The participation rate of girls in sport has increased dramatically in North America over the last 3 decades (24), resulting in an escalating number of musculoskeletal injuries in this population. Of particular concern is injury to the anterior cruciate ligament (ACL) of the knee, which has been found to occur more commonly in female athletes, as compared with male athletes. It has been suggested that the ACL injury rate in female athletes is 4–6 times that of male athletes (1,12).
Noncontact mechanisms produce the majority of ACL injuries and frequently involve a sudden change of direction or landing from a jump (34). During these activities, female athletes tend to adopt the “position of no return” (14), which entails landing with minimal knee and hip flexion, and a valgus or “knock-kneed” posture. These joint positions have been suggested to place the ACL at increased risk of injury (15). Low knee flexion angles have been associated with higher anterior tibial shear forces, which increase the strain on the ACL (19). However, injury to the ligament cannot occur with sagittal loading only (19,21). An abduction moment applied to the knee has been shown to increase strain in the ACL (6,19). Transverse plane movement has also shown the capacity to increase ACL strain (19). It is likely that ACL injury occurs as a result of multiplanar forces and not one in particular (30). The planes of movement at the knee, and how they interact with each other to cause ACL injury, are an essential area of research. However, because it is possible that each plane contributes significantly to the injury mechanism, affecting change in only 1 or 2 planes may prove to be beneficial.
Recently, there has been increased attention directed at studying ACL injury prevention strategies, as researchers aim to minimize risk factors for injury that are modifiable. A few studies have investigated the effects of verbal and video feedback on improving jump landing biomechanics. Some authors (22,29) found a significant acute improvement in jump landing biomechanics, as determined by a lower peak ground reaction force, after augmented feedback. Mizner et al. (23) showed immediate positive results in jump landing kinematics in female collegiate athletes after 5 minutes of instruction. Athletes significantly increased their peak hip and knee flexion angles, whereas peak knee valgus concurrently decreased. A recent volleyball-specific study demonstrated that after 2 minutes of verbal feedback, Division I female collegiate athletes were able to decrease their vertical ground reaction forces by 23% when landing from spiking a tossed volleyball (4). These studies all involved young, college-aged adults with years of sport experience.
To date, there has been a paucity of research involving younger female athletes completing a jump landing in a sports-specific situation. Because the increased risk of ACL injury in female athletes becomes apparent around puberty (37), it is relevant to examine athletes of that age. However, the verbal feedback instructions given to novice athletes needs to be different from those used for older, experienced players and should be of limited scope. Beginners need a broader, overall picture to be successful with the new task (18). Unfortunately, this can limit the number of risk factors for injury that are addressed during a feedback session and can limit the ability to explore multiplanar movements. It is also important within this population to identify whether augmented feedback, including verbal and visual instruction, is effective in improving jump landing technique in a sport-specific situation, not only in a controlled laboratory setting as examined in previous studies (22,23,29).
In some university and club-level sport settings, when biomechanical analysis is requested by a coach, an analysis group visits the athlete or team in their own environment and collects video data during a regular practice or training session. Each athlete is then kinematically analyzed and feedback is delivered to them on the spot. The athlete then has the opportunity to try the skill again using the newly acquired information. Mainly because of budget constraints, it is customary for the athlete or sports team to receive only one visit from the biomechanical analysis group per competitive season. Therefore, it is valuable to examine whether one educational session is sufficient to bring about positive change in athletes' movement patterns. There is also a need to establish whether there is a long-term change in motor patterns after verbal and video feedback, as previous studies have mainly examined the immediate changes in behavior after a feedback intervention (22,23,29).
The efficacy of using one verbal and video feedback session with young female athletes to improve jump landing mechanics over a 4-week time period has not been previously investigated. If found to be effective, feedback would be an inexpensive, easily implemented method of decreasing injury risk. Therefore, the purpose of the present exploratory study was to examine whether spike jump landing mechanics in adolescent female volleyball players could be improved during a single practice session using video and verbal feedback and whether that change could be sustained over the last 4 weeks of their competitive season. It was hypothesized that an improvement in jump landing would be observed after feedback, with the immediate effects demonstrating more significance than the changes at 4 weeks.
