For a brief overview of knee anatomy, physiology, and biomechanics, please click here.

Kinematics and Electromyography of Landing Preparation in Vertical Stop-Jump -- Risks for Noncontact Anterior Cruciate Ligament Injury, Jonathan D. Chappell et al.; American Journal of Sports Medicine; February 2007; Volume 35, pages 235-241. Comments: This study, done using state-of-the-art technology (3D infrared camera, floor-mounted force plate system, EMG sensors) confirms what has long been suspected: the jump-landing movement parameters are determined prior to the actual landing itself. In other words, these parameters are "firmware settings". The authors compared males and females in jumping landing, and noted that the females have less knee and hip flexion at jump landing, decreased hamstring activation (i.e. more stress on the ACL), and increased quadriceps activation (again more ACL stress). Fortunately, this disconcerting state of affairs can be rectified through a targeted, diligently pursued knee-injury-prevention training program. Other good articles on these topics are Noyes-AJSM-Feb05.shtml, Ahmad-AJSM-Mar06.shtml, BarberWestin-AJSM-Mar06.shtml, Mandelbaum-AJSM-Jul05.shtml, and Chappell-AJSM-Mar02.shtml.

Sex-based differences in the tensile properties of the human anterior cruciate ligament, Naveen Chandrashekar et al.; Journal of Biomechanics; December 2006; Volume 39, pages 2943-2950. Comments: This brilliant and penetratingly insightful study shows that the mechanical properties of the female ACL are distinctly inferior to those of the male ACL. Chandrashekar notes that the female ACL has an 8.3% lower strain at failure (i.e. it deforms less prior to disintegrating, so there is less warning), 14.3% lower stress at failure (i.e. less applied loading, hence less force, is required to get it to tear), 9.43% lower strain energy density at failure (in practice, this means less force is required to deform a given unit of the female ACL, i.e. strength per unit volume, and therefore strength per unit cross-sectional area, is lower), and, most important, 22.49% lower modulus of elasticity (i.e. less stretchiness, therefore more brittle, so it stretches less and fails sooner -- and less stretching means less of a timeframe in which the ligament's embedded tension-sensitive nerve endings can signal pain and trigger the ACL-protective hamstring reflex). So, from a materials-science point of view, the female ACL is of poorer quality. Comparing these attributes of a male and female ACL is analogous to comparing a cotton rope to a high-strength nylon one. This study builds on the equally important findings of its predecessor, which showed that the female ACL has a markedly smaller cross-section than that of its male counterpart. This structural difference, combined with the aforementioned differences in mechanical properties, helps explain why females have a 2-8 times higher ACL-injury incidence.

Lack of Effect of a Knee Ligament Injury Prevention Program on the Incidence of Noncontact Anterior Cruciate Ligament Injury, Ronald P. Pfeiffer et al.; Journal of Bone and Joint Surgery (American Edition); August 2006; Volume 88, pages 1769-1774. Comments: These authors found that a simple 20-minute exercise routine based on plyometric (ballistic jumping) exercises is insufficient inasmuch as preventing ACL injuries is concerned. Indeed, a more rounded, broad-based training program is needed, as is more research. It would have been interesting to compare the relatively new KLIP program with a more established, broad-based, extensively research-backed program such as Cincinnati Sportsmetrics.

