Keywords

1 Introduction

Core stability is a popular term in both the scientific literature and popular media because it is almost universally believed that better core stability will result in enhanced performance as well as improve injury prevention and treatment strategies [1, 2]. However, the term itself is vague and is used in various ways in the literature, making it difficult to synthesize the current state of knowledge in this area. In order to properly discuss core stability and its relation to anterior cruciate ligament (ACL) injury, the terms core, stability, and core stability must first be defined.

1.1 Core Defined

The extent of the core region of the body itself has not been defined consistently across the literature. Some authors include all of the muscles crossing the hip joint, lumbar spine, and inferior thoracic spine, colloquially referred to as nipples to knees. This latter definition may be too broad to isolate and examine function, because in sports where ACL injury risk is high, the hip muscles often act as prime movers of the lower extremity, while the muscles crossing the lumbar spine primarily act to stabilize the spine in a relatively static position. For example, the gluteal muscles contribute significantly to total power generation and absorption in jumping, landing, and lateral sliding, while the hip flexors contribute to kicking. In contrast, the muscles crossing the lumbar spine limit its motion, thereby allowing the athlete to maintain proper posture and appropriately transfer energy between the arms and legs. Therefore, we define the core as the region of the body bounded by the pelvis and diaphragm which includes the muscles of the abdomen and lower back. By this definition, the muscles of the core are responsible for the position and movement of the trunk.

1.2 Core Stability Defined

According to the Merriam-Webster Dictionary, stability may be defined as “the property of a body that causes it when disturbed from a condition of equilibrium or steady motion to develop forces or moments that restore the original condition” [3]. It follows, then, that “core stability” is the ability of the core, or the muscles of the abdomen and lower back, “to maintain or resume a relative position [or trajectory] of the trunk after a perturbation” [4]. This perturbation can come from sources external to the body (such as other players, obstacles, or equipment) or from movement of the extremities. Core stability is a dynamic quality rather than an intrinsic property of the system; that is, the muscles involved must continually react to changing loading conditions and postures to maintain stability [5]. To achieve this stability and maintain position or trajectory in the presence of external forces that are continuously changing during athletic maneuvers, appropriate strength, endurance, and muscle activation timing and intensity are all required. Without any one of these attributes, core stability cannot be efficiently maintained.

The core is responsible for positioning the trunk and upper body over the lower extremity. Since the trunk and upper body account for more than half an individual’s body weight [6], poor trunk control and core stability could place this mass in a position that results in adverse loading of the knee, leading to injury (see Sects. 10.4 and 10.5 for further discussion). Core stability may also contribute to athletic performance, by providing “proximal stability for distal mobility” [1]. Hodges and Richardson demonstrated that core muscle activation precedes activation of muscles responsible for moving the lower extremities [7]. This sequence of muscle activation has, therefore, been thought to provide a stable base for limb movement, making the movement more efficient and effective. This theory is further supported by recent studies of professional baseball pitchers by Chaudhari et al. which showed that lumbopelvic control was associated with pitching performance [8] and risk of time-loss injuries [9]. Pitchers with better control of the lumbopelvic region in the first study pitched significantly more innings and had significantly fewer walks plus hits per inning pitched over one season [8]. In the second study, pitchers with the poorest lumbopelvic control were 3.0 times as likely to miss 30 or more days due to injury during the season [9]. While these data supported the connection between core and upper extremity movement, they also suggest that the role of the core in providing that same stable base for better lower extremity control and reduced ACL injury risk merits further exploration.

Critical Points

  • Core: the region of the body bound by the pelvis and diaphragm, which includes the muscles of the abdomen and lower back.

  • Core Stability: the ability of the core to maintain or resume relative position of the trunk after a perturbation or disturbance.

  • Core stability is a result of the combined effects of strength, endurance, and muscle activation timing and intensity.

  • Concentration of half the body’s mass in the upper body creates a theoretical basis for the core to contribute to both function and lower extremity injury risk.

2 Traditional Core Assessments and Training

Because strength, endurance, and activation patterns all contribute to core stability, measurements and training related to core stability vary widely in the literature and in practice. Due at least in part to the lack of appropriate tools to measure stability in the clinical setting, most assessments of the core have focused instead on strength and endurance. Both the United States Army Physical Fitness Test and Presidential Physical Fitness Test quantify how many times a subject can perform a sit-up in a given amount of time [10, 11]. The Army Physical Fitness test determines how many sit-ups a soldier can perform in 1 min, while the Presidential Physical Fitness test measures how many curl-ups or partial curl-ups a subject can perform in 1 min. These tests theoretically require both strength and endurance of the core to achieve a high score, but they do not provide any objective assessment of the muscle activation patterns that would contribute to core stability.

