Chapter objectives
- •
Explain the biomechanical and neuromuscular factors that predispose females to lower extremity injury.
- •
Relate biomechanical and neuromuscular factors to the lower extremity injuries more commonly experienced by females.
- •
Describe specific rehabilitation interventions targeting the neuromuscular and biomechanical risk factors associated with lower extremity injury in female athletes.
Women have become more involved in both recreational and competitive sports since enactment of Title IX and are therefore receiving more attention in the sports medicine literature. Recent studies suggest that some lower extremity injuries are encountered more often by female athletes. Most impressive is the two to eight times higher risk for noncontact anterior cruciate ligament (ACL) tears in females than in male basketball and soccer players. Women runners are also reported to sustain twice as many overall lower extremity injuries as their male counterparts. In addition, studies indicate that stress fractures and patellofemoral dysfunction are twice as likely to develop in female runners as male runners. The injury patterns in women may be a consequence of structural, mechanical, neuromuscular, hormonal, or some combination of these factors.
Rehabilitation programs for the injured female athlete, in addition to resolving impairments, should include therapeutic exercises to alter the factors that may have led to the injury. Although it may not be possible to alter structure, normalization of mechanical and neuromuscular problems is necessary to decrease the possibility of reinjury. In this chapter, an overview of the structural, biomechanical, and neuromuscular factors that may predispose females to injury and a summary of gender differences in movement are presented, followed by descriptions of specific rehabilitation programs and techniques that may be useful in minimizing the influence of these factors.
Gender differences
Structural Differences
Several differences in lower extremity structure have been noted between males and females ( Fig. 9-1 ). When compared with men, women exhibit greater amounts of static external knee rotation alignment and active hip internal rotation, which may result in greater rotational motion and work at both the hip and knee. In addition, women have been shown to exhibit a greater interacetabular distance and increased hip width when normalized to femoral length than seen in men. Greater pelvic width and increased knee external rotation have been suggested to contribute to greater amounts of standing genu valgum alignment in women than in men (see Fig. 9-1 ). In addition, the structural combination of increased hip adduction, femoral anteversion, and genu valgum may explain, in part, the well-documented larger Q angle in women than in men ( Fig. 9-2 ).
A more valgus knee position has been suggested to increase frontal plane motion at the knee and increase the risk for lower extremity overuse injuries. Cowan et al found that subjects with a Q angle greater than 15° had a significantly increased risk for overuse injury and that the risk for lower extremity stress fractures was significantly higher in participants with the most valgus knee positions. An increased knee valgus angle also increases the Q angle, which is thought to lead to patellofemoral disorders (see Fig. 9-2 ). Mizuno et al investigated the relationship between Q angle and patellofemoral kinematics. Through manipulation of the Q angle in vitro, these authors reported that a larger Q angle may lead to greater lateral patellar contact forces and increase the potential for patellofemoral pain syndrome and lateral patellar dislocations. In support of this hypothesis, runners with patellofemoral pain were found to exhibit a significantly greater Q angle than did a group of healthy control subjects.
Intercondylar notch size and shape ( Fig. 9-3 ) have been proposed as a potential contributor to the greater incidence of ACL injury in women. It has been suggested that women have a narrower notch width than men do, and some investigators have reported that this contributes to a smaller and potentially weaker ACL. Lund-Hanssen et al found that women with a narrow notch width were six times more likely to rupture their ACL than were women with larger notch widths. However, more recent work has suggested that there are no differences in intercondylar notch width between males and females. Ireland et al performed a retrospective investigation and measured the notch width of males and females with and without ACL injuries and reported that smaller notch widths were associated with previous ACL injury regardless of gender or notch shape. Thus, further investigation is warranted to better determine the influence of femoral intercondylar notch width and shape on the incidence of ACL injury.
Gender differences in lower extremity structure include increased pelvic and hip width, static knee external rotation, genu valgum, increased Q angle, and decreased intercondylar notch size in females.
Differences in Mechanics
It is believed that structural differences may lead to different movement patterns, which in turn place women at risk for injury in comparison to their male counterparts. Although relatively few studies have been performed in the area of gender differences in lower extremity mechanics during functional tasks, the studies that have been conducted show that females may perform some tasks differently from males. The following sections address each of these activities with respect to possible gender-related differences.