The Anterior Cruciate Ligament Tear Rate Varies by Race in Professional Women’s Basketball, Thomas H. Trojian and Seamus Collins; American Journal of Sports Medicine, Baltimore; June 2006; Volume 34, pages 895-898. Comments: This brilliant and penetratingly insightful study shows that the mechanical properties of the female ACL are distinctly inferior to those of the male ACL. Chandrashekar notes that the female ACL has an 8.3% lower strain at failure (i.e. it deforms less prior to disintegrating, so there is less warning), 14.3% lower stress at failure (i.e. less applied loading, hence less force, is required to get it to tear), 9.43% lower strain energy density at failure (in practice, this means less force is required to deform a given unit of the female ACL, i.e. strength per unit volume, and therefore strength per unit cross-sectional area, is lower), and, most important, 22.49% lower modulus of elasticity (i.e. less stretchiness, therefore more brittle, so it stretches less and fails sooner -- and less stretching means less of a timeframe in which the ligament's embedded tension-sensitive nerve endings can signal pain and trigger the ACL-protective hamstring reflex). So, from a materials-science point of view, the female ACL is of poorer quality. Comparing these attributes of a male and female ACL is analogous to comparing a cotton rope a high-strength nylon one. This study builds on the equally important findings of its predecessor, which showed that the female ACL has a markedly smaller cross-section than that of its male counterpart. This structural difference, combined with the aforementioned differences in mechanical properties, helps explain why females have a 2-8 times higher ACL-injury incidence.

Comparing the Incidence of Anterior Cruciate Ligament Injury in Collegiate Lacrosse, Soccer, and Basketball Players -- Implications for Anterior Cruciate Ligament Mechanism and Prevention, Leanne C.S. Mihata et al.; American Journal of Sports Medicine, Baltimore; June 2006; Volume 34, pages 899-904. Comments: Mihata et al. looked at the injury rates in intercollegiate (varsity) soccer, basketball, and lacrosse, for both males and females. They found a very high rate of ACL injuries in men's lacrosse, and noted that this sport is high-risk for ACL injuries, just like soccer and basketball. They note that lacrosse is contact for males, but noncontact for females. They note that there is a strong correlation between the level of contact in sports and the ACL-injury risk. So, for example, a high-contact sport can bring a high likelihood of ACL injuries via both contact means and non-contact means. The level of contact of the sport is merely indicative of its aggressiveness. Contact knee injuries (e.g. knee forced sideways or injuriously hyperextended) routinely occur via unintentional contact in noncontact sports. And, non-contact (i.e. twisting-type, thus pivoting-type) knee injuries often occur in contact sports. The authors originally thought that the carrying of a stick in lacrosse reduces the risk of knee injury, but this does not seem to be the case. (In ice hockey, the risk of twisting-type knee injuries is reduced not because of the stick, but rather because it is inherently very difficult to plant a skate on ice in a torque-resisting fashion.) The authors note that "the key to injury prevention is an accurate understanding of biomechanical risk factors, followed by prospective identification and demonstrable changing of those factors presumed to be modifiable, culminating in proven decreases in long-term ACL injury rates." In practice, the best knee-injury-prevention methodologies seem to be good training (including for both strength and endurance), along with learning to pivot only on the front portion of the foot (instead of planting the entire shoe sole prior to twisting) and, for athletes with previous knee-injury histories, the use of functional knee bracing.

Jump-Land Characteristics and Muscle Strength Development in Young Athletes -- A Gender Comparison of 1140 Athletes 9 to 17 Years of Age, Sue D. Barber-Westin et al.; American Journal of Sports Medicine, Baltimore; March 2006; Volume 34, pages 375-384. This penetratingly insightful and very highly thought-provoking study looked the effects of gender and chronological age on muscle strength and neuromuscular characteristics in a group of over a thousand athletes in the 9-17 age bracket. Barber-Westin et al. found that most of athletes in this age group land with knees angled inwards (which can be a predisposing factor to ACL injuries); however, this was seen not only in females, but in males as well. It was noted that maximum hamstring strength (as a function of body weight) in females occurs by age 11, whereas this same milestone is reached 3 years later in males. The similar intergender lower-limb-alignment upon landing (taking into account that females do have a wider pelvis on account of childbirth enablement) means that some other factors must underlie the 4- to 8-fold higher incidence of ACL injuries in female athletes. Possible factors noted by various researchers, and discussed in detail in other articles, include vertibular-system "firmware" issues, too-narrow intercondylar notches, a proclivity towards lax joints, and a monthly hormonal cycle with ligament-weakening estrogen spikes. (Note that female-athlete knee-injury-prevention training programs such as Cincinnati Sportsmetrics focus on developing hamstring strength, improving jump height, and landing jumps with knees well-flexed and with knees angled outwards. Such training has been proven highly effective in preventing ACL injuries in females, and so it is clear that the aforementioned aspects do play a significant role in some way.) The authors note that further study is needed, especially longitudinal studies which follow a given group of athletes for extended periods of time. Ideally, such a study would entail about 1000 athletes, starting at about age 9 and carrying through until about age 30 or so. It would be very intriguing to correlate the data collected in such a study to any subsequent knee injuries in the group. (Another excellent article dealing with female athletes, and focusing especially on the topic of hamstring strength in proportion to quadriceps strength, is Ahmad-AJSM-Mar06.shtml.)