Considering the primary role of the core in maintaining postural stability (i.e., holding proper posture over time), several researchers have employed tests where subjects must maintain a static position against gravity for as long as possible. Variations of trunk endurance assessment include the prone-plank, side-bridge, and flexor endurance tests (Fig. 10.1) [12, 13]. In the clinical setting, these tests are relatively easy to administer and ensure that subjects are using proper technique. In the prone-plank test, subjects lie prone (face down) with their feet and/or legs secured to a table. The upper body is cantilevered off the edge of the table and parallel to the floor. During the side-bridge test, subjects support themselves on their feet and one elbow, keeping the body in a straight line with the supporting elbow side facing down. Finally, during the flexor endurance test, subjects sit with knees and hips flexed at approximately 90° and their upper body 60° from the test bed. For each endurance test, subjects are instructed to hold the desired position as long as possible, and the hold time is recorded.

Fig. 10.1
figure 1

Prone-plank (a), side-bridge (b), and flexor endurance (c) tests used to assess trunk endurance

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Positional control of the lumbar spine, another aspect of core stability, is also commonly tested in a supine position (lying face up) using what is commonly known as the “Sahrmann test” [14]. In this test (Fig. 10.2), an air bladder is placed under the lumbar curve, and changes in air pressure are observed while subjects perform arm and leg raises of increasing difficulty [13,14,15]. Subjects are instructed to maintain the curve throughout the movements. Increased or decreased air pressure in the bladder indicates a lack of control of spine movement. The test can also be performed with the tester’s hand substituted for the air bladder, where the tester qualitatively assesses whether the lumbar spine rises off or presses into the table. This test comes closest to measuring core stability as defined in Sect. 10.1, but it has the drawback of only measuring core stability in the supine position for specific, controlled movements. Nevertheless, this test has become extremely common in the clinical setting because it is easy to administer and can be performed by all patients, even if they lack the core strength to perform the previously mentioned sit-ups or endurance tests.

Fig. 10.2
figure 2

Clinical test of supine trunk control. An air bladder is placed under the lumbar curve (red arrow) and inflated to a known pressure as indicated by a pressure gauge (green arrow). Subjects perform arm or leg lifts while attempting to maintain the curve, and increased or decreased air pressure in the bladder indicates a lack of control of spine movement. This apparatus can also be used as biofeedback during training exercises where the goal is to minimize changes in pressure

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By design or coincidence, typical training programs for core stability have followed similar principles to the abovementioned tests for core stability. In the US Army, soldiers train with sit-ups to be able to perform well on their regular fitness tests, in spite of the recommendations to limit sit-ups due to dangerously high spinal compression loads, especially when performed for speed [16,17,18]. In other settings, similar movements have been popular in training regimens throughout sports and in the general population, so much so that hundreds of products are currently available on the market that tout their ability to help people improve their abdominal strength and endurance by assisting the individual to perform the exercise correctly even if he/she lacks the strength and endurance to perform unassisted sit-ups [19].

Training programs for low back rehabilitation and for running injury prevention often focus on stabilization exercises similar to the prone-plank, side-plank, and flexor endurance tests [20,21,22], though hold times are not usually the main objective in these cases. Typically challenging progressions in the exercises include the addition of limb movement and decreased support or increased weight while keeping the requirement of a stable, fixed trunk [22,23,24]. Increasingly difficult leg raises are often incorporated into exercise programs in conjunction with the bladder to provide feedback on lumbar position to increase strength and control of the core [20, 25]. In the latter stages of training for performance, athletes are asked to simultaneously move their upper and lower extremities with increasing range of motion and velocity while maintaining that stable, fixed trunk [21].

Several successful ACL injury prevention programs have included components of core stability training [26,27,28,29,30,31,32,33]. However, it remains controversial whether improving core stability is a necessary component to reduce lower extremity injury risk, as well as whether improving core stability can be effective in isolation or only when incorporated into a comprehensive injury prevention program. A meta-analysis of ACL injury prevention programs [34] concluded that preventative neuromuscular training programs that included proximal control exercises were more effective at reducing ACL injury rates than programs that did not include these exercises (odds ratio, 0.33 and 0.95, respectively). The proximal exercises in the training programs varied from sit-ups and push-ups to upper body resistance training. In addition to the proximal exercises, these training programs also incorporated exercises focused on balance, plyometrics, and strength. While a reduced incidence of ACL injuries was observed with these programs, it is impossible to attribute the reduction of injury to any single component of the exercise programs because they were designed in a trial-and-error fashion rather than by systematic addition/removal of individual exercises to determine each exercise’s role on injury incidence or functional changes. Nevertheless, the evidence accumulated through an increasing number of intervention studies that have differences in exercises suggests that training of the core is an important component in reducing the incidence of ACL injury.

Critical Points

  • A large variety of core training and assessment protocols are being used to evaluate and train aspects of the core.

