Decreasing the Risk of Anterior Cruciate Ligament Injuries in Female Athletes

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Chapter 19 Decreasing the Risk of Anterior Cruciate Ligament Injuries in Female Athletes

Sue D. Barber-Westin, BS, Frank R. Noyes, MD

SCIENTIFIC RATIONALE AND SUPPORTING INVESTIGATIONS FOR SPORTSMETRICS NEUROMUSCULAR RETRAINING PROGRAM

The purpose of this chapter is to present the scientific rationale, supporting data, and specific strategies for the implementation of a neuromuscular knee ligament injury prevention–training program, Sportsmetrics. Although many knee ligament injury prevention–training programs have been published,* few have presented scientific justification that the training effectively improved neuromuscular deficiencies and reduced the incidence of noncontact anterior cruciate ligament (ACL) injuries in female athletes (Table 19-1). Some programs had small sample sizes and were thus underpowered to avoid the potential for a type II statistical error.^7,^11,^28,^29,^33,³⁵ Most studies were not randomized, and several did not contain a control group studied concurrently with a trained group. Although some investigations cited a reduction in noncontact ACL injury rates, others failed to find a statistically significant effect.^11,^28,^29,³³^–³⁵Some programs have been published even though the investigators did not follow athletes over a season or a period of time to determine whether a reduction in ACL injuries occurred as a result of preventive training. ^14,^16,²¹

TABLE 19-1 Anterior Cruciate Ligament Injury Prevention Training Programs

The Sportsmetrics program is effective in inducing changes in neuromuscular indices in female athletes, because studies have shown improved overall lower limb alignment on a drop-jump test,²⁴ increased hamstrings strength, ^13,^24,³⁶ increased knee flexion angles on landing,^13,³¹ and reduced deleterious abduction/adduction moments and ground reaction forces.¹³ Sportsmetrics also significantly reduces the risk of noncontact ACL injuries in female athletes participating in basketball, soccer, and volleyball.¹²

A pilot laboratory study was conducted at the authors’ institution using 11 high school female volleyball players to determine the effect of Sportsmetrics training on landing mechanics and lower extremity strength.¹³ The mean age of the female subjects was 15 ± 0.6 years, and they had participated in organized volleyball for an average of 2 ± 1 years before the study was initiated. A control group of 9 male subjects was selected and matched to the females according to height, weight, and age. The Sportsmetrics training program, described in detail later in this chapter, was performed 3 times a week (2-hr sessions) for 6 weeks. The athletes were taken through a series of tests before and after the training program. These tests included isokinetic muscle testing at 360°/sec and force analysis testing with a two-camera, video-based, optoelectronic digitizer for measuring motion and a multicomponent force plate for measuring ground reaction force. The subjects performed 10 jumps on the force plate that simulated volleyball jumping and landing maneuvers.

After 6 weeks of training, peak landing forces from a volleyball block jump decreased 22% (P = .006), or an average of 456N (103lb); all but 1 of the female subjects demonstrated a decrease in these forces (Fig. 19-1). Knee adduction and abduction moments, which induce a lateral or medial torque to the knee joint, decreased approximately 50% (Fig. 19-2). These moments were significant predictors of peak landing forces. The importance of this finding is that decreased abduction or abduction moments may lessen the risk of lift-off of the medial or lateral femoral condyle, improve tibiofemoral contact stabilizing forces,²⁰ and reduce the risk of knee ligament injury.

FIGURE 19-1 Bar graph shows decrease in peak landing forces with training in female subjects before and after training and relative to age- and weight-matched athletic male subjects. *P < .01.

(From Hewett, T. E.; Stroupe, A. L.; Nance, T. A.; Noyes, F. R.: Plyometric training in female athletes. Decreased impact forces and increased hamstring torques. Am J Sports Med 24:765–773, 1996.)

FIGURE 19-2 Bar graph shows peak knee adduction and abduction moment data at landing before and after training. The female subjects were grouped according to the dominant moment (adduction or abduction) and all the female subjects grouped together (all). *P < .05 compared with untrained females.

Female athletes demonstrated lower landing forces than male athletes, and lower adduction and abduction moments after training compared with pretraining values. External knee extension moments (which are balanced by an internal flexion, hamstring muscle) of male athletes were three times higher than those of female athletes. The training did not significantly alter these extension moments.

After training, hamstring-to-quadriceps muscle peak torque ratios increased 26% on the nondominant side and 13% on the dominant side, correcting side-to-side imbalances in the female athletes. In addition, hamstring muscle power increased 44% on the dominant side and 21% on the nondominant side. Whereas peak torque ratios of male athletes were significantly greater than those of the female athletes before training, the ratios were similar between genders after training.