Effect of Gender and Maturity on Quadriceps-to-Hamstring Strength Ratio and Anterior Cruciate Ligament Laxity, Christopher S. Ahmad et al.; American Journal of Sports Medicine, Baltimore; March 2006; Volume 34, pages 370-374. In this study, Ahmad et al. found that after menarche (where menarche is defined as the first ovulation, hence the first period), females demonstrate a greater quadriceps-strength increase than hamstring-strength increase. This disproportionate hamstring strength seems to be a major factor in the 2-8 times higher ACL risk of females. For this reason, a cornerstone of all female-athlete knee-injury-prevention programs, such as Cincinnati Sportsmetrics and Girls-Can-Jump, is hamstring strengthening. (Another excellent article dealing with female athletes is BarberWestin-AJSM-Mar06.shtml.)

Sex-Based Differences in the Anthropometric Characteristics of the Anterior Cruciate Ligament and Its Relation to Intercondylar Notch Geometry A Cadaveric Study, Naveen Chandrashekar et al.; American Journal of Sports Medicine, Baltimore; October 2005; Volume 33, pages 1492-1498. Comments: The authors found that gender makes a substantial difference in terms of the ACL's form and structural characteristics. They note that ACL mass increases with height in men, but not in women, and also that ACL size increases proportionally to intercondylar-notch width in men, but not in women. The authors astutely note that more research is needed in order to determine gender-based differences in ACL anatomy and physiology.

Isokinetic profile and differences in tibial rotation strength between male and female athletes 11 to 17 years of age, Frank R. Noyes and Sue D. Barber-Westin et al.; Isokinetics and Exercise Science; September 2005; Volume 13/5, pages 1-9. Comments: This penetratingly thought-provoking article found substantial differences in various kinesiological characteristics (as measured with a Biodex machine), both as a function of age and gender. But some of the attributes varied only with age, and then only in males. (With males, muscle strength was much greater in the 14-17 age group than in the 11-13 age group.) It was noted that males have faster times to peak torque, for internal rotation, compared to females. The study notes the importance of dynamic muscular stabilization of the knee joint in protecting the ACL. The authors also comment on the lasting and worrisome reduction in internal-rotation strength (ranging from 13% to a staggering 31%) that results from hamstring-type (semitendinosus-gracilis, hence STG/DLSTG) ACL-graft harvesting.

Effectiveness of a Neuromuscular and Proprioceptive Training Program in Preventing Anterior Cruciate Ligament Injuries in Female Athletes: 2-Year Follow-up, Bert R. Mandelbaum et al.; American Journal of Sports Medicine, Baltimore; July 2005; Volume 33, pages 1003-1010. Comments: This study shows that neuromuscular and proprioceptive training (for example, an organized program such as Cincinnati Sportsmetrics) can be expected to bring direct and very tangible benefits. These benefits extend not only to the realm of injury prevention, but include improved athletic performance (better endurance, higher jump height, etc.). Although this study focuses on females, the same benefits apply to males as well.