  • Training programs incorporating core-specific exercises have been successful at reducing ACL injury risk, but the extent to which the core-specific exercises influenced the reduction in injury risk in these studies remains unclear.

  • Many different core-specific exercises have been utilized in successful training programs, but it is unknown which of these exercises are effective at training core stability or at reducing ACL injury risk.

3 Prospective Evidence Linking the Core to ACL Injuries

A growing number of prospective studies have investigated the link between aspects of core stability and ACL injuries. The first, conducted by Leetun et al. [35], examined the prone-plank and side-bridge core endurance tests in intercollegiate athletes as potential predictors of lower extremity injury. These investigators found no significant association between core muscle endurance and future injury status.

In contrast to Leetun et al.’s examination of core endurance, two prospective studies conducted by Zazulak et al. [4, 36] examined core stability as defined in Sect. 10.1. These studies highlighted a potential connection between core stability and female ACL injuries. A total of 140 female and 137 male varsity athletes with no prior history of knee injury were included. Knee injury was defined as any ligament, meniscal, or patellofemoral injury diagnosed by a university sports medicine physician. One study characterized the ability of the individual to return the torso to its starting position after being rotated in the transverse plane, while the other characterized the ability of the individual to halt movement of the torso after a rapid perturbation.

In the first of these two studies, Zazulak et al. quantified core proprioception using an apparatus originally designed by Taimela et al. to produce passive motion of the lumbar spine in the transverse (horizontal) plane (Fig. 10.3) [4, 37]. In this test, subjects sat on a seat driven by a motor that generated motion in the horizontal plane while their upper body was fixed to a backrest that did not rotate with the seat. The seat was rotated 20° by the experimenter, held for 3 s, and then released. After release, subjects were asked to rotate to the original, neutral position. When subjects perceived that they had reached this position, the error between the actual original position and the current position was calculated.

Fig. 10.3
figure 3

Apparatus used by Zazulak et al. [4] to test core proprioception. The stepper motor rotates the seat 20°, holds for 3 s, and then is released using the clutch. Subjects rotate themselves back to what they perceive is the original, neutral position. The error between the perceived and actual positions is measured (Reproduced from Zazulak et al. [4]; with permission from SAGE Publications, Inc)

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In the second study, Zazulak et al. quantified isolated trunk control following sudden unloading in three directions (flexion, extension, and lateral bending; Fig. 10.4) [36]. In this investigation, subjects were placed in a semi-seated position with their pelvis secured while still allowing their upper body to move freely. A cable was attached to a chest harness at approximately the level of the fifth thoracic vertebra. Subjects pulled isometrically against the cable at a constant force level corresponding to 30% of the maximum isometric trunk strength for an average healthy man (108 N) or woman (72 N). The resisted force was suddenly released at random time intervals by deactivating an electromagnet anchoring the cable. Angular displacement of the trunk after the release was calculated. Subjects were instructed to minimize movement post-release so increased displacement was associated with a decrease in trunk control.

Fig. 10.4
figure 4

A subject positioned in a multidirectional, sudden force release apparatus used by Zazulak et al. and Jamison et al. to quantify trunk control [23, 36, 70]. Subjects pulled in trunk flexion (a), extension (b), and lateral bending (c) against the cable with a prescribed force. Resisted force was suddenly released by deactivating the magnet, and the subsequent trunk angular displacement was recorded (Reproduced from Zazulak et al. [36]; with permission from SAGE Publications, Inc)

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During the 3-year posttest follow-up period for these two studies [4, 36], of the 277 (140 female) athletes that participated in both studies, 25 (11 female) sustained knee injuries, and 6 (4 female) sustained ACL injuries confirmed with magnetic resonance imaging. The trunk proprioception study included both meniscus and ligament injuries in a third injury category (16 total, 7 female), while the trunk control study only included ligamentous injuries (11 total, 5 female).

Results from the proprioception study indicated that women who later experienced knee and ligament/meniscus injuries had significantly greater repositioning errors than uninjured females (P = 0.006 and P = 0.007, respectively) [4]. Further, the authors found that for every degree of increased error, a 2.9-fold increase in the odds ratio of knee injury (P = 0.005) and a 3.3-fold increase in the odds ratio of ligament/meniscus injury (P = 0.007) were observed. No significant difference was observed between repositioning error of ACL injured and uninjured females, but the sample size (only four ACL injuries) was too small to draw definitive conclusions.

Results from the trunk control experiment indicate that ligament-injured female athletes demonstrated greater maximum lateral displacements than uninjured female athletes (P = 0.005) [36]. When the results were collapsed across gender, maximum lateral displacement was significantly greater in all three injury classifications, including ACL injuries.