A second study was conducted in a group of 62 high school female athletes to determine whether Sportsmetrics training altered lower limb alignment on a drop-jump task and isokinetic lower limb muscle strength.²⁴ This investigation is described in detail in Chapter 16, Lower Limb Neuromuscular Control and Strength in Prepubescent and Adolescent Male and Female Athletes. Before training, 80% of the female athletes had a valgus overall lower limb alignment on landing from a drop-jump. After training, 34% of the females demonstrated this alignment (P < .0001). Significant increases were also noted after training in knee flexion peak torque in both the dominant and the nondominant limbs (P < .0001).

In order to determine whether Sportsmetrics training reduced the incidence of noncontact ACL injuries in female athletes, a controlled, prospective investigation was conducted in 1263 high school athletes.¹² One group of 366 females underwent 6 weeks of Sportsmetrics training and a second group of 463 females was not trained. Included in the study was a group of 434 untrained male athletes. All athletes were followed throughout a single soccer, volleyball, or basketball season. Weekly reports submitted by athletic trainers included the number of practice and competition exposures and mechanism of injury. All ACL injuries were confirmed by arthroscopy, and medial collateral ligament (MCL) injuries were confirmed by manual valgus testing.

The total number of athlete-exposures were 23,138 for the untrained group, 17,222 for the trained group, and 21,390 for the male control group. There were 14 serious knee ligament injuries, which were sustained in 10 of 463 untrained female athletes (8 noncontact), 2 of 366 trained female athletes (0 noncontact), and 2 of 434 male athletes (1 noncontact). The knee injury incidence per 1000 athlete-exposures was 0.43 in untrained female athletes, 0.12 in trained female athletes, and 0.09 in male athletes (P = .02, chi-square analysis). Untrained female athletes had a 3.6 times higher incidence of knee injury than trained female athletes (P = .05) and 4.8 times higher than male athletes (P = .03). The incidence of knee injury in trained female athletes was not significantly different from that in untrained male athletes (Fig. 19-3; P = .86).

FIGURE 19-3 Serious knee injuries per 1000 player-exposures in soccer and basketball players.

The risk factors that have been hypothesized to increase the potential for an ACL injury are discussed in Chapter 15, Risk Factors for Anterior Cruciate Ligament Injuries in the Female Athlete. The final position of the knee joint on landing, pivoting, and cutting is influenced by the center of gravity of the upper body and the trunk over the lower extremity. Equally important are trunk-hip adduction or abduction, foot-ankle pronation-supination, and foot separation distance. These effects add together to produce either a varus or a valgus moment about the knee joint that must be balanced by the lower limb musculature. If the athlete is off-balance, or contacts another player, a loss of trunk and lower limb control and position may occur and the knee joint may go into a hyperextended or valgus position. Although there are many potential mechanisms for ACL injury, it has been postulated that excessive valgus moments about the knee joint with subsequent high anterior tibial shear forces may be one of the most common to occur in females.^8,^15,¹⁸ Excessive valgus loading may result in decreased tibiofemoral contact, or condylar lift-off,²⁰ and a reduction in the normal joint contact geometry that contributes to knee joint stability. This position, coupled with a highly activated quadriceps muscle that produces maximum anterior shear forces with the knee joint at low flexion angles (0°–30°),^5,^10,²⁶ and a relatively inactive hamstrings musculature, could potentially lead to ACL rupture. The function of the hamstrings is dependent on the angle of knee flexion and degrees of external tibial rotation.²⁵ The mechanical advantage of these muscles increases as the knee is flexed. At 0° of extension, the flexion force is only 49% of that generated at 90° of flexion. Withrow and coworkers³⁷ measured the relative strain in the anteromedial region of the ACL in cadaver knees during a simulated jump landing maneuver. Increased hamstring muscle forces during the knee flexion phase significantly reduced the peak relative strain in the ACL by greater than 70% compared with the baseline condition (P = .005). The authors concluded that it may be possible to limit ACL strain by accentuating hip flexion during knee flexion phase of jump landings because this increases the tension in the active hamstring muscles.