Experimental Approach to the Problem
A repeated measures design, as depicted in Figure 1, was used to determine whether one session of video and verbal feedback could be effective in improving jump landing kinematics in adolescent girls. Kinematic data relating to the girls who received augmented feedback were collected on 3 separate days, with 2 weeks in between each data collection day. The girls in the control group (CG) were filmed on 2 occasions only, at the beginning and at the end of the 4-week study period. A 4-week time frame was deemed sufficient, as no previous studies have looked at jump landing changes beyond 1 week postfeedback (26). The competitive season for the teams was ending in 4 weeks as well, preventing data collection beyond that point. To test the hypothesis, the data collection was done during regularly scheduled volleyball practices at the girls' home gyms. One of their coaches tossed the balls for the girls to hit as they completed their spike jump landings so as to create a sports-specific experience.
Trunk, hip, and knee flexion angles when landing were chosen as dependent variables because of previous work suggesting a more erect body position may put the ACL at increased risk of injury (15,19). Augmented feedback, including both verbal and visual feedback, has been shown to improve jump landing mechanics in some populations (4,23). However, there is limited information on using this intervention with younger individuals. If found to be effective, feedback could be implemented at an earlier age to reduce injury risk.
A sample of convenience of 19 healthy female athletes, comprising 2 grade 8 volleyball teams, was recruited from 2 middle schools to participate in this study. The team from one school was randomly designated as the intervention group (IG, n = 10), whereas the team from the other school was designated as the CG (CG, n = 9). The individuals on the 2 teams were not randomly selected for the control and IGs because of the likelihood of contamination using that design. If only a few girls on a team received feedback regarding their landing mechanics, their instructions and experiences would likely be passed on to the rest of their teammates, who would be members of the CG. This would make any statistical comparison between groups difficult.
All subjects were between 12 and 14 years of age (IG: mean ± SD age = 13.2 ± 0.4 years; CG: 13.1 ± 0.3 years). No significant differences existed in age, playing experience (IG: mean ± SD playing experience = 1.8 ± 0.6 years; CG: 1.9 ± 0.6 years), or hand dominance between the groups (all girls were right handed). Three additional subjects were excluded because they failed to attend all of the data collection sessions (IG = 1; CG = 2). None of the subjects had received previous ACL injury prevention education or specific jump landing training. At the time of filming, the teams were in the last 4 weeks of their competitive season. Neither team was participating in a resistance training program at the time of the study.
Ethics approval was obtained from the University Education & Nursing Research Ethics Board, and informed written consent was obtained from each of the subjects and their parents, before beginning the study.
All athletes were filmed using 4 Canon digital camcorders (Canon U.S.A. Inc., Lake Success, NY, USA). One camera (Canon GL2) was placed 2 m from the end position of the spike to capture a frontal view of the spike landing (Figure 2). An additional camera (Canon GL2) was situated 6 m to the left of the athlete to capture the left side of the body during the spike landing in the sagittal view. The remaining 2 cameras (Canon ZR500 and Canon ZR700) were placed 2 m behind the athlete's start position, and 6 m to the right, capturing the rear frontal view and the right sagittal view, respectively. All cameras were affixed to tripods to ensure they remained stationary for the duration of the trials. Footage from the camera to the left and from the anterior view of the athlete was uploaded directly to a laptop computer during filming using Dartfish motion analysis software (Dartfish TeamPro 5.0; Fribourg, Switzerland). This allowed immediate viewing of the video clips by the athlete and principal researcher. The video footage from the additional 2 cameras was manually uploaded after each data collection session, at a rate of 30 frames per second, using Dartfish motion analysis software.
Control Group Protocol
The CG was filmed for biomechanical analysis during regularly scheduled, after-school volleyball practice times in their school gymnasium. Before filming commenced, the athletes participated in the team's warm-up, which consisted of 5–10 minutes of light jogging and hitting a volleyball back and forth between partners.
Two position markers were placed on the volleyball court, designating a start point and an end point for the spike landing task that was to be recorded (Figure 2). These points were confirmed by the coach as following the path of the spike approach taught during previous practices. At the initial filming session, each subject in the CG was filmed completing 5 successful spikes as the coach tossed the volleyball. A spike was considered successful if the approach and landing occurred within the start and end points and the ball passed over the net and landed in the court. The subjects completed the spikes at their own pace, taking a rest break between trials as needed.