An Austrian study of competitive ski racers by Raschner et al. [38] demonstrated the potential for isometric core strength as a factor in ACL injuries. By combining regular prospective screening and injury tracking over a 10-year period of members of the Skigymnasium (a junior development program for elite alpine skiers), the authors identified isometric supine trunk flexion and prone extension strength as significant factors in multivariate logistic regression models for both male and female athletes aged 14–19. In both males and females, the absolute flexion force to absolute extension force ratio (FLE:EXT R) was a significant though moderate contributor (male odds ratio 0.24, female odds ratio 0.54). In group comparisons between injured and noninjured males, FLE:EXT R was significantly different (P = 0.007), as were summed trunk flexion and extension forces relative to body mass (P = 0.013). However, in the female athletes, the absolute sum of trunk flexion and extension forces was significantly different between groups (P = 0.009). Given the likely importance of having adequate strength to provide core stability during high-speed ski racing where sudden and extreme external forces are present, the significant core strength findings in this study are consistent with a need for good core stability in ski athletes. The future addition of core stability measures may provide additional insight and additional predictive value for ACL injuries in this population.

Most recently, Dingenen et al. [39] prospectively followed 50 elite female soccer, handball, and volleyball athletes for 1 year to examine whether their single-leg drop vertical jump technique was associated with future knee injuries. Using two-dimensional frontal video, these investigators measured and summed the knee valgus angle and the trunk angle at peak knee flexion of the landing relative to horizontal from the ipsilateral side (KVLTM). A perfectly vertical trunk and neutral knee would give a value of 180°, while greater knee valgus and greater ipsilateral trunk lean would each lead to smaller values. Seven athletes suffered noncontact knee injuries (four ACL ruptures, three other), and these athletes demonstrated significantly smaller KVLTM than noninjured athletes, indicating either greater ipsilateral trunk lean, greater knee valgus, or both. Receiver operating characteristic analysis demonstrated that the KVLTM had significantly greater area under the curve than random prediction with both the future injured and noninjured legs. In this experiment, the KVLTM measure was considered a measure of core stability, because the external impulse from foot impact created a perturbation to both the hip and trunk and the amount of deflection before the participant was able to arrest frontal motion was assessed.

Results from these four studies [4, 36, 38, 39] suggest a role for trunk proprioception and strength and motor control (all contributing factors to core stability) in knee and ligamentous injury risk for both male and female athletes. Zazulak’s and Dingenen’s tests were designed as surrogates for how an athlete responds to the conditions and demands of play in his/her sport. However, it remains unknown whether these factors have a direct role in ACL injury risk or whether they are surrogates for systemic proprioception and control. Under the former hypothesis, core stability could have a direct role in ACL injury risk by leading to increased or decreased strain on the ACL. Under the latter hypothesis, an individual with superior trunk proprioception and control would also have better proprioception and control of the hip, knee, and ankle, which could be the direct cause of reduced biomechanical loading on the knee and reduced ACL injury risk.

Critical Points

  • Decreased transverse plane trunk proprioception has been associated with increased knee and ligament, not ACL, injury risk in females.

  • Decreased lateral trunk control has been associated with increased risk of injuries to the knee, including the ACL, in female athletes.

  • Decreased lateral trunk control has been associated with increased ACL injury risk in male and female varsity athletes.

  • Decreased sagittal trunk isometric strength and flexion-extension strength imbalances have been associated with increased ACL injury risk in male and female elite alpine ski racers.

  • It remains unknown whether trunk control, proprioception, and strength cause ACL injury risk to increase or whether the association is merely coincidental.

4 Video Observations of Core Motion During ACL Injury Events

Most clinicians and researchers have observed an ACL injury occur at some point in time, either live or on video. It is impossible to know exactly when the injury occurred and rare to have the ideal view(s) of the event to accurately reconstruct the body kinematics during the event. Nevertheless several authors have reported comparisons between observed motions of the core during injury events versus noninjury events.

Hewett et al. [40] reported lateral trunk angles in female and male athletes during 23 ACL injury events where the camera angle approximated a coronal view, the foot was clearly visible contacting the ground, the athlete was unobscured, and minimal contact with other players was observed. They compared the trunk angles to control athletes performing similar tasks. The measurement was made by first choosing the frame of video that approximated initial foot contact with the ground and the five subsequent frames. In each of the five frames, trunk angle was estimated by the angle between a line connecting the greater trochanter to the ipsilateral acromioclavicular joint and a vertical line. This measurement was compared between female injured and male injured and between female injured and female controls using a repeated measures ANOVA including all five time points. The authors reported that female injured experienced greater lateral trunk lean over the injured leg than male injured and trended to greater trunk lean when compared to female controls.

