Injuries to the anterior cruciate ligament (ACL) are serious, costly, and unfortunately common in many different sports, including basketball, soccer, lacrosse, European handball, and Australian football (9,31,33). To return to sport from a ruptured ACL, an injured athlete usually requires surgery, followed by 9-12 months of rehabilitation (32). The approximate cost of an ACL reconstruction is $17,000, with the total cost of all ACL reconstructions in a given year in the United States estimated at $850 million (14). Individuals who have suffered an ACL injury also have significantly increased risk of developing knee joint osteoarthritis by the age of 50 yr (11).
Anterior cruciate ligament injuries can be classified into two broad groups: contact and noncontact. Across various sports, noncontact injuries have been found to make up 50-80% of ACL injuries (1,6,9). Because a large percentage of injuries are noncontact, this indicates that there is potential to reduce the number of ACL injuries occurring in sports. This may be achieved by changing how the person performs the injury-prone maneuvers with appropriate training. Lloyd (22) states that training programs to prevent ACL injuries should include balance, plyometric, and technique components. Although there have been several studies examining the effect of balance and plyometric training on the risk of ACL injury (7,16,18,33), only a few recent studies have investigated changing participants' performance techniques on knee loading, and these have been restricted to landing tasks (10,35). Because more ACL injuries occur during sidestep cutting compared with landing, changing technique in this maneuver has greater potential to reduce ACL injury rates (9,34).
Injuries to the ACL occur when the loads being applied to the ligament are larger than the ligament's capacity to sustain them. The ACL's primary function is to prevent anterior tibial translation, but cadaveric studies have shown that the ligament is also loaded by valgus and internal rotation moments at the knee (15,25,36). Previous laboratory studies have shown that when compared with running the knee has larger valgus and internal rotation moments during sidestep cutting, the authors suggesting that the valgus and internal rotation moments are major contributing factors to ACL injury (3,4). Results from a prospective study of landing by Hewett and colleagues (17) supports this, finding that females who had large peak valgus loads were at a greater risk of suffering an ACL injury. Video analyses in several sports have also reported that when the ACL ruptures during sidestep cutting, the knee usually collapses into valgus (6,9,34). Recently, it has been shown that the knee also gives way in internal rotation when the ACL ruptures during Australian football games (9). Collectively, these results suggest that high valgus and internal rotation moments are a main cause of ACL injuries during side stepping and should be reduced if injury risk is to be lowered.
Cadaveric studies have found that the resultant strain experience at the ACL in knees for anterior forces, rotation, and abduction/adduction moments is modified by the knee flexion angle (15,25). In general terms, as knee flexion angle increases, there is a reduction in the resultant strain at the ACL. This seems to be reflected in vivo. Studies of actual injuries have found that athletes tend to have knee flexion angles of less than 30° at foot strike (9,34). It would, therefore, seem that increasing knee angle may reduce the resultant load on the ACL for the same applied load at the knee, therefore reducing the risk of injury.
Previous studies have indicated that there are differing techniques employed to perform a sidestep cut. Besier and colleagues (3,4) have identified two groupings within their subjects, one exhibiting mean valgus moments and one exhibiting mean varus moments during the weight-acceptance phase of sidestep cutting. McLean and colleagues (30) have observed intersubject variability in knee angles during sidestep cutting, but they do not report knee loads. It has been shown that by constraining arm movements during sidestep cutting, valgus loads at the knee are increased (8). Increased valgus loads have also been linked to increased hip flexion, hip internal rotation, and knee abduction angles (28). However, no study has investigated the effect of imposing a range of different sidestep cutting techniques on knee loads. Therefore, the aim of this study was to identify whether modifying sidestep cutting technique creates substantial and functionally important changes to knee loading. It was hypothesized that varus/valgus and internal/external rotation moments, and knee flexion angle, would be affected by changes in sidestep cutting technique.
Fifteen healthy, male, experienced amateur team sport athletes, with no history of major lower-limb injury, volunteered to participate in this study (mean age 21.1 ± 2.8 yr, height 182.5 ± 7.1 cm, mass 73.3 ± 10.4 kg). Experienced team sport (Australian football, rugby union, and soccer) athletes were selected to ensure that they had sufficient skill in performing a sidestep cut. Our previous work comparing the differences between planned and unplanned sidestepping revealed effect sizes of about 0.8 (3). In the current study design, to achieve similar effect sizes, which represented substantial functional differences, seven subjects were required for an 80% power and alpha of P < 0.05. For the same power and alpha, we decided to recruit 15 subjects, which gave us the power to detect a smaller effect size of 0.65. All test procedures were approved by the human research ethics committee at the University of Western Australia (UWA), and before data collection, written informed consent was obtained from all subjects.