The athletes in the CG received no feedback, other than notification about whether the spike had been successful or not. They continued participation in their competitive season for 4 weeks, after which time they were again filmed, using an identical protocol as the initial filming session.
Intervention Group Protocol
The initial filming of the IG began exactly as for the CG. However, after completion of all 5 successful spikes, each individual athlete was brought over to the side of the court where feedback was provided on her landing mechanics. To begin, the researcher reviewed a checklist of desired movements and positions for landing safely from a jump (Table 1). This checklist was adapted from previous studies involving augmented feedback for improving jump landing mechanics (11,26). The same researcher read out the instructions to each individual athlete at each filming session to ensure consistency of delivery.
The researcher then directed the athlete's attention to her video. Video clips of the last 2 landing trials were shown to the athlete, once at full speed and once in slow motion, stopping at relevant frames that illustrated where the athlete could improve on her jump landing performance. Both the sagittal and frontal plane views of the 2 landing trials were shown to the athlete. This method of video feedback delivery was based on previous work by Herman et al. (11). The athlete was allowed time to ask questions of the researcher to clarify the jump landing instructions. The duration of augmented feedback averaged 3–4 minutes per athlete. Every effort was made to maintain consistency in the delivery of the feedback. However, each individual's feedback varied somewhat depending on her performance of the jump landing task. Verbal feedback given while viewing the video footage was limited to those items on the checklist, but concentrated on each athlete's unique deficiencies. For example, some girls demonstrated adequate knee flexion, but allowed their knees to assume a valgus position, whereas others did not demonstrate knee valgus but landed in a very upright position. Feedback appropriate for each of these specific situations was provided. After viewing the videotape and hearing the instructional checklist, the athlete then repeated 5 spike jumps while being videotaped. Therefore, at week 1, data collected from the IG consisted of 5 preintervention jump landings and 5 postintervention jump landings. Filming of 5 volleyball spikes for each athlete in the IG was repeated after 2 and 4 weeks.
Landing from the spike jump was divided into 2 key positions for biomechanical analysis. Initial touchdown was designated as the first frame of video in which the principal researcher could confidently identify that the athlete's foot was in contact with the floor. If there was doubt as to which of 2 frames of video initial touchdown occurred, the second, or later, frame was chosen to ensure that foot contact with the floor had occurred. The position of maximum flexion was the position of maximum range of motion of the movement of interest reached by the athlete during the spike jump landing. All variables were measured using Dartfish motion analysis software. Dartfish was used as the data analysis tool because of its wide use in the sporting community. This is the system often used when biomechanical analysis teams are hired by sport organizations to analyze athletes because it is portable and easy to use with a minimum of equipment required. Although a 3-dimensional (3-D) motion analysis system is the gold standard for measuring kinematic variables, it was not possible to access such a system for this exploratory study because of time and budget restraints. As well, using a 3-D system would require moving the study into a laboratory setting, when the intent of this investigation was to examine jump landings in an authentic environment.
Kinematic variables from the 3 best spike jump video clips were analyzed for each athlete. The “best” spike jump video clips were chosen by the quality of the video clips and the ability to visualize and measure the variables accurately. Because of the presence of windows in the gymnasiums, the light varied during filming, affecting the clarity of some clips. Also, because of individual variability, the athletes did not always land directly square to the camera. As this would affect the accuracy of the joint measurements, the video clips with the best positioning of the athlete relative to the cameras were chosen.
In total, 14 variables were measured and statistically analyzed to attain a holistic description of jump landing task kinematics. Joint variables measured included trunk, hip, knee, and ankle flexion angles at initial touchdown, and at the position of maximum flexion. Initially, it was the authors' intent to measure knee valgus angle pre- and postfeedback. However, upon examination of the video, it was determined that the angle could not be reliably measured using Dartfish. The valgus angle to be measured was comparatively small (<5°), and the athletes did not consistently land square to the frontal view camera to afford accurate measurement of such a small angle. In addition, because 3-D motion analysis was not used, measured knee valgus angles may not have reflected true frontal plane motion. As other authors have noted (2), valgus measurement taken from a frontal view video camera may be influenced by both tibial and femoral rotation. The current study had the additional challenge of recording and measuring a functional sport-specific task, where the landing was not tightly controlled. Sagittal measurements are less likely to be affected by transverse plane rotations, and because sagittal plane angles were comparatively large (>20°), it was felt that they could be measured with sufficient reliability to determine the change in angle over the study period. A recent study found that 2-D video analysis, when measurements are done by one researcher, can indeed be used to successfully determine changes in sagittal joint position after an intervention (8). Therefore, it was decided to drop valgus measurements from the analysis and focus on sagittal plane kinematics.