Sheehan et al. [41] examined sagittal trunk angles, as well as the distance between the base of support and estimated center of mass in female and male athletes during 20 ACL injury events where a sagittal view was available, following similar criteria for inclusion of videos as Hewett et al. [40]. Again, these events were compared to athletes performing similar maneuvers in noninjury events. For this analysis, the authors drew ellipses to approximate the trunk, thigh, shank, and foot in the video frame closest to initial foot contact. Sagittal trunk angle was estimated as the angle between the major axis of the trunk ellipse and vertical. The center of mass was estimated as the center of the trunk ellipse. The horizontal distance between the center of mass and point of contact between foot and ground (COM_BOS) was also estimated. Both trunk angle and COM_BOS were observed to be significantly different between injured and uninjured athletes, with the injured athletes having more upright posture and stretching the foot further in front of the center of mass.

Stuelcken et al. [42] examined trunk and knee angles in female netball athletes during 16 ACL injury events that occurred during televised games at the ANZ championship competitions from 2009 to 2015. Using previously reported criteria and consensus scoring among biomechanists, a skill acquisition specialist, and a physiotherapist, the authors identified key characteristics of motions. These included the movement just before injury, the task the athlete was attempting to achieve, the trunk motion just before the injury, and the knee motion just before, during, and after the injury. All 16 injuries involved either no contact with the injured athlete, indirect contact to another part of the body, or contact just before the injury event. In 13 cases, the athlete was attempting to receive or intercept a pass or compete for a loose ball. In 11 cases, the athlete performed transverse trunk rotation away from the leg about to be injured, and in 7 of those 11 cases, the athlete also tilted the trunk laterally toward the side of the leg about to be injured (ipsilateral trunk lean).

The results of these studies must be considered in light of the limitations inherent to two-dimensional video including video quality, video angle, and measurement accuracy, as well as the limitation that the timing of the ACL injury itself is unknown. Nevertheless, these results are consistent with the theoretical basis for the role of the core that placement of the relatively large mass of the upper body may influence knee loading and thereby injury risk. Moreover, these findings suggest the need for biomechanical studies to determine whether position of the core influences knee loading.

Critical Points

  • Video analysis of ACL injury events provide a unique opportunity to observe kinematics that may be related to the injury.

  • Limits to spatial and time resolution of standard video make it impossible to conclusively determine injury mechanisms.

  • Greater lateral trunk lean may be related to ACL injury in women based on video observation.

  • More upright posture and position of the trunk center of mass further behind the foot in the sagittal plane may be related to ACL injury in both men and women based on video observation.

5 Biomechanical Evidence Linking the Core to Knee Loading: Cross-Sectional Studies

As detailed in Sect. 10.3, most prospective research on core stability and ACL injury has focused on empirically identifying associations between core stability measurements and ACL injury incidence. Studies like these are critical to establish the extent of the injury problem, which is commonly accepted as the first step in preventing sports injuries [43]. However, as previously mentioned, these studies still leave unanswered the question of whether core stability has a direct mechanical effect on the knee joint and the ACL. Recent video observations of ACL injury events suggest that position of the trunk may influence ACL injury risk [40,41,42], but due to the limitations of two-dimensional video analysis, they serve best as a motivation for developing hypotheses that can be tested more rigorously using more sophisticated techniques. Along these lines, recent work using motion analysis and computer simulation has begun to explore the direct biomechanical connection between core stability and knee loading in greater detail.

Several cross-sectional studies have linked positioning of the upper body to knee loading parameters which have been identified as risk factors for ACL injury. In particular, these studies examined peak external knee abduction moment (pEKAbM) as the knee loading outcome of greatest interest during side-step cutting maneuvers and drop landings, which are known to be high risk for ACL injury in field and court sports [40,41,42, 44, 45]. An external knee abduction moment occurs when the forces generated between the ground and the lower limb act to push the knee medially into a more valgus alignment. Increases in pEKAbM were associated with ACL injuries during a prospective study in a population of female adolescent athletes [46]. Increasing knee abduction moments have also been associated with increased strain (elongation) of the ACL in both cadaver knees [47, 48] and computer simulations [49, 50].

Chaudhari et al. [51] used markered-motion capture techniques and inverse dynamics to estimate the pEKAbM of 11 subjects (6 women, 5 men; mean age of 22.3 ± 3.5 years) performing 90° cuts away from the plant-side foot for 4 arm conditions (holding a lacrosse stick with both hands, holding a football with the cut-side arm, holding a football with the plant-side arm, and a control condition where nothing was held). Results indicated that constraining the arms during a cutting maneuver can increase pEKAbM when compared to a baseline condition where the arms are not constrained. When the plant-side arm was forced to hold a football, the pEKAbM increased 29% (P = 0.03). When subjects held the lacrosse stick with both hands, the pEKAbM increased 60% (P = 0.03).