All trials were performed on a 20 × 15-m runway with markers tracked by a 12-camera VICON MX motion-analysis system operating at 250 Hz (VICON Peak, Oxford, UK), and ground-reaction forces synchronously recorded at 2000 Hz from a 1.2 × 1.2-m force plate (Advanced Mechanical Technology Inc., Watertown, MA). Subjects were asked to perform repeated trials of both normal and nine imposed sidestep cutting tasks during one testing session. Before commencing trials, subjects selected the preferred foot with which they would perform the sidestep cut. This foot was determined by subjects performing a sidestep cut with each leg and selecting their preferred side.
Subjects were required to perform five successful trials of each sidestep cut, which was to 45 ± 5° from the approach direction, with all subjects running at 4.5 ± 0.2 m·s−1 during the stride before the force plate. This speed was monitored using VICON Workstation (VICON Peak, Oxford, UK) to identify the average linear velocity of a marker on the left posterior superior iliac spine across the final approach stride. Cut angle was monitored through tape markings on the ground signifying 45 ± 5°, with subjects required to land with their next foot contact within these markings. All subjects performed their normal sidestep cut (NS), then sidestep cuts with nine different imposed techniques, categorized into four extreme postural groupings (Fig. 1):
- Torso lean: leaning in the same direction (TSame) and leaning in the opposite direction (TOpposite) to the direction of the sidestep cut;
- Knee: knee straight (KStraight) (as close to full extension as possible) and knee flexed (KFlexed) (as flexed as possible);
- Frontal plane foot placement: foot placed close to the body (FClose) and foot placed away from the body (FWide); and
- Transverse plane foot placement: foot turned in (FIn) and foot turned out (FOut).
In addition, we had one extra technique modification involving the trunk rotating in the opposite direction (TRotated), to which we found it was not possible to have a functional opposite for this posture. The NS was performed first, followed by the imposed tasks presented in random order within the functional groupings.
The imposed postures were demonstrated to the subjects using a previously prepared video and standard instructions. A trial was then captured using a digital video camera, and subjects were given both visual and auditory feedback on their performance. This step was repeated until the subject could successfully perform the imposed sidestep cut. This was assessed by same experimenters for each subject, using demonstration video as a reference. Once they were capable of performing the imposed sidestep, subjects undertook the trials immediately. After undertaking the trials, the subject was then trained and tested on the next imposed posture. This step was repeated until all imposed postures had been completed.
A trial was considered successful if the subject performed the required sidestep cut with the appropriate technique, achieved a cut angle of 45° ± 5 with the foot of the leg of interest landing on the force plate, and did not target the force plate. Subjects were aware of the location of the force plate, but to avoid targeting they were instructed to look ahead during their approach run. Targeting was identified by either a "stutter step" during approach or by "reaching" towards the force plate with the last stride. To assist in this, a run-up marker was used to modify the approach distance to ensure that the correct foot was striking the force plate.
Data collection and analysis.
To facilitate the motion analysis, retroreflective markers were affixed to the whole body to conform to requirements of the UWA marker set (5,23) (Fig. 2), which consisted of 50 markers placed on either bony landmarks or as part of three-marker clusters. Single markers were placed on the left and right forehead, left and right rear head, left and right acromion process, sternal notch, spinous process of C7 and T10, xiphoid process, left and right anterior superior iliac spines, left and right posterior superior iliac spines, left and right head of first and fifth metatarsals, left and right head of third metacarpal, and left and right calcaneus. Three-marker clusters were placed on the upper arm, forearm, thigh, and leg, and a two-marker cluster was placed on the dorsal surface of the hand. In addition, the ankle-, wrist-, and shoulder-joint centers were respectively defined using markers on the left and right medial and lateral malleoli, left and right radial and ulnar styloid processes, and left and right anterior and posterior shoulder. These markers were removed during the dynamic trials. A six-marker pointer was used to identify three-dimensional location of the medial and lateral humeral epicondyles of both elbows, and medial and lateral femoral epicondyles of both legs (5). Functional knee and hip tasks were carried out to identify knee joint axes and hip joint centers, as was a trial with the subject standing on a foot-calibration rig (5). The latter trial was used to establish the position of the foot markers and to measure foot abduction/adduction and rear foot inversion/eversion angles (5).