The time in which an ACL rupture occurs was recently estimated to range from 17 to 50 msec after initial ground contact.¹⁷ It is evident that there is not enough time for an athlete, upon sensing a knee injury about to occur, to alter the body or lower extremity position to prevent the injury. In order to have a significant impact on reducing the incident rate of this injury, a training program must teach athletes to control the upper body, trunk, and lower body position; lower the center of gravity by increasing hip and knee flexion during activities; and develop muscular strength and techniques to land with decreased ground reaction forces. In addition, athletes should be taught to pre-position the body and lower extremity prior to initial ground contact in order to obtain the position of greatest knee joint stability and stiffness. The program should consist of a dynamic warm-up, plyometric jump training, strengthening exercises, aerobic conditioning, agility, and risk awareness training.⁹

SPORTS INJURY TEST

The authors developed a sports injury test (SIT) in order to obtain a profile of each athlete before and after Sportsmetrics training. The SIT measures (1) knee and ankle position on landing and take-off from a drop-jump during a video test (described in detail in Chapter 16, Lower Limb Neuromuscular Control and Strength in Prepubescent and Adolescent Male and Female Athletes), (2) lower limb symmetry^1,²³ and a general assessment of lower limb control on single-leg hop tasks (Fig. 19-4), (3) isokinetic lower extremity muscle strength, (4) hamstring flexibility, and (5) vertical jump height. Athletes participating in sports-specific Sportsmetrics training programs, discussed later in this chapter, also undergo a 10-yard dash, a 20-yard dash, a T-drill run, and a 1-mile run. A history is collected for each athlete regarding prior injuries to the knees and ankles. The SIT is completed just prior to the initiation of training and within 1 week of completion of training.

FIGURE 19-4 Single hop for distance video screening allows a qualitative assessment of an athlete’s ability to control the upper and lower extremities upon landing, which may be rated as either good (A), fair (B), or complete failure, fall to ground (C).

SPORTSMETRICS NEUROMUSCULAR TRAINING PROGRAM COMPONENTS

The essential elements of the Sportsmetrics neuromuscular training program are a dynamic warm-up, plyometric jump training, strengthening, and flexibility. It is recommended that this training program be conducted either during the off-season or just prior to the beginning of the athlete’s sport season. The training is conducted 3 times a week on alternating days for 6 weeks.

As is described later, sports-specific speed and agility components may be incorporated into the training program if desired by an athlete or team. Sports-specific programs have been developed for soccer (Sportsmetrics Soccer), basketball (Sportsmetrics Basketball), and tennis (Sportsmetrics Tennis) players. A maintenance-training program (Sportsmetrics Warm-up for Injury Prevention and Performance [WIPP]) may be performed upon completion of the 6 weeks of formal Sportsmetrics to continue to instill the skills and techniques learned during the athlete’s sports season. For athletes who have already suffered an injury or had ACL reconstruction, Sportsmetrics Return to Play is recommended as end-stage rehabilitation training.

Training may be accomplished either in classes led by an instructor who has completed certification training from the authors’ foundation or from an instructional step-by-step videotape series. Athletes who do not train with a certified instructor are encouraged to perform the training either in front of a mirror or with a partner so that mistakes in technique, form, and body alignment may be detected and corrected. A list of certified instructors and sites is available at http://www.sportsmetrics.net.

Dynamic Warm-up

The purpose of the dynamic warm-up is to prepare the body for activity with functional-based activities that incorporate sports-specific motions. The goals are to raise core body temperature; increase heart rate; increase blood flow to the muscles; and improve flexibility, balance, and coordination. Upon completion of this warm-up, the athlete will be physically prepared for training. All of the exercises should be performed across the width of a court or field or for approximately 20 to 30 seconds.

1 Toe walk. The athlete walks up on the toes with the legs straight. The heel of the foot should not touch the ground and the hips should remain neutral throughout.

2 Heel walk. The athlete walks on the heels with the legs straight. The toes should not touch the ground, the knees should not lock, and the hips should remain neutral.

3 Straight leg march. The athlete walks with both legs straight, alternating lifting up each leg as high as possible without compromising form. The knees are kept straight and posture is erect; the body should not lean backward. The arms are swung in opposition.

4 Leg cradle (Fig. 19-5). The athlete walks forward, keeping the entire body straight and neutrally aligned. One leg is lifted in front of the body, bending at the knee. The knee is rotated outward and the foot inward. The foot is held with both hands, standing on one leg, and held in this position for 3 seconds. The foot is placed back down and the exercise is repeated with the opposite leg.

5 Dog and bush (hip rotator) walk (Fig. 19-6). The athlete is instructed to pretend that there is an obstacle directly in front of her or him. The athlete faces forward and keeps the shoulders and hips square. One leg is extended at the hip and the knee kept softly bent. The leg is externally rotated out at the hip and the knee is bent to approximately 90°. The leg is rotated and brought up and over the obstacle and then placed back on the ground. The exercise is repeated with the opposite leg.

6 High knee skip. The athlete begins to skip. One knee is driven up in the air as high as possible, while the other leg hops off the ground. As the athlete lands, the exercise is immediately repeated on the opposite side. The opposite arm of the high knee is swung up in the air to help gain height.