Angular variables in the extremities in the present study were measured using relative angles. This technique involved measuring the angle between the long axis of one body segment and the long axis of the adjacent body segment at the joint of interest. The measurement was taken using the 180° scale. In anatomical position, according to the 180° system, all joints are in a position of zero degrees. Any deviation from anatomical position is measured and designated the joint angle (10). Trunk, hip, and knee flexion were designated as a positive number for the purposes of statistical analysis. Extension was designated as a negative number. Plantarflexion of the ankle was measured as a positive number and dorsiflexion (DF) as a negative number.
Angular variables were measured using Dartfish software by the use of the angular measurement tool. An average value from the 3 trials was calculated for each variable for each athlete and used in the statistical analyses. An average of 3 trials was used to reduce the magnitude of the error component contributing to the observed measurement, as each individual measurement inherently includes some random error (28). Also, because of the young age and relative inexperience of the subjects in the study, some girls demonstrated large variation in performance from trial to trial. This, along with measurement error as a result of body position relative to the camera, resulted in moderate to poor within-session reliability measurements (intraclass correlation coefficients [ICCs] < 0.75) between the 3 trials within each filming session (28). Therefore, using an average value was felt to be the best representation of the athletes' performance at each time point. Although reliability analyses between filming sessions was not incorporated into the present investigation, recent work in the Journal of Strength and Conditioning Research has found Dartfish measurement to be reliable and independent of the rater (33). Previous studies from our laboratory have also found good between session reliability when using Dartfish to measure angular kinematics (7,36).
Statistical analyses were undertaken in an attempt to quantify any changes in landing patterns in the 2 groups that were observed subjectively by the researcher. However, the data must be considered preliminary, because of the small sample size and relatively large number of variables measured. Results and discussion of the investigation will therefore be discussed in both quantitative and descriptive terms.
A Shapiro-Wilks test for normality was performed for all variables, and parametric or nonparametric tests were employed as appropriate. A 2-way (group × time) repeated measures analysis of variance (ANOVA) was used to determine if there was any significant difference between IG and CG from week 1 to week 4. If a main effect or interaction was significant, a Holm-Sidak post hoc test was employed to determine where the difference existed. A 1-way repeated measures ANOVA (parametric) or Friedman repeated measures ANOVA on ranks (nonparametric) was used to compare the preintervention variable mean with each of the 3 postintervention variable means to determine if there was a significant change over time within the IG. A critical p value of ≤0.05 was chosen for all statistical tests.
No differences in landing mechanics existed between the 2 groups at baseline. However, statistical analyses revealed significant differences between the 2 groups in the position of maximum flexion 4 weeks after augmented feedback was provided (Figure 3). Maximal hip flexion in the IG was approximately 30° greater than in the CG at week 4. Maximal trunk flexion was 15° greater in the IG. Even though the IG demonstrated maximal knee flexion values 20° or greater than the CG, this variable was not found to be statistically significant. A number of shorter term changes in the position of maximum flexion were found within the IG immediately and 2 weeks after the feedback session (Table 2). Right and left ankle DF, right knee and hip flexion, and trunk flexion all significantly increased after 1 session of augmented feedback. Left knee and hip flexion did not reach statistical significance despite a 20° increase over time, indicating an asymmetrical landing pattern. Minimal differences were apparent when comparing the 2 groups at the instant of initial touchdown (data not shown). Trunk flexion showed an interaction of main effects. The CG at week 4 demonstrated an average of 8° trunk extension, whereas the IG at week 4 demonstrated an average of 3° trunk flexion (p < 0.05). Right knee flexion in the CG decreased slightly, from 21° to 17° (p < 0.05) between week 1 and week 4. There were no other significant differences between groups at the instant of initial touchdown.