Kinematic and inverse dynamic calculations were performed in VICON Workstation and Bodybuilder (VICON Peak, Oxford, UK) using the UWA full-body model, a combination of the UWA upper- and lower-body models (5,23). Before modeling, both the ground-reaction force and position data were filtered using a fourth-order, 18-Hz, zero-lag, low-pass Butterworth filter, with the filter frequency selected by performing a residual analysis and visual inspection of the data. The UWA lower-body model uses a functional method to identify both the knee joint and hip joint centers (5). The knee joint axis was located by calculating a mean helical axis using a custom MATLAB (Mathworks Inc., Natick, MA) program, with the knee center identified as the midpoint of the femoral epicondyles along this line (5). Spheres were fitted to each thigh marker trajectory to find a hip joint center relative to the pelvis anatomical coordinate system, constraining it to within a 100-mm cube around a regression-calculated hip joint center (5). The foot coordinate system was established using the data from the foot-calibration rig trial, which overcame errors in placing markers while incorporating the person's measured foot abduction/adduction and rear-foot inversion/eversion angles (5). External moments were calculated with inverse dynamics (5,20) using the body segment parameters calculated according to values in de Leva (12).
A custom MATLAB program was used to identify a weight-acceptance phase during stance. This phase was from heel strike to the first trough in the unfiltered vertical ground-reaction force (Fig. 3). Although we have previously analyzed multiple phases of the sidestep cut, weight acceptance was selected as the sole phase to analyze in this study because the maximum magnitude valgus and internal rotation moments were found within this phase, indicating that this may be the period of high injury risk (Fig. 4) (2-4). Peak valgus (PV), peak internal rotation (PI), and mean flexion/extension (FE) moments were identified within the weight-acceptance phase. Peak valgus and internal moments, rather than means, were chosen because peaks in both moments were exhibited during weight acceptance, and large peaks could constitute dangerous loading patterns (Fig. 3). When analyzing the loads experienced in sidestep cutting, other groups have also used peak valgus moments (17,28). Mean flexion/extension moments were used because there was no definite peak within the weight-acceptance phase. Knee flexion angle was identified at heel strike for all tasks. Joint angle data representing the imposed technique performed in the trial were determined and analyzed at heel strike to ensure that the subjects had successfully achieved each required technique. If this was not the case, the trial was rejected. In all cases, three or four trials for each technique were available for analysis. A subject average was calculated from these trials.
Because we were interested in comparing the differences in knee moments and flexion angle between the extreme postures within each technique group and with the NS, aone-way repeated-measures ANOVA was performed on the following groupings: torso lean: TOpposite-NS-TSame; knee: KFlexed-NS-KStraight; transverse-plane foot placement: FIn-NS-FOut; and frontal-plane foot placement: FWide-NS-FClose. Because the TRotated did not have an extreme opposite posture, it was only compared with NS using a paired t-test. For the paired t-test and the four ANOVA, we use an alpha level of P < 0.05 with no correction, because all comparisons were specified a priori. However, in the post hoc comparisons within the four ANOVA, a Sidak correction applied to an original alpha level of P < 0.05, in preference to Bonferroni corrections, which can be very conservative. To examine whether relevant segment posture angles were changed in the extreme postural groupings, we compared posture angles across all tasks using a repeated-measures ANOVA for each variable, using the same procedure described above. All statistical procedures were performed using SPSS 14.0 (SPSS Inc., Chicago, IL).
There were significant differences in the relevant position data between each of the extreme postural groupings (Table 1). This indicates that the positions represent the extremes of a particular posture. In addition to this, four techniques also reported values significantly different to the NS: TOpposite had greater trunk lateral flexion away from the direction of sidestepping, TRotated had greater trunk rotation in the opposite direction of the sidestep, KFlexed had greater knee flexion, and FWide returned a greater foot distance from the pelvis.
All tasks returned a mean FE moment with a value in the flexion range. The FWide condition returned a mean FE moment (−0.94 ± 0.36 N·m·kg−1·m−1) with the highest flexion value (Fig. 5). This was significantly greater than the FClose (−0.72 ± 0.38 N·m·kg−1·m−1, P = 0.024) technique. The mean FE moment displayed during the NS (−0.78 ± 0.44 N·m·kg−1·m−1) was significantly greater than the FIn (−0.59 ± 0.37 N·m·kg−1·m−1, P = 0.021), and the mean FE moment displayed during the FIn was also significantly smaller than its pair task of FOut (−112.96 ± 39.29 N·m·kg−1·m−1, P = 0.001). All other pairs produced moment values of similar magnitude except for TSame, which tended to generate lower moments than during both the NS and TOpposite.