Running
Gender differences during running have received little attention in the scientific literature, but the studies that have been conducted show that women have different running mechanics than men. Similar to the results of landing studies (see later), women tend to run with less knee flexion. Women also exhibit a significantly greater knee valgus angle throughout stance than men do and demonstrate significantly greater peak hip adduction and hip internal rotation angles, which may be the result of a greater Q angle. Hip frontal and transverse plane negative work is greater in women than in men, which suggests that the muscles controlling hip adduction and internal rotation are under greater eccentric loading while running. These gender differences in running mechanics may lend insight into the greater incidence of specific lower extremity injuries seen in women, such as patellofemoral pain.
The greater femoral internal rotation and tibial external rotation exhibited by female runners may also place them in a biomechanical dilemma with respect to the patellofemoral joint. Tiberio suggested that excessive femoral internal rotation might result in malalignment of the patellofemoral joint and lead to anterior knee pain. It has also been suggested that abnormal tibial rotation results in subsequent interruption of the normal tibiofemoral rotational relationship and an alteration in normal patellofemoral mechanics. The increased femoral internal rotation observed in female athletes, coupled with the increased tibial external rotation position, may result in a greater Q angle, thereby increasing the risk for patellofemoral disorders in females.
The hip adduction and hip internal rotation angles and the frontal and transverse planes work are greater in females than in males during running.
Cutting
Another common noncontact mechanism of injury is performance of a cutting maneuver while running. It has been suggested that when the knee approaches a valgus position, the load experienced by the ACL may be five times greater than that when the knee is aligned in the frontal plane. Women tend to exhibit less knee flexion and greater knee valgus during side-cutting and cross-cutting tasks, which in turn may place the knee in a position in which significant ACL strain can occur. No differences were found between males and females in knee rotation during cutting; however, females have greater intertrial variability in femoral rotation patterns. This amount of variability was most strongly influenced by the level of experience, with less experienced female athletes exhibiting greater knee rotation variability. Thus, it may be possible to train female athletes to use specific lower extremity movement patterns that do not predispose them to lower extremity injury or reinjury.
During cutting maneuvers, females exhibit greater dynamic valgus and a dependence of knee rotation variability on the level of experience in performing cutting.
Landing
Landing from a jump results in forces between 3 and 14 times body weight that must be attenuated by the lower extremity. One of the most common noncontact mechanisms of lower extremity injury is landing from a jump, and it has been reported that women are more likely than men to injure themselves during sports that involve jumping and landing. Huston et al studied the landing patterns of males and females and reported that females land with less knee flexion than men do and thus experience greater ground reaction force vertical loading rates. Specifically, women experienced a 9% increase in loading rate per unit of body weight when compared with men. Lephart et al found that females land with less knee flexion and with greater femoral internal and tibial external rotation than males do. The decreased amount of knee flexion exhibited by females may reduce their ability to attenuate the impact forces experienced during landing. Furthermore, because ACL strain has been found to be greatest when the knee is near or at full extension, the extended knee position during landing may predispose female athletes to greater ACL strain. Recent video analysis of the mechanics of ACL injury reveals four common components, especially apparent in women. As a female athlete lands, her knee buckles inward into a valgus position, the injured knee is relatively straight, and her trunk tends to be tilted laterally with the center of mass displaced from the plantar aspect of the foot. Female athletes display different neuromuscular strategies than do male athletes during landing. These gender differences in muscle recruitment and timing of muscle activation may affect dynamic lower extremity stability and contribute to injury.
Females experience higher ground reaction force and have increased femoral rotation motion during landing.