Both long-term and short-term changes in landing biomechanics were observed in a group of young female volleyball athletes after one session of verbal and video feedback, providing support for the hypothesis of this study. The long-term changes observed in the IG compared with the CG after 4 weeks concur with previous studies, which found that the positive effects of verbal and video feedback on skill performance can last beyond the immediate (9,26,27). However, the results are contrary to other studies, which have failed to find a significant long-term change in performance after video and verbal feedback (5,35,39). In the present study, hip flexion measures were at least 50% greater in the IG as compared with the CG at week 4. These joint angles were similar to those found by Salci et al. (34) when analyzing college-aged volleyball athletes landing from a jump. Comparable hip flexion range of motion was also found immediately after augmented feedback in another previous study, but no longer term testing was completed by those authors (23). Although hip flexion has not been independently linked to ACL injury mechanisms when landing from a jump, increased hip flexion usually accompanies an increase in knee flexion, which has been suggested as a safe landing position (15).
It is notable that the other variable that significantly differed between groups after 4 weeks was trunk flexion, both at the instant of initial touchdown and at the position of maximum flexion. Trunk flexion is a variable that has not often been reported in the literature in relation to ACL injury prevention, and there is conflicting opinion on whether more or less trunk flexion during jump landing is desirable (3,15,31). Advocates of increased trunk flexion suggest that the movement may help produce increased flexion in the joints of the lower extremities, as found by Blackburn and Padua (3). In addition, a flexed trunk position may decrease the quadriceps force required during a jump landing, because of a reduction in size of the moment arm for the weight of the body about the knee joint. Indeed, a recent study demonstrated that by moving the center of gravity of the body anteriorly, as would occur during trunk flexion, knee extensor torque was decreased (38). A decrease in the magnitude of quadriceps contraction torque has been shown to decrease the anterior shear force acting on the ACL (16,20).
During the present study, it was observed that the subjects showed uncertainty regarding proper landing biomechanics at the filming sessions at weeks 2 and 4, and oftentimes required prompting on good landing form before completing their 5 landing trials. This indicated that the newly acquired skill had not been completely integrated into the athletes' movement patterns and still required practice. Because of this lack of integration, it is questionable whether the desired levels of trunk, hip, and knee flexion would be achieved if, at 4 weeks, jump landing biomechanics had been assessed in a game situation, when the girls were not thinking specifically of proper landing form. Recent work has found that kinematics during a volleyball block jump landing differ depending on the presence of an opponent during the block (13). In that study, athletes displayed less knee flexion during a game-specific blocking task, which involved an opponent hitting the ball from the other side of the net. The findings of the present study and those of Hughes et al. (13) underscore the importance of repetition of feedback for proper jump landing biomechanics. It suggests that one feedback session may not be sufficient to effect long-term change in a game-specific situation, when the athlete must rely on subconscious thought to accomplish the movement correctly and safely.
In addition to the long-term changes noted after 4 weeks, more immediate alterations to jump landing mechanics were also apparent. Right knee and bilateral ankle kinematics were significantly improved immediately after the feedback intervention. Right knee flexion increased over time, from 66° preintervention to approximately 85° at all 3 times postintervention. This change in knee flexion was almost double the 11° difference found previously by Mizner et al. (23), but less than the 35° found by Onate et al. (26) after verbal and video feedback. Increased knee flexion after augmented feedback may be a key improvement in terms of injury prevention, as smaller knee flexion angles during jump landing have been found in athletes who went on to rupture their ACL (12). However, it is acknowledged that the lack of knee valgus data in the present study is a limitation. It restricts the ability to assess the effectiveness of the augmented feedback intervention specifically in terms of ACL injury, as valgus knee angles, rather than sagittal plane movements, have been found to be the primary predictors of injury (12).
Although right knee and hip angles significantly increased after feedback, the same did not hold true for left knee and hip measurements. This may reflect the asymmetric nature of the spike task. A recent study found that right-handed volleyball athletes tended to land on their left foot first when hitting from the left side of the court (17). All the athletes in the current study were right handed and hit the ball from the left side of the court. A number of the girls did land on the left leg first after they completed their hit. Frequencies varied from 20% of landings in the IG at week 2 to 67% of landings in the CG at week 1. However, no significant differences were found between groups in terms of 1 or 2 foot landings. Because it appears that the left leg is the primary contributor to controlling the landing in this situation, it may be that one augmented feedback session is not adequate to cause adaptation to this essential role.