Dempsey et al. [52] examined 15 healthy males performing a 45° side-step cutting maneuver using their own technique but also when attempting to lean/twist in the frontal plane or transverse plane and attempting to alter foot placement in the frontal plane or transverse plane. When altering motion of the trunk, several differences in knee moments were observed. Trunk lean in the opposite direction from the cut resulted in 38% higher pEKAbM than leaning in the same direction as the cut (P < 0.05). Trunk twist resulted in 53% higher peak tibial internal rotation moment (pTIRM) than the natural condition (P < 0.05). pTIRM is an external moment that would act to rotate the tibia internally with respect to the femur, and increases in pTIRM have also been associated with increases in ACL strain [50] and force [53].

In another study using markered-motion capture and inverse dynamics to estimate knee moments, Jamison et al. [54] used similar data collection and reduction techniques on a similar population (14 female, 15 male, no prior history of ACL injury). However, in this study, an unanticipated 45° cut was examined to better mimic the environment on the field when an athlete’s movements are dictated by the game play. Unanticipated cutting situations have also been shown to lead to higher knee abduction moments than preplanned movements [55]. In addition to calculating pEKAbM and pTIRM, ipsilateral trunk lean (lateral angle away from direction of the cut, termed “outside tilt” in the study) was calculated. Using multiple regression analysis to examine the relationship between moments and ipsilateral trunk lean as continuous variables within each individual, this study observed that pEKAbM was positively associated with ipsilateral trunk lean (P = 0.002), while pTIRM was negatively associated with ipsilateral trunk lean (P = 0.021). A positive association between ipsilateral trunk lean and pEKAbM suggests that as torso angles increase, so does pEKAbM, which would be expected to increase strain in the ACL and therefore place it a greater risk for rupture. The negative association between ipsilateral trunk lean and pTIRM suggests that as ipsilateral trunk lean increases, pTIRM decreases, protecting the ACL from strain and danger of rupture. However, these peaks in pEKAbM and pTIRM did not occur at the same time, so the effect of increased ipsilateral trunk lean on pEKAbM would be expected to increase the risk of ACL injury through an excessive valgus moment mechanism. More recently, other cross-sectional studies using similar measurement techniques that examined between-subject differences during 90° cutting maneuvers [56], lateral reactive jumps [57], and single-leg drop vertical jumps [58] all found similar results, with ipsilateral trunk lean having a significant association to pEKAbM.

Donnelly et al. [59] applied computer simulation techniques to baseline data from markered-motion analysis of nine male athletes with high pEKAbM during unanticipated 45° side-step cutting maneuvers to estimate how an athlete might optimize his whole-body movement to reduce pEKAbM. The open-source simulation software OpenSim with a scaled generic model was used to perform the simulations. In the simulations, adjustments to motions of all joints were permitted as long as they reduced pEKAbM while not altering foot position relative to the ground more than 30 mm. While each of the nine subject-specific simulations began with unique kinematics and kinetics, the optimization resulted in the “strategy” of repositioning the whole-body center of mass medially and anteriorly in all nine simulations.

The above findings suggest that subjects may be capable of using their arms and core to protect their knee from adverse loading patterns and, potentially, from ACL injury. Conversely, trunk lean or twist away from the direction of cutting, an upright posture, and constrained arms may all lead to increased knee loading and, therefore, increased risk of ACL injury. These biomechanical results are consistent with the video observations of ACL injury events described in the previous section: lateral trunk lean [40, 42], upright posture [41], and a more posterior center of mass relative to the foot [41].

Frontal motion is not the only dimension in which cross-sectional studies have shown an association between trunk motion and ACL injury risk. In vivo, simulation, and cadaveric studies have demonstrated that both trunk flexion and transverse plane trunk rotation can contribute to adverse knee loading, ACL force, and ACL strain. Shimokochi et al. [60] demonstrated that, during a single-leg drop landing, landing with a forward trunk lean decreased anterior shear force at the knee and increased knee flexion, both protective of the ACL. Kulas et al. [61], using in vivo and modeling techniques, showed that an increase in forward trunk lean during a single-leg squat decreased ACL ligament force by 24% and ACL strain by 16%. Frank et al. [62], in an in vivo three-dimensional analysis of side-step cutting, corroborated Stuelcken’s two-dimensional video results previously discussed [42]. In the three-dimensional analysis, an increase in transverse plane trunk rotation away from the stance limb was associated with a decrease in pEKAbM, which would serve to protect the ACL. Contrary to previous findings, however, an increase in forward trunk lean was associated with an increase in pTIRM, which would suggest an increase in ACL injury risk. In an in vivo and simulation study of ski jump landing, trunk orientation accounted for 60% of the variance in ACL force, with increased trunk extension being associated with increased ACL force [63]. Finally, in a study combining cadaveric, in vivo, and simulation biomechanics, trunk flexion uniquely accounted for nearly half the variance of ACL strain by extrinsic factors, with an increase in trunk flexion being associated with a decrease in ACL strain [64]. This collection of studies suggests that a moderate forward trunk lean can help protect the ACL during dynamic motion.