7 High knees. The athlete begins to jog forward. With each step, the knees are driven up as high as possible using short, choppy steps. The athlete remains facing forward, keeping the shoulders and hips square throughout the exercise.

8 Glut kicks. The athlete begins to jog forward. Each step, the athlete kicks the feet back as if trying to reach the gluts with the heel, using short, choppy steps. The athlete remains facing forward, keeping the shoulder and hips square throughout the exercise.

9 Stride out. The athlete begins jogging forward with an exaggerated running form. The knees are driven as high as possible and the feet are kicked back, as if trying to make a large complete circle with the legs. The athlete should stay up on the balls of his or her feet throughout the exercise.

10 All-out sprint. The athlete sprints forward as fast as possible while keeping proper technique and running form.

FIGURE 19-5 A–C, Leg cradle.

FIGURE 19-6 A–D, Dog and bush walk.

Plyometrics/Jump Training

In reference to ACL injury prevention–training programs, Griffin and associates ⁹ stated, “The rationale to include plyometric exercises is based on evidence that the stretch-shortening cycle activates neural, muscular, and elastic components and, therefore, should enhance joint stability (dynamic stiffening).” Plyometrics help develop muscle control and strength considered critical to reduce the risk of knee ligament injuries. These exercises are well known to increase muscular power, vertical jump height, acceleration speed, and running speed.^3,^4,³² However, if done improperly, these exercises would not be expected to have a beneficial effect in reducing the risk of a noncontact ACL injury. Therefore, the philosophy of the plyometric and jump-training component in the Sportsmetrics program is to place a major emphasis on correct jumping and techniques throughout the 6 weeks of training. Specific drills and instruction are used to teach the athlete to pre-position the entire body safely when accelerating or decelerating on landing. The selection and progression of the exercises entail neuromuscular retraining and proceed from simple jumping drills (to instill correct form) to multidirectional, single-foot hops and plyometrics with an emphasis on quick turnover (to add movements that mimic sports-specific motions). The jump training is divided into three 2-week phases. Each 2-week phase has a different training focus and the exercises change correspondingly (Table 19-2).

TABLE 19-2 Sportsmetrics Neuromuscular Training Program: Jump-Training Component

Phase 1, termed the technique development phase, aims to teach the athlete proper form and technique for eight different jumps. Four basic techniques are stressed: correct posture and body alignment throughout the jump (spine erect, shoulders back, chest over knees), jumping straight up with no excessive side-to-side or forward-backward movement, soft landings including toe-to-midfoot rocking and bent knees, and instant recoil preparation for the next jump. The athletes are taught these techniques through verbal queues from instructors such as “on your toes,” “straight as an arrow,” “light as a feather,” “shock absorber,” and “recoil like a spring.” Constant feedback is offered by instructors, and mirrors are used in the training sessions to provide direct visual feedback as frequently as possible. Throughout each session, exercises are increased by duration or repetition. If the athlete becomes fatigued and cannot perform the jumps with the proper technique, she or he is encouraged to stop and rest. Approximately 30 seconds of recovery time is allowed between each exercise.

Phase 2, called the fundamentals phase, continues to emphasize proper technique and quality jumping form. Six jumps from phase 1 are continued, but these are performed for longer periods of time. In addition, three new jumps of increased difficulty are incorporated. Phase 3, the performance phase, increases the quantity and speed of the jumps to develop a truly plyometric exercise routine. The athlete is encouraged to complete as many jumps as possible with proper form and to concentrate on the height achieved in each jump.

1 Wall jump (wk 1–6). Also known as ankle bounces, this jump is performed with the knees slightly bent and both arms raised overhead (Fig. 19-7A). From this position, the athlete bounces up and down off his or her toes (see Fig. 19-7B). The knees should be soft and the hips, knees, and ankles in neutral alignment. This jump is performed first to prepare the athlete mentally and physically for plyometric training. This also provides the trainer an opportunity to observe and begin positive feedback and instruction.

FIGURE 19-7 A and B, Wall jump.

Mistakes to correct include slouched posture, too much knee flexion, and eyes watching the feet with the head down. The athlete should be told to keep the eyes and head focused up, bend the knees slightly, and maintain neutral alignment.

2 Tuck jump (wk 1–6). The athlete begins in an upright neutral stance with the feet shoulder-width apart (Fig. 19-8A). The athlete jumps up, bending the knees together to bring the thighs up toward the chest as high as possible (see Fig. 19-8B).

FIGURE 19-8 A and B, Tuck jump.