Neuromuscular Differences
Ligament Dominance
A potential contributor to the greater incidence of lower extremity injury in females than in males is the gender difference in joint stability as a result of active muscle stiffness. Stiffness has been defined as the resistance exerted by the soft tissue structures in response to a force that may result in joint stress, and active joint stiffness can be modulated through voluntary muscle contraction. In addition, it has been suggested that individuals who are less able to voluntarily contract their muscles in response to an external force or perturbation may be more likely to sustain an injury. Wojtys et al used a dynamic stress test to measure anterior tibial translation and simultaneous muscular response to an externally applied stress. It was reported that muscle cocontraction significantly decreases anterior tibial translation in both men and women. However, men exhibited significantly greater stiffness than women, thus suggesting that women have a reduced ability to actively protect the knee in response to an anteriorly applied force. Other studies measuring active muscle stiffness in males and females during isometric knee flexion and extension contractions and during functional hopping tasks have been conducted. During either task, females demonstrated reduced active muscle stiffness in comparison to males. Thus, it is possible that reduced active muscle stiffness may lead to reduced joint stability and predispose females to lower extremity joint injury. Because the muscles in female athletes do not adequately absorb ground reaction forces, the joints and ligaments are forced to absorb high amounts of force over short periods. This ligament dominance creates higher impulse forces and probably results in ligament injury. Ligament dominance is characterized by the use of anatomic bone structures, articular cartilage, and ligament to absorb ground reaction forces during athletic maneuvers rather than the active musculature of the lower extremity. Especially important for lower extremity dynamic control is the musculature of the posterior kinetic chain (gluteals, hamstrings, gastrocnemius, and soleus).
Quadriceps Dominance
Considerable attention has been focused on how the muscles surrounding the knee joint react during joint loading and to unexpected perturbations. This area of research is particularly important because failure of the ACL occurs when large mechanical loads exceed the capacity of the stabilizing ligaments. In addition, whereas the ACL provides significant static restraint to anterior tibial translation, active muscle recruitment can assist the ACL in maintaining joint stability and preventing injury. Thus, gender-related differences in muscle activation in response to joint loading and unexpected perturbations may shed light on the gender bias of lower extremity injury. Previous work by Huston and Wojtys indicated that female athletes initially use their quadriceps muscles for knee stabilization in response to anterior tibial translation whereas female nonathletes and males initially rely on their hamstring muscles. In addition, female athletes took significantly longer to generate maximum hamstring muscle torque during isokinetic testing than did males. Females, unlike males, do not increase hamstring-to-quadriceps torque ratios at velocities approaching those of functional activities. Female athletes exhibit reduced hamstring and greater quadriceps electromyographic activity during landing, running, side-cutting, and cross-cutting tasks. These muscle recruitment patterns, in combination with less knee flexion and greater loading rates exhibited by female athletes, certainly increase the potential for ACL strain. The combination of these muscle activation patterns may produce excessive strain on the ACL. In addition, it has been reported that females exhibit hamstring muscle onset that is less coincident with anterior tibial shear forces during landings, thereby increasing the potential for ACL injury.
Leg Dominance
Female athletes tend to be more one leg dominant than their male counterparts. Leg dominance refers to a measurable imbalance between right and left lower extremity muscle strength or relative recruitment. This imbalance is particularly important in tasks that normally require side-to-side symmetry of the lower extremities. In single-leg jump tasks, it is known that most athletes have a favored plant or kick leg. However, the difference between limbs in muscle recruitment patterns, muscle strength, and muscle flexibility tends to be greater in women than in men. Hewett et al have shown that those who have greater asymmetry in these force and torque profiles have greater risk for future injury and should be targeted for interventional neuromuscular training to equalize bilateral lower extremity muscular recruitment, strength, and flexibility.
Trunk Dominance
Impaired trunk neuromuscular control, or trunk dominance, appears to increase the risk for lower extremity injury in female athletes. Prospective evidence suggests a correlation between trunk neuromuscular control and lower extremity injury. In studies performed by Zazulak et al in which the association between trunk neuromuscular control and injury was examined in 277 collegiate varsity athletes, trunk proprioception and trunk displacement after quick release predicted risk for future ligament injury in female athletes but not in male athletes. Trunk dominance may be exaggerated by growth and maturation factors. After puberty, females have increased trunk mass with a higher center of gravity but do not experience a “neuromuscular spurt” of muscular development and increase power as males do. Video analysis of the mechanism of knee injuries reveals a common position of trunk leaning, with the center of mass being displaced from the plantar aspect of the foot, thus indicating an inability to precisely control the trunk in three-dimensional space.
This excessive uncontrolled lateral motion creates medial-lateral torque on the knee, which increases the risk for injury.