The highest PV moment (Fig. 6) was again returned by the FWide condition (0.79 ± 0.38 N·m·kg−1·m−1), which was significantly higher than both NS (0.45 ± 0.32 N·m·kg−1·m−1, P = 0.000) and FClose (0.51 ± 0.37 N·m·kg−1·m−1, P = 0.003) techniques. The PV moment generated during the TOpposite was significantly higher than its paired TSame (0.65 ± 0.36 vs 0.47 ± 0.36 N·m·kg−1·m−1, P = 0.030), and it tended to be greater than the NS. All other pairs returned moment values of similar magnitude to each other.
Two techniques produced high PI moments in relation to the other tasks (Fig. 7). As with PV and mean FE, the FWide (−0.33 ± 0.23 N·m·kg−1·m−1) technique resulted in the highest PI, significantly greater than the NS (−0.19 ± 0.10 N·m·kg−1·m−1, P = 0.048). The NS also generated significantly lower PI moments than the TRotated (−0.29 ± 0.10 N·m·kg−1·m−1, P = 0.001). All other techniques returned PI moment values of similar magnitude.
As can be seen from Table 1, there was a significant difference in knee flexion angle between KFlexed and KStraight (P = 0.000) as well as between KFlexed and NS (P= 0.006). There were also significant larger knee flexion angles recorded in the TRotated technique (23.6 ± 6.5°) compared with the NS condition (17.6 ± 5.5°, P = 0.010). The TSame technique (22.3 ± 1.7°) returned a knee angle that was significantly larger than at both NS (P = 0.010) and TOpposite (18.2 ± 1.7°, P = 0.004). All other groupings returned similar values.
The aim of this study was to identify if modifying sidestep cutting technique creates substantial and functionally important changes to knee loading. It has been shown that externally loading the knee with valgus and internal rotation moments results in high loading of the ACL (25). Two of the imposed postures (FWide and TOpposite) in the present study resulted in significantly higher peak valgus moments compared with their functional pair, with FWide also significantly higher than NS. In peak internal rotation moments, no techniques were significantly greater than the functional pair. However, FWide and TRotated were significantly higher than NS. Markolf and colleagues (25) found that the combination of the two aforementioned loading directions significantly increased the strain being experienced by the ACL. Both peak moments occur in close to the same time point during the weight-acceptance phase across the techniques (Fig. 4); therefore, FWide is the technique most likely to endanger the ACL, because it returned significantly greater PV and PI moments.
For all three moments, there was a general increase in magnitudes compared with the normal sidestep. The average effect sizes for all moments were 0.48 for PV, 0.57 for PI, and 0.45 for F. However, in the tasks where a significant difference was identified, there was a large effect size, with a mean value of 0.81. The smaller increases may not be "bad" in terms of ACL injury; rather, they may reflect the subjects' inexperience at the new task. A large, significant difference between a pair of tasks indicates that a functionally important increase mayhave been caused by the body posture, and, therefore, the technique that produced the high loading should be avoided.
With reference to body posture, three conditions were significantly different from NS: FWide, TOpposite, and TRotated. The normal sidestep always occurs at some point between the two extreme postures, which are always significantly different from each other. Nonsignificant positional change may limit the ability to identify whether technique changes modify the knee moments, but because all the extremes are significantly different, it is possible to identify the moment changes from these positions.
There is currently some debate as to whether a high external flexion moment is good or bad in terms of ACL injury. A high external flexion moment, as exhibited in the FWide technique, indicates a high level of quadriceps activation to prevent the knee from flexing. Some groups argue that this increase in quadriceps activation is bad because it will increase anterior translation at the knee and, therefore, increase ACL load (14). The other argument is that an increase in quadriceps contraction will protect the ACL, because the quadriceps have moment arms that provide support for the knee in varus/valgus and internal/external rotation (2,24). In addition, McLean et al. (27) have shown that when modeling sidestep cuts, the level of quadriceps action causing anterior translation of the tibia was not sufficient to rupture the ACL. The stated reasons for this were that the quadriceps were not strong enough, and the action of quadriceps was counteracted by the action of the hamstrings and posteriorly directed forces on the tibia resulting from the deceleration experienced during the first half of stance. Nevertheless, when the anterior translation produced by the quadriceps is combined with valgus and internal rotation moments, this is probably the loading condition that constitutes the greatest risk of noncontact ACL injury.