Mistakes to correct include lowering the chest to the knees rather than lifting the knees to the chest, bringing the knees together during take-off or landing, double-bouncing between jumps, and producing a loud landing with a lack of muscle control. The athlete should be told to lift the knees up to the chest, control and keep the landing quiet, land on the balls of the feet, keep the knees bent when landing in order to go immediately into the next jump, keep the knees and ankles at shoulder/hip width at all times, keep the back straight, shoulders back, and keep the head/eyes up with each jump.

3 Squat jump (wk 1–6). The athlete begins in a fully crouched position as deep as comfortable (Fig. 19-9A). The knees and feet are directed forward and are in alignment with the hips. The upper body is upright with the chest open. The hands touch or reach toward the ground on the outside of the heels. The athlete jumps up, reaching as high as possible (see Fig. 19-9B), and then returns to the crouched position with hands reaching back toward the heels (see Fig. 19-9C).

FIGURE 19-9 A–C, Squat jump.

Mistakes to correct include landing with body/knees forward and/or off-balance, bringing the knees together during take-off or landing, and producing a loud landing with a lack of muscle control. The athlete should be told to keep the hands reaching back toward the heels (not forward), keep the knees tracking under the hips on take-off and landing, maintain the knees and ankles at shoulder/hip width at all times, keep the back straight and the shoulders back, and keep the head/eyes up with each jump.

4 Barrier jump side-to-side (wk 1–2). A cone or barrier approximately 6 to 8 inches in height is used. The athlete stands in a modified squat position (Fig. 19-10A). The athlete then jumps from one side of the barrier to the other side with the feet kept together (see Fig. 19-10B), and lands in the same amount of knee flexion as the starting position (see Fig. 19-10C).

FIGURE 19-10 A–C, Barrier jump side-to-side.

Mistakes to correct include landing or taking off with stiff, straight, or wobbly knees; jumping side-to-side quickly without bringing the entire body over the barrier; double-bouncing on landing and take-off; and not landing with the feet together. The athlete should be told to bend the knees up to clear the barrier, land quietly on the balls of the feet and rock back to the heels, control the landing to be able to immediately take off again, keep the back straight with the shoulders back, keep head/eyes up with each jump, keep the knees tracking under the hips and toes pointed forward on take-off and landing, and keep the feet parallel to each other.

5 Barrier jump forward-backward (wk 1–2). A cone or barrier approximately 6 to 8 inches in height is used. The athlete stands facing the barrier in a modified squat position (Fig. 19-11A). The athlete jumps forward and backward with the feet kept together (see Fig. 19-11B), and lands in the same amount of knee flexion as the starting position (see Fig. 19-11C).

FIGURE 19-11 A–C, Barrier jump forward-backward.

Mistakes to correct include landing or taking off with stiff, straight, or wobbly knees; jumping forward-backward quickly without bringing the entire body over the barrier; double-bouncing on landing and take-off; and separating the feet to clear the barrier. The athlete should be told to lift the knees up to the chest to clear the barrier, land quietly on the balls of the feet and rock back to the heels, control the landing to be able to immediately take off again, keep the back straight with the shoulders back, keep the head/eyes up with each jump, and keep the knees tracking under the hips and the toes pointed forward on take-off and landing.

6 180° jump (wk 1–2). The athlete begins from an upright neutral stance with the feet shoulder-width apart (Fig. 19-12A). The athlete jumps with both feet straight up into the air and makes a 180° turn in midair (see Fig. 19-21B) before landing (see Fig. 19-12C). The landing is held for two seconds, and then the direction is reversed and the jump repeated.

FIGURE 19-12 A–C, 180° jump.

Mistakes to correct include over- or underrotating; not turning the body 180°; not rotating the body as a unit; landing loud, straight, stiff-legged, with staggered feet or one foot landing before the other; always jumping in the same direction (in a circle); rotating back and forth with minimal height during jump; and separating the feet beyond the desired “feet shoulder-/hip-width” distance.

The athlete should be told to jump straight up and rotate the body as a unit from the head to the toes; land with soft, slightly flexed knees; always jump in opposing directions (one jump over the right shoulder, the next over the left); keep the knees tracking under the hips on take-off and landing; maintain knees and ankles at shoulder/hip width at all times; keep the back straight with the shoulders back, and keep the head/eyes up with each jump.

7 Broad jump (wk 1–2). The athlete starts from an upright neutral stance (Fig. 19-13A) and jumps forward as far as possible (see Fig. 19-13B), taking off with both feet. The athlete lands on both feet, remains in a deep crouch position (see Fig. 19-13C) for 5 seconds, and then repeats the jump.