The Drop-Jump Screening Test: Difference in Lower Limb Control By Gender and Effect of Neuromuscular Training in Female Athletes, Frank R. Noyes and Sue D. Barber-Westin et al.; American Journal of Sports Medicine, Baltimore, Maryland; February 2005; Volume 33, pages 197-207. Comments: This highly insightful study looks at the biomechanics of landing jumps -- and, ultimately, the resulting influence on ACL-injury proclivity. Because valgus knee alignment (i.e. knees too close together) upon landing has been correlated with noncontact-type ACL injuries, using a video camera to record the knee alignment upon jump landing makes sense. This video capture would then make it easy to screen for and target valgus-knee-landing athletes (both male and female) with special remedial jump-training programs; the use of video recording also makes it easy to track the progress of the athletes. (The video camera used in this study was an ordinary one, which makes this kind of work easy to do in the field. No ultra-expensive high-specialized laboratory equipment is needed here. The authors note that a second camera could additionally be used, so as to enable the knee-flexion angles to be recorded directly [i.e. views from the side, not just from the front]. However, they elected to use only one camera because of the goal of making this a screening test which could easily be done by coaches and athletic trainers with equipment one would find at home.) The injury-prevention training used in this study was Cincinnati Sportsmetrics, a program whose injury-prevention benefits are very well-supported by peer-reviewed research.

The Effect of Estrogen on Ovine Anterior Cruciate Ligament Fibroblasts Cell Proliferation and Collagen Synthesis, Aruna Seneviratne et al.; American Journal of Sports Medicine, Baltimore, Maryland; October 2004; Volume 32, pages 1613-1618. Comments: This is a very interesting investigation into the effect of estrogen on ligaments. The authors caution about extrapolating from sheep to humans. It is also interesting to note that estrogen, when released prior to and during labour, serves to weaken ligaments and therefore to facilitate childbirth. The authors conclude, however, that the two-to-three-day estrogen spike during the luteal phase of the human female period would probably not be of sufficient duration to result in significant weakness of the ACL in the knee. But they also note that further investigation is needed. (Note, too, that the biomechanics of the human body are quite different from those of the sheep. A sheep walks on four legs, and so there is less stress at the knee when it changes direction. If a sheep were to run at the same speed and change direction at the same rate as an athletic human, the sheep would be at far less risk of tearing an ACL. Additionally, the sheep's centre of gravity is substantially lower than that of a human.)

Knee Laxity Does Not Vary With the Menstrual Cycle, Before or After Exercise, Michael J. Belanger et al.; American Journal of Sports Medicine, Baltimore, Maryland; July 2004, Volume 32, pages 1150-1157. Comments: Irrespective of how ligaments are influenced by the menstrual cycle, keep in mind that there are many factors behind female athletes having a manyfold higher ACL-injury rate than their male counterparts. For quantification of knee-ligament properties, this article used only anterior-drawer laxity, as measured by the Lachmann Drawer Test via the KT-2000 device. The problem is that such testing is not sophisticated enough to accurately measure strength of the ligament. The main reason is that such testing necessarily involves forces much lower than those which tear ligaments. So, it is entirely possible for the ACL in a female during the luteal (high-estrogen) phase to have a lower maximum tensile stress than that of the same person during the rest of the menstrual cycle. The authors compare their study results to studies done on animals. (With animals, the advantage is that it is relatively easy to find the tensile strength of ligaments: all that needs to be done is to sacrifice the animal and place the ligament-and-bone assembly in a tensile-testing machine.) This study is, in many ways, similar to the one by Van Lunen, which can also be found here in the Knee Library. Belanger et al did find very small differences in knee laxity throughout the menstrual cycle, but the variation did not correlate unequivocally to any certain part of the menstrual phase. It must also be kept in mind that KT-2000 measurements are not really fraction-of-a-millimetre-precise anyway. Results are affected by the technique of the examiner (and in fact, whether the examiner is right-handed or left-handed makes a difference too, as Van Lunen also noted), and will naturally vary slightly over time (and so, taking KT-2000 measurements of a male athlete, over the course of a month, will probably also show a variation of about a millimetre or so). The real value of KT-2000 measurements lies in their comparative ability: that is, when examining an ACL-injured knee, it is necessary to compare the laxity in that knee to the laxity in the other knee, which is presumed to be uninjured. (Typically, only the difference in laxity is recorded.)