One of the many questions left unanswered by these studies is the role of muscle coordination patterns in the observed associations between upper body movement and knee and ACL loading. One potential explanation of the observed outside torso tilt is that muscle activation of the core lags behind the lower extremity, while an alternative explanation is that athletes actively choose to pull the torso into the outside tilt position as preparation for a change of direction. Jamison et al. [65] attempted to answer this question by examining muscle activation differences between left/right and anterior/posterior pairs of muscles during cutting maneuvers. Left/right activation patterns of the obliques and lumbar extensors were not associated with outside torso tilt during unanticipated cutting maneuvers, suggesting that the athletes were not actively attempting to achieve that position. In contrast, coordinated contraction of both left and right lumbar extensors was associated with a stiffer torso and higher pEKAbM, suggesting an active strategy by some athletes to maintain an upright trunk that may be detrimental to the knee. These latter results were consistent with another study by Haddas et al. [66] in which athletes were asked to consciously contract the abdominal muscles just before landing from a jump. Increased volitional preemptive abdominal contraction (VPAC) was associated with decreased pEKAbM and with a relative increase in anterior trunk muscle activations compared to posterior trunk muscles. This result suggests that some athletes are actively trying to maintain an upright posture in the absence of VPAC, which, again, may be detrimental to the knee. The results from these studies further support the theory mentioned previously that a forward trunk lean, or at least muscle activation that would facilitate a forward trunk lean, is protective of the ACL [60, 61, 63, 67]. However, future studies are needed that incorporate electromyography of the core musculature to gain a better understanding how the core muscles might activate to better control the trunk, reduce knee loading, and prevent injury. With this information, more efficient and effect training programs may be developed that can be incorporated across large populations of athletes to alter knee loading in a positive way and potentially reduce ACL injury risk.

Critical Points

  • Constraining the arms close to the body during a cutting maneuver increases knee loading patterns associated with ACL injury risk.

  • Increased trunk angles away from the direction of cutting are associated with increased knee moments and may lead to increased ACL injury risk.

  • Reducing trunk angles, medializing the center of mass, and shifting the center of mass anteriorly more over the foot are all associated with reduced knee moments and may lead to reduced ACL injury risk.

  • Studies investigating the role of the core musculature using electromyography provide preliminary evidence of a connection between active and reactive activations of the core muscles and knee loading patterns relevant to ACL injury.

6 The Core-ACL Connection: Causation or Just Correlation?

Although the evidence from cross-sectional, epidemiological, and interventional studies previously discussed shows associations between measures of the core and contributors to ACL injury risk, they fall short of demonstrating that core stability alters ACL injury risk. Cross-sectional and epidemiological studies cannot establish causation, and previous interventional studies have lacked the systematic approach necessary to determine if the core-directed exercises are an essential part of the training programs or if they could be removed without reducing efficacy. Moreover, perhaps the most relevant question to answer for the at-risk athlete is whether, on an individual level, improving core stability can reduce knee loading and thereby reduce ACL injury risk. However, three interventional studies that focused narrowly on core stability interventions and outcomes shed some light on the direct role that core stability training may play in reducing ACL injury risk and provide direction for future investigations in this area.

Pedersen and colleagues [68] studied soccer participation as a novel way to elicit changes in trunk control, hypothesizing that unanticipated perturbations to the trunk due to repeated directional changes and other movements during soccer would improve trunk control. Previously inactive women were recruited and allocated to 16 weeks of either playing soccer, running, or continuing to not train (negative control). Before and after the week training period, displacement after sudden trunk loading was assessed for all participants. A weight attached to a pulley was dropped suddenly, providing an anterior tug on the trunk that the subject was asked to resist (Fig. 10.5). Members of the soccer group significantly reduced trunk displacement after sudden loading, indicating that this group improved their trunk control. In contrast, no significant change in trunk displacement was observed in either the running group or negative control group. To examine the differences between soccer and running in greater detail, nine players were filmed during three soccer training sessions to assess their movement patterns. Over these 27 hours of soccer training, the women made 191.5 ± 63.3 specific movements per hour on the field, including heading, dribbling, shoulder tackling, stopping, and turning. These movements present challenges to the core musculature over and above those required for the running control group, suggesting that sudden, unanticipated perturbations of the trunk may be important in eliciting changes in trunk control.