Neuromuscular responses that may predispose females to injury include ligament dominance, quadriceps dominance, leg dominance, and trunk dominance.
Hormonal Influences
Although not a structural difference, the most obvious differences between males and females are reproductive hormones and the menstrual cycle exhibited by women. The menstrual cycle ranges from 24 to 35 days, with an average of 28 days, and can be broken down into three phases with varying levels of reproductive hormones within each stage. The first phase is the menstrual phase (days 1 to 5) and is marked by low estrogen and progesterone levels. The follicular phase (days 6 to 13) is marked by rising levels of estrogen and leads to ovulation, which occurs immediately after a surge in estrogen. Finally, the luteal phase lasts approximately 14 days, during which progesterone is the dominant hormone, but estrogen is also increased to approximately half the value of the late follicular phase surge.
The physiologic link between surges in estrogen levels and ACL laxity has been the topic of recent research because some studies have identified estrogen receptors on the human ACL. Thus, it has been suggested that based on in vitro data, the higher rates of ACL injury in women may be due to decreased fibroblastic proliferation and decreased procollagen synthesis in the human ACL caused by increases in estrogen concentration. Whether increasing levels of estrogen cause females to have a greater risk for ACL injury remains debatable. A systematic review of the literature analyzed the effect of the menstrual cycle on anterior knee laxity. Six of nine studies reported no significant effect of the menstrual cycle on anterior knee laxity in women. However, three studies observed significant associations between the menstrual cycle and anterior knee laxity. These studies all reported that laxity increased during the ovulatory or postovulatory phases of the cycle. A metaanalysis that included data from all nine reviewed studies corroborated this significant effect of cycle phase on knee laxity (F value = 56.59, P = .0001). In the analyses, the knee laxity data measured at 10 to 14 days were greater than that at 15 to 28 days, which in turn were greater than that at 1 to 9 days. Although hormone confirmation was provided in many of the studies that selected specific days to depict a particular cycle for all women, it is unknown from these data whether they truly captured times of peak hormone values in all women. This combined systematic review and metaanalysis of the literature indicate that the menstrual cycle may have an effect on anterior-posterior laxity of the knee; however, further investigation is needed to confirm or reject this hypothesis.
Several studies have investigated the effect of the menstrual cycle on ACL injury. In a prospective study, Myklebust et al reported that 14 women suffered injury in the late follicular or menstrual phases in a group of 17 female handball players with ACL injuries. The most notable limitation of this study was the lack of stratification of the athletes taking oral contraceptives. However, a more recent study by this same group analyzed the distribution of ACL injuries in 46 subjects and found that most ACL injuries occurred during the menstrual phase. Arendt et al corroborated the trend of increased ACL injury during the first half of the menstrual cycle in two studies that retrospectively studied collegiate female athletes. Wojtys et al also attempted to determine the menstrual phase in which ACL injury occurred by using a retrospective approach up to 3 months after injury. These authors reported a significantly higher frequency of ACL injury than expected during the late follicular phase and a significantly lower frequency of ACL injury than expected in the early follicular phase. However, the phase of the menstrual cycle was self-reported in these investigations and not confirmed by direct hormone measurements. More recently, Slauterback et al studied the relationship between ACL injury and serum estrogen levels in women within 48 hours after ACL injury. These authors stated that a significant number of ACL injuries occurred on days 1 and 2 of the menses, when estrogen levels were lowest. A systematic review of these seven studies revealed a consistent trend of increased ACL injury during the preovulatory phase of the menstrual cycle.
The presence of estrogen receptors on the ACL and the prevalence of ACL injuries in certain phases of the menstrual cycle may indicate a link between hormones and injury in female athletes.
Summary
Many differences exist between males and females, and the differences appear to influence injury patterns in females. Females have a different structural alignment of the lower extremities, and this alignment may predispose them to specific injuries. The structural alignment also has the potential to influence mechanics, thus further predisposing females to specific injuries. Although relatively little information is available on gender differences during functional, sport-specific activities, it appears that females may perform tasks differently, regardless of their alignment, and the manner in which they perform these tasks may make them susceptible to injury. Two areas receiving greater attention in recent years are the influence of neuromuscular responses and hormones on injuries in female athletes. Further research is warranted to determine the magnitude of their influence.