Association of Menstrual-Cycle Hormone Changes with Anterior Cruciate Ligament Laxity Measurements, Bonnie L Van Lunen et al.; Journal of Athletic Training, Dallas, Texas; Oct-Dec 2003; Vol 38/4, pages 298-304. Comments: This study examines laxity measured at the female knee (with a MedMetric KT-series arthrometer, which duplicates the manual Lachmann Drawer test) throughout the menstrual cycle, and it provides an excellent overview of previous research. It also gives good insight into the structure of the ACL, and the different types of collagen and their role. (The authors note that because the knee ligaments are alive, their structure is a consequence of the continual remodelling of the tissue in response to conditions. New collagen molecules are continually being synthesized, as old ones are dismantled. Type I collagen imparts greater mechanical strength to connective tissue, whereas type 3 collagen is responsible for tissue elasticity. The authors note that the relative decrease in type I procollagen synthesis with increasing estradiol concentrations may translate into ligament weakening, but as progesterone increases and estrogen is held constant, fibroblast proliferation and type I procollagen synthesis are increased significantly.) Note that this study does not investigate actual ACL-injury rates. The drawback with this type of study is that the amount of force exerted by the KT-series measuring device is unlikely to be enough to distinguish the ACL-weakening effect of the reproductive hormone estrogen. Furthermore, note that simply because estrogen tends to decrease the rupture strength of connective tissues (including knee ligaments such as the ACL) does not automatically guarantee that measurable laxity will be observable at the rather low stressing levels typical of manual Lachmann/anterior-drawer testing. This article should be read in conjunction with the studies by Wojtys (Sep/Oct 1998 and Mar/Apr 2002 issues of the American Journal of Sports Medicine; both articles are here in the Knee Library), given that Wojtys delves penetratingly into the connection between the female menstrual cycle and actual ACL-injury occurrence. Van Lunen et al. also note the ACL-injury incidence in female athletes is influenced by neuromuscular, structural, and biomechanical factors, in addition to the hormonal ones studied. (And, there are other hormones circulating in the body besides the reproductive ones.) They also point out that with regards to testing for knee laxity with the KT-series devices, the examiner (due to right-handedness or left-handedness) is often more comfortable with testing either the right or left limb; this, in turn, can significantly affect the readings. (Also, it is difficult to ensure that a given knee is at exactly the same flexion angle in each text, because the likelihood that a given person will always be lying with exactly the same positioning on the examination table [at examinations that are several weeks apart] is rather low. The tension in the ACL changes throughout the range of motion, and so the laxity measurements will be affected by knee flexion angle.)

Gender Differences in Surface Rolling and Gliding Kinematics of the Knee, John H. Hollman et al.; Clinical Orthopedics and Related Research, August 2003. Comments: This fascinating article will intrigue anyone who is interested in the fundamental research which targets the female-ACL-injury conundrum (i.e. why female athletes are 2-8 times as likely to incur ACL injuries as their male counterparts). Hollman et al. use only a modicum of basic mathematics to express the kinematics of the knee. This study shows that females very clearly have a different knee motion from that of males. This finding is to be expected, given that females are inherently more knock-kneed than males (on account of the childbirth-enabling wider pelvis). It seems safe to say that the female proclivity towards ACL injuries is due to a complex composite of interdependent factors, including not only biomechanics and musculoskeletal anatomy, but also vestibular-system firmware issues, hamstring-strength insufficiency, insufficient knee extension upon jump-landing (and also inwards-angled knees upon jump-landing, a problem which is exacerbated by knockkneedness), hormonal aspects, and other factors.