Fig. 10.5
figure 5

Setup for generating a sudden forward pull to the upper part of the subject’s trunk used by Pedersen et al. [68] to quantify trunk control. A cable is fastened to a rigid bar at the back by means of a harness attached to the upper part of the trunk. At a random time, an electromagnet is used to increase the weight W suddenly from 0.5 to 5.9 kg, creating an anterior tug on the torso. Movement of the trunk is measured by a potentiometer mounted on the pulley (Modified with permission from Pedersen et al. [68])

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In a study aimed at determining if altering trunk control leads to altered knee loading and thereby altered ACL injury risk, Jamison and colleagues [23] analyzed the effectiveness of two different 6-week training programs (whole-body resistance program and trunk stabilization program). Both regimens included traditional, whole-body bilateral strength training exercises with free weights (such as bench press, deadlift, squats, lat pulldowns); however, the trunk stabilization program replaced one set of the traditional free weight exercises with trunk stabilization exercises. Peak external knee abduction moments (pEKAbM) during an unanticipated 45° cut and displacement after a sudden force release were assessed pre- and post-testing for 22 men who completed the training programs (11 per intervention group). Athletes completing the whole-body resistance training-only program significantly worsened their knee loading (pEKAbM during the cut) and worsened their trunk control (lateral trunk displacement following the sudden force release). The trunk stabilization group did not demonstrate any changes in these variables. The changes in the two programs were not significantly different from each other either, although the study was underpowered to prove this to be true. Nevertheless, the results suggest that the whole-body resistance training program’s lack of any challenge to core stability negatively affected trunk control, which may have in turn negatively affected pEKAbM. While the trunk stabilization program was not able to improve trunk stabilization in this population, it is possible that the inclusion of the trunk stabilization exercises had a protective effect on trunk control, limiting any potential negative effects of the resistance training. The observed coupling of negative changes in trunk control and pEKAbM in the whole-body training group is consistent with the theory that the trunk may have an influence on knee loading that endangers the ACL, though this intriguing observation deserves further study.

A third study by Dempsey et al. [69] explored the effectiveness of whole-body cutting technique modification training in reducing pEKAbM and pTIRM during cutting maneuvers. Nine male football, rugby, and soccer athletes completed a 6-week training program that focused specifically on reducing lateral reaching of the plant foot and decreasing lateral lean of the trunk away from the change of direction by providing immediate oral and visual feedback on cutting technique. Peak external knee abduction moments (pEKAbM) were estimated during a 45° unanticipated side-step cut before and after the task-specific training. Significant reductions in pEKAbM (P = 0.034), lateral trunk lean (P = 0.005), and lateral reaching of the plant foot (P = 0.039) were observed following training. While lateral trunk positioning and pEKAbM both improved, the simultaneous improvement in foot position makes it difficult to conclude whether it was the change in foot position or the change in trunk lean that led to the reduction in knee loading. In addition, no crossover tests were done in this study to determine whether improvements in the 45° cutting task carry over to other common high-risk activities. This study does demonstrate, however, that trunk positioning changes can be attained through task-specific training using visual and audio feedback, which may be useful when considering ACL rupture risk reduction training in the future.

In summary, soccer training, which by nature includes many sudden perturbations to the trunk, appears to be effective based on current reports in the literature in improving trunk control [68]. Running and static trunk stabilization exercises do not appear to improve trunk control, although they may assist in maintaining control [23, 68]. Eliminating core-directed exercises in whole-body resistance training appears to negatively influence trunk control and knee loading [23]. Lastly, task-specific side-step cutting training may improve cutting mechanics, including lateral trunk position and foot placement, as well as reduce adverse loading of the knee during the cut [69].

While these studies provide modest insight into the best ways to train the core and the connection between trunk control and ACL injury risk factors, further work is required in these areas to better understand this connection. Well designed, randomized control trials that mechanistically identify which components of exercise interventions are effective and efficient in triggering improvements in knee loading and reducing ACL injury incidence are critical to easing the challenge of identifying those at greatest risk of injury and those who would benefit the most from ACL injury prevention interventions. Emphasis should also be placed on reducing the burden of these interventions so that more athletes comply with and benefit from them. Moreover, most of the tests for core stability described in this chapter are only feasible to perform in the laboratory. More clinically feasible tests also need to be developed to assess core stability both for screening individuals at risk and determining which exercise interventions are most effective for the large prospective populations necessary for ACL injury incidence research.

Critical Points

  • Soccer training, which incorporated many sudden, unanticipated trunk perturbations, was effective at improving trunk control.

  • Whole-body resistance training alone (i.e., with no core training component) may have negative effects on both trunk control and knee loading which endangers the ACL.

  • Task-specific cutting training can improve lateral trunk and foot positioning as well as reduce knee loading which endangers the ACL.

  • More studies are needed to determine if core stability is a main factor in the ACL injury mechanism.

  • Training programs targeted at improving core stability need to be clinically feasible in both scope and equipment.