Knee flexion angles have been shown to alter the resultant ACL strain for the same load in cadaveric studies (15,25). In the current study, the significantly increased PI found for the TRotated may not be as "bad" for the ACL as it first seems, because the resultant load on the ligament may be lowered by the significant increase in knee flexion. However, whether the 6° of increased knee flexion is sufficient to reduce ACL loads is unknown. The lateral hamstrings' support of applied internal rotation loads at knee angles of less than 30° can reduce applied ACL load; therefore, it would be expected that increased knee flexion would moderate the increased PI (26). However, the loads occurring at the knee are in three dimensions. The resultant ACL load from a valgus load increases to 30° of knee flexion, even with muscular support (13,21). Therefore, although PV is of similar magnitude to NS (Fig. 6), the resultant load at the ACL caused by the PV moment may cancel out the reduction in PI because of the increased knee flexion. With the present position of the literature, it is difficult to draw a conclusion as to the moderating impact of knee flexion.
Previous research investigating possible relationships between techniques and ACL injury has used video analyses of injuries occurring during games (6,9,34). One major drawback in this type of analysis is that there is no information about the loads being experienced at the knee, which can be assessed in laboratory studies. However, the limitation of laboratory analysis is that although the knee loads can be calculated, they cannot be clearly linked to the actual injury. Although a prospective laboratory study such as Hewett et al.'s (17) allows for better links between the laboratory results and actual injury, the positions achieved in the laboratory do not necessary reflect those that occur during the injury. Coupling the results from the laboratory studies and in-game injury analysis can overcome these limitations. Video analyses have suggested that an abducted hip, straight leg, foot rotated out, rotated torso, and lateral torso flexion are often characteristic of noncontact ACL injuries (6,14,19,34). Three of these postures are represented in the high loading techniques identified in this study: FWide-abducted hip; TOpposite-lateral torso flexion; and TRotated-rotated torso. During FWide, the foot was also turned out more than in NS, with TRotated also having more lateral flexion and hip abduction than NS (Table 2), consistent with the postures causing ACL injury suggested by video analyses. Therefore, the current work supports the previous video analyses of ACL injury and provides the actual knee loads that may be related to the injury. It is recommended that sidestep cutting techniques that exhibit these postures should be avoided, to reduce the risk of injury.
Athletes do not suffer an injury each time that they perform a sidestep cut, as evident by the fact that no injuries were sustained during the present testing. This is the result of the external knee loads being supported by the muscles crossing the knee (2,24). This study did not analyze the effect technique had on muscular support and is an area of future research. Previous work has found that when sidestep cutting tasks are performed under an unanticipated condition, the loads experienced at the knee in both valgus and internal rotation increase significantly, with possibly compromised muscular support (2,3). Unanticipated sidestep cuts are common during team sports, often to avoid a defender-a task that has been shown to change the kinematics of a planned sidestep cut (29). During the current protocol, all sidestep cuts were performed in anticipated conditions. Should an individual perform an unanticipated sidestep cut with a FWide technique, the knee loads experienced may be even higher and place the athlete at a high risk of injury. However, this notion requires further investigation.
Having identified sidestep cuts with techniques that may highly load the ACL, the next step is to identify whether athletes can be trained to avoid using these techniques. If technique modification is successful in changing technique and reducing knee loads, it can be added to current training protocols aimed at noncontact ACL injury reduction. However, to be accepted by the sporting community, it would also need to be shown that the technique modification is not detrimental to the ability of an athlete to use his or her sidestep cut to avoid or intercept the opposing player. There also needs to be a long-term, prospective, randomized control study similar to those performed by Caraffa et al. (7), Hewett et al. (16), and Myklebust et al. (33) to identify whether technique changes aimed at reducing ACL injuries are successful or have any effect on other injuries. Athletes are unlikely to accept training that will increase their risk of another injury, because there are other training protocols that have been shown to be effective at preventing ACL injuries and that do not carry this risk (7,16,18,33). If a technique-modification study is unsuccessful, it may also be appropriate to look at the ability to modify the technique of young, developing athletes. The motor patterns of adult, particularly elite, athletes may be harder to change, especially in unanticipated situations. This may not be true of younger, developing athletes.
In summary, sidestep cutting techniques have a significant effect on peak valgus, peak internal rotation, and mean flexion/extension moments at the knee. With the identification of high-risk techniques, it be can speculated that it may be possible to develop training protocols that modify an athlete's sidestep cutting technique, specifically by bringing the foot to the midline and keeping the torso upright with no rotation, to reduce their knee loads and, therefore, potentially reduce their risk of ACL injury.
This project was funded by a grant from the Australian Football League.
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