Effectiveness of a neuromuscular and proprioceptive training program in preventing the incidence of ACL injuries in female athletes: two-year follow-up, Bert R. Mandelbaum, Holly J. Silvers, et al.; Amateur Athletics Foundation, July 2003. Comments: This article shows the clear benefits attainable using a sedulously pursued injury-prevention-training program: approximately a halving of ACL-injury rate. The program studied was known as PEP (Prevent Injury, Enhance Performance). Because this training program is quite similar to other female-athlete knee-injury-prevention programs (most notably Cincinnati Sportsmetrics, as well as programs which follow in its wake, for example the Girls Can Jump series), the advantages of pursuing PEP would also be obtained by using these other programs.

A Comparison of Knee Kinetics between Male and Female Recreational Athletes in Stop-Jump Tasks, Jonathan D. Chappell et al.; The American Journal of Sports Medicine, Baltimore; March 2002, Vol 30, p. 261-267. Comments: This absolutely superb article is a must-read for all female athletes who have endured ACL injuries and for everyone who is interested in preventing such injuries. In this study, done in a biomechanics laboratory equipped with a three-dimensional video-camera system and floor-mounted force plates, the authors looked at the knee forces in athletes during rapid acceleration-deceleration activities associated with knee injuries. They found that during jump landing, females had a greater proximal anterior tibial shear force (i.e. the same type of anterior-drawer forcing as is used during the Lachmann and anterior drawer tests for ACL injuries). The authors also found that females, upong landing, also had greater knee extension and valgus moments (i.e. inwards torquing at the knee). The authors note that these findings show the females have different motor-control firmware setups than males, and that female-athlete knee-injury-prevention training should aim to reduce the proximal tibial anterior-shear forces at jump landing. Training for increased knee flexion and keeping the knees pointing somewhat outwards would also be helpful. Such training is already part of some training programs, most notably Cincinnati Sportsmetrics. Other good articles on these topics are Noyes-AJSM-Feb05.shtml, Ahmad-AJSM-Mar06.shtml, BarberWestin-AJSM-Mar06.shtml, and Mandelbaum-AJSM-Jul05.shtml.

The effect of the menstrual cycle on anterior cruciate ligament injuries in women as determined by hormone levels, Edward M Wojtys et al.; The American Journal of Sports Medicine, Baltimore, Maryland; Mar/Apr 2002, Vol 30/2, pages 182-189. Comments: This absolutely superb article provides penetrating insight into the effect of the female menstrual cycle on ACL-injury proclivity. (Estrogen is known to decrease connective-tissue strength, and it has other impacts on musculoskeletal components as well. This hormone is at very high levels during ovulation.) The authors note, however, that more research remains to be done. More hormones are involved than merely estrogen. And, the central nervous system is also involved (given that it controls musculature; this, in turn, also affects injury propensity). The reasons behind female athletes being 2-8 times as likely as their male counterparts to incur ACL injuries are clearly extremely complex. Although the authors of this study conclude that taking contraceptives seems to decrease the likelihood of female ACL injuries (yet they note that a larger group [in particular of the contraceptive users] would be needed in order to ensure statistical validity), it should also be kept in mind that pursuing a dedicated female-athlete knee-injury-prevention training program (which would include ballistic jump training, hamstring strengthening, and various proprioceptive exercises) is well worthwhile. Such training has been very well-proven to reduce the female athlete's predilection towards knee-ligament injuries. (In other words, the value of contraceptive-pill-popping is unproven, while injury-prevention training programs are a safe bet.)