Screening for the high-risk athlete
Measures related to lower extremity valgus angles at the knee were found to be highly predictive of risk for noncontact ACL injury in female athletes. More recently, biomechanical measures during landing studied in subjects after ACL reconstruction were found to be predictors of a second ACL injury in athletes after being released to return to sport. Furthermore, Zazulak et al discovered that measures of trunk proprioception and displacement are predictive of knee injury with high sensitivity and moderate specificity . The predictive screening tools used in these studies included motion analysis to measure lower extremity kinematics, stabilimetry to measure postural sway, a trunk proprioception apparatus that measures the athlete’s ability to detect trunk rotation, and a trunk quick-release apparatus that measures trunk displacement. These are sophisticated laboratory tools that many clinicians may not be privy to. Hence, Hewett et al developed a clinic-based neuromuscular screening tool. The assessment algorithm identifies female athletes with neuromuscular deficits in the clinic or in the field ( Fig. 9-4 ). This algorithm delineates five biomechanical factors, including tibia length, knee valgus motion, knee flexion range of motion, mass, and quadriceps-hamstring ratio, and has been validated by the highly accurate laboratory assessment instruments. Although a valuable tool, it does not readily allow immediate feedback for correction of neuromuscular deficits.
The functional performance and technique assessment is a useful clinician-friendly, real-time, field-based tool for identifying the four neuromuscular flaws that can occur during a functional task. The criteria evaluated are knee and thigh motion, foot position during landing, and plyometric technique. The athlete performs repeated tuck jumps for 10 seconds, which allows the clinician to visually grade the criteria. The athlete may initially focus cognitive efforts on simply completing the difficult task, so faulty technique can readily be revealed to the examiner. An assessment tool was developed to allow clinicians to monitor the athlete’s performance before, during, and after training ( Fig. 9-5 ). The athlete’s deficits are rated as present (checked) or not and then tallied for a total score. The deficits found should be the focus of correction, and improvement can be tracked by repeated assessments at the midpoint and conclusion of training or rehabilitation programs. If improvements are made in neuromuscular control and biomechanics during the tuck jump and landing sequence, the learned skill may be transferred to competitive play. Empiric laboratory evidence suggests that athletes who demonstrate six or more flawed techniques should be targeted for further training in technique. Pilot work has shown intrarater reliability of the tuck jump assessment to be high, r = 0.84 (range, 0.72 to 0.97).
Rehabilitation interventions
The majority of rehabilitation programs developed specifically for female athletes have been designed to decrease risk for ACL injury. Although these programs have been designed with injury prevention in mind, the principles behind these programs can be implemented during postinjury rehabilitation for females as a means of preventing future injury. This section presents specific exercises, along with published programs, that can be implemented during rehabilitation for the injured female athlete to address the four predominant deficits in neuromuscular control: ligament dominance, quadriceps dominance, leg dominance, and trunk dominance.
Core Stability Training
Core stability training is gaining popularity in rehabilitation as clinicians become more aware of the influence of weakness in the “core” of the body on lower extremity mechanics and performance. The lumbar, pelvic, and hip region together are considered to be the core of the body and are collectively called the lumbopelvic-hip complex (LPHC). Optimal core function involves both trunk mobility and stability. When the core is functioning efficiently, advantageous length-tension relationships are maintained that allow the athlete to produce strong movements in the extremities. Core stability may also be important in allowing an athlete to maintain the center of gravity over the base of support.
Core stability training may be particularly important for the female athlete because weakness in the core could alter posture, thereby exacerbating factors that are believed to contribute to injury. For example, weakness of the hip abductors and external rotators could result in greater hip adduction and femoral internal rotation, which could contribute to increased knee valgus and thus possibly result in patellofemoral injury. In addition, weakness of the gluteal musculature has been hypothesized to cause tightness in the tensor fasciae latae and a more erect hip and trunk, which could result in greater loads across the knee. To create a comprehensive core stability training program, the practitioner must first understand the functional anatomy of the core.
A weak and underrecruited core may result in inefficient movements and altered postures that can lead to injury.
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
