of ACL injuries.It appears that plyometric power and biomechan-ics technique training specifi c to landing, cutting,fiand jumping activities can induce neuromuscular changes and prevent ACL injury, at least in female

bones, ligaments, and tendons after training. For example, Lehnhard et al. (97) signifi cantly reducedfiinjury rates with a strength training regimen inmen’s soccer. Th e studies by Hewett Th et al. (85,86)and Mandelbaum et al. (87) incorporated strength training in their intervention protocols. Myklebustet al. (1), Heidt et al. (88), Soderman et al. (89),and Petersen et al. (98) did not include strength training in their interventions. Th e designs thatThincorporated strength training were among the most eff ective at decreasing ACL injury rates. But ffffstrength training in isolation may not be a prereq-uisite for prevention, as the Myklebust et al. (1)study was eff ective in reducing ACL injury risk, ffffand it did not incorporate strength training. In the fi nal analysis, weight training alone has not been fireported to be eff ective at decreasing ACL injury ffffrates and may not need to be incorporated into a successful intervention. It is important for us topoint out that this may apply only to those sub-jects who have adequate strength prior to enteringan injury prevention program.

Targeting participation in ACL injury interventions to individuals or teams

Athletes at the greatest risk of ACL injury shouldparticipate in neuromuscular training interven-tions to decrease the risk of injuring this importantligament. This approach is specifiTh c to the individualfiathlete. Th e neuromuscular training protocol should Thpreferably be designed and instituted specifi cally forfiand with athletes selected for neuromuscular training based on identified neuromuscular defifi ciencies andfiimbalances. The authors realize that the individual-Thized approach may not be tenable for team athletes and their coaches. Although we do not know if the generalizations from our review of the literature dis-cussed above apply to all athletes or just those who participate in jumping, landing, and cutting sports, we can presume that there will be positive effects of ffffneuromuscular training programs designed around these basic commonalities in effective interventions. ffffTh ese broad generalizations for the athlete in cuttingThand landing sports, for “team training,” from thestudies discussed above, should be eff ective if plyo-ffffmetric training and landing, cutting, and jumpingtechnique training are included in the intervention.

The proper “training dose”: how much and how often eeinterventions should be performed

Neuromuscular power can increase within 6 weeks of training and may result in decreases in peak impact forces and knee abduction torques. TheThevidence from the literature indicates that train-


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athletes. Balance, core stability, and strength train-ing may be useful adjuncts. However, we do not yet know which of these components are most effec-fffftive or whether their eff ects are combinatorial.ffffFuture research eff orts will assess the relative effiffff -fficacy of these interventions alone and in combina-tion in order to achieve the optimal effect in the ffffmost effi cient manner possible. Selective combina-ffition of neuromuscular training components may provide additive eff ects, further reducing the risk ffffof knee and ACL injuries in female athletes.Neuromuscular training interventions designed toprevent injury should be based on the previously published literature. Final conclusions are that neuromuscular training and educational trainingreduce ACL injury risk in female athletes if plyo-metric exercises and biomechanical techniques areincorporated into the protocol, training sessionsare performed more than one time per week, and the duration of the training program is a minimum of 6 weeks in length. Th e studies by Hewett Th et al.(85,86), Myklebust et al. (1), and Mandelbaum et al. (87) all incorporated plyometrics and biome-chanical movement analysis and feedback intotheir injury prevention programs and applied thesebasic rules of proper intervention “dose.”

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