Correlation of anthropometric measurements, strength, anterior-cruciate-ligament size, and intercondylar notch characteristics to sex differences in anterior-cruciate-ligament-injured athletes, Allen F. Anderson et al.; The American Journal of Sports Medicine, Baltimore; Jan/Feb 2001, Vol 29/1, p. 58. Comments: This article provides an interesting overview of the factors underlying the two-to-eight times higher incidence of ACL injuries in women. Their findings support the view that the sex difference in ACL tear rates is due chiefly to factors such as subtle structural/physiological and biomechanical differences, relative/proportional weaknesses of the hamstring and quadriceps muscles, and ligament size. The net result is that (even when body mass is taken into account) the female ACL is under greater stress than its male counterpart. Anderson et al. comment on the potential value of pursuing a strength-and-conditioning program at an early age.

Anterior Cruciate Ligament Injury in the Female Athlete, Lori Thein Brody; Knee Ligament Rehabilitation, edited by Todd S. Ellenbecker. Philadelphia, Pennsylvania: Churchill Livingstone (Harcourt), 2000. Pages 262-275. Comments: Female athletes are more prone (as compared against their male counterparts) to ACL tearing for a number of reasons, and many of these reasons are still being researched. Factors that have been implicated include more inwards-angled knees (due to the childbirth-enabling wider pelvis), more lax joints to begin with (especially knees that hyperextend grotesquely), vertibular-system-firmware attributes, the monthly hormonal cycle with its ligament-weakening estrogen spikes, a narrow intercondylar notch, proportionally weak hamstrings, and tendencies to land jumps with inadequate knee flexion. The author quote study findings that the likelihood of ACL injury is four times greater in females than in males, and also notes that a greater percentage of women’s injuries tend to be noncontact (i.e. planting-and-twisting of the knee). This finding is consistent with other studies that suggest jumping activities are a major source of ACL injuries in female athletes. Brody also discusses some of the research that has been done in the realm of female-athlete ACL-injury prevention, and that has spawned a number of injury-prevention training programs (such as Cincinnati Sportsmetrics). For example, instead of planting and cutting in order to change directions, an accelerated rounded-turn technique can be used. This guards against ACL-risky sudden deceleration and allows the athlete to continue accelerating through the turn instead of abruptly stopping and changing direction. Also helpful are learning bent-knee landings, especially given that females often tend to land with knees straight, thus ruinously hyperextending the knee. Another very helpful technique is to make a three-step stop when decelerating, as opposed to the standard single-step stop. This prevents sudden deceleration with a straight knee, another major cause of ACL devastation. (Griffis, one of the authors quoted here by Brody, found that these three techniques alone cut female-athlete ACL injuries by 89 percent.) This eminently readable, easy-to-understand and highly thought-provoking chapter is a must-read for any female athlete or parent of a female athlete. There are numerous interrelated factors germane to the four-fold greater incidence of ACL injuries in female athletes, and this article provides penetrating insight into these factors and their inherently complex nature. Knee-injury-prevention training can easily be incorporated into standard coaching regimens, but the training must go beyond standard strength-and-endurance exercises and canned sets of sports drills.

The effect of neuromuscular training on the incidence of knee injury in female athletes: A prospective study, Timothy Hewett et al.; The American Journal of Sports Medicine, Baltimore; Nov/Dec 1999, Vol 27/6, p. 699. Comments: Hewett et al. follow up diligently on Chandy and Grana's 1985-vintage recommendations of functional evaluation and training of knee-surrounding musculature. This study found that female athletes tend to have proportionally weak hamstrings, and that they tend to land jumps with inadequate knee flexion. Emphasis is placed on the importance of hamstring-strengthening exercises and plyometric (ballistic-jumping) training. These recommendations have been embodied in the renowned Cincinnati Sportsmetrics training program, with the result that female athletes who underwent said training experienced 2 to 8 times less ACL injuries (as compared to female athletes who did not undergo the training). The resounding success and widespread acceptance of Cincinnati Sportsmetrics has spawned a plethora of similar training programs, including the heavily promoted Girls Can Jump series.

Knee-joint laxity and neuromuscular characteristics of male and female soccer and basketball players, Susan L. Rozzi et al.; The American Journal of Sports Medicine, Baltimore; May/Jun 1999, Vol 27/3, p. 312. Comments: Rozzi et al. found that female athletes (when compared to their male counterparts) have proprioceptive deficits and excessive joint laxity. These athletes exhibit muscle-activiation patterns which, albeit adequate for obtaining stable joint functioning during sports, predispose them to severe knee injuries (e.g. via landing jumps with insufficient knee flexion). The authors note that training which preprograms desirable muscle-activation patterns (for example, plyometric training) can pay big dividends with regards to injury prevention.

Hormonal changes throughout the menstrual cycle and increased anterior cruciate ligament laxity in females, Ned A. Heitz et al.; Journal of Athletic Training; Dallas; Apr-Jun 1999; Vol 34/2, p. 144. Comments: This article provides good insight into the effect of female-menstrual-cycle hormones on connective-tissue strength. Heitz states that the greatest ACL laxity occurs during the luteal phase (i.e. days 15-28 in a 28-day menstrual cycle). The luteal phase is characterized by a rise in estrogen and progesterone levels, as well as a drop in follicle-stimulating hormone and luteinizing hormone. It appears that high levels of estrogen and progesterone cause increased ligamentous laxity.

Association between the menstrual cycle and anterior cruciate ligament injuries in female athletes, Edward M Wojtys et al.; The American Journal of Sports Medicine, Baltimore, Maryland; Sep/Oct 1998; Vol 26/5, pages 614-619. Comments: Wojtys et al. studied the connections between the female menstrual cycle and ACL-injury occurrence. They found a significant statistical association between the menstrual-cycle stage and the likelihood for ACL injury. They observed more ACL injuries during the ovulation phase, and fewer ACL injuries during the follicular phase. The authors note that further study is needed before unassailable conclusions can be drawn with regards to whether or not the connective-tissue-weakening estrogen spikes (which occur during ovulation) are the definitive, principal, directly causative factor in the high female ACL-injury incidence. (So far, the only sure-fire way for a female athlete to reduce her knee-injury risk is to diligently pursue exercises geared towards strengthening hamstring musculature, increasing knee-flexion angle during landing of jumps, and improving overall proprioception, balance, and co-ordination.)

A Rigorous Comparison Between the Sexes of Results and Complications After Anterior Cruciate Ligament Reconstruction, Sue Barber-Westin et al.; The American Journal of Sports Medicine, Baltimore; Jul/Aug 1997, Vol 25/4, p. 514-526. Comments: The staggeringly high incidence of ACL injuries in female athletes is something which many orthopedists bear in mind when confronted with knee-ligament injuries, and the issue continues to be an ongoing stimulus for clinical research. In this penetratingly insightful and thought-provoking study, Barber-Westin et al found that a gender-based selection bias for (or against) ACL reconstruction, as a function of complications and outcome, is not warranted. This means that whether the patient is male or female should have absolutely no bearing on whether or not a ruined ACL is considered amenable to reconstruction.

Plyometric Training in Female Athletes -- Decreased Impact Forces and Increased Hamstring Torques, Timothy Hewett et al.; The American Journal of Sports Medicine, Baltimore; Nov/Dec 1996, Vol 24/6, p. 765-773. Comments: This study shows that a ballistic-jumping-training program improved jump performance (10% increase in jump height) as well as jump biomechanics (e.g. improved side-to-side uniformity, lowered peak forces on the knee upon landing, and improved technique). Hewett et al. provide a thought-provoking and insightful discussion of jump kinematics and dynamics, and the review the research into the disparity in ACL-injury incidence in males versus females. They point out that the mechanical mechanisms that male athletes employ in order to compensate for high landing forces are different from those used by their female counterparts. (Remember that kinetic energy increases with the square of speed. So, if we double the speed of a given incident in which an athlete loses control, then the resulting injury would be expected to be four times as severe. ) Note: The term "adduction" denotes movement towards the centre-line of the body. The term "abduction" denotes movement away from the centre-line of the body.

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