Both left and right ankle DF increased immediately postintervention, to similar values demonstrated by college-aged athletes in other studies of jump landing biomechanics (23,34). However, right ankle DF subsequently decreased at week 2, whereas left ankle DF decreased significantly at both weeks 2 and 4, which was not expected. DF decreased to baseline values at weeks 2 and 4, whereas knee, hip, and trunk flexion values remained relatively stable in a flexed position.
The only means by which an athlete can land in a deeply flexed position and not have a concurrently high degree of ankle DF is to possess excellent “squat” technique. It appears that over time, the IG adopted good squat technique when landing from a spike approach. Keeping the knees directly over top of, or posterior to, the toes is generally accepted as being a safe squat position and is accomplished by limiting DF while concurrently flexing the hips and knees (25). This movement pattern is taught extensively by exercise and rehabilitation professionals as good form (32). Although DF returned to baseline values after the feedback intervention, a high degree of hip and knee flexion was preserved by these athletes when landing from a jump. Hip and knee flexion were maintained within a range of motion similar to that exhibited by male college-aged athletes in a recent study (34). In that investigation, male athletes demonstrated 67° of hip flexion and 80° of knee flexion when landing from a jump. Because male athletes have a decreased risk of ACL injury, the similar position of the IG athletes in this study at weeks 2 and 4 could be considered a safe, desirable landing position.
Left knee and bilateral hip flexion increased by a wide margin immediately postfeedback and at week 2; however, the movements were not found to be significantly different from prefeedback values. These variables demonstrated greater variability than the ankle and right knee measurements, and so despite increasing 20° on average, statistical significance was not attained. The large diversity of responses noted in the athletes suggested that their interpretation of and reaction to the feedback was individualized. Some girls subsequently landed with much more knee and trunk flexion, whereas other girls responded by only flexing their knees and not their trunk. Individual differences in musculoskeletal development may also have played a role in the large SDs calculated from the data. Landing from a jump with a high degree of flexion requires adequate strength to control the movement. Some of the girls may not have possessed that strength, because of a relatively low physical maturity level compared with their teammates. The fact that developmental level was not recorded is a limitation of this study. The information could have provided further insight as to the reasons for large variation both within and between individuals when landing from a jump. Nevertheless, short-term changes in jump landing biomechanics were apparent in the present study and support the findings of a number of previous studies (4,22,23,29) that found immediate positive results in jump landing mechanics after a single session of feedback.
It is acknowledged that designating teams as groups invites additional confounding factors such as different coaches and team schedules. The 2 teams were in the same middle school league, and therefore played the same number of games and had similar practice schedules during the 4-week study period. It was felt that coaching would not affect the outcome of the study for a couple of reasons. First, jump landing mechanics were not taught by either team coach before the start of the study. As well, even though good jump landing technique was discussed with the coach of the IG during the first data collection session, he reported afterward that he did not coach the girls in their landing mechanics during the study period, as he was “too busy and had too many other things to cover in a practice.” Therefore, it was up to the girls to remember the proper jump landing instructions and practice them. This is a reality of volleyball teams receiving biomechanical analysis and adds credibility to the “real-world” experience the present authors were aiming for. However, the limitations of using teams as groups must be kept in mind.
The observations from this preliminary study suggest that lower extremity kinematics can be positively influenced by just one session of verbal and video feedback. It also appears that the trunk may play a larger role than previously thought in improving jump landing biomechanics and deserves further study. Future research should be conducted using 3-D motion analysis in a sports-specific situation to verify the findings of this study. It would also be beneficial to determine if the improved jump landing technique observed in practice carries over into a competitive situation.
The outcomes of this study suggest that a single session of verbal and video feedback may have a positive effect on the jump landing patterns demonstrated by adolescent female volleyball players. This could result in decreased risk of ACL injury. Volleyball coaches and physical education teachers may want to incorporate jump landing feedback into their daily practices and classes. Verbal feedback can be initiated at no financial cost and can be done simultaneously with practice of other specific skills, such as blocking or hitting. It can be accomplished by simply telling girls to land softly in a more flexed position, on both feet, with knees above the toes. Introducing video feedback would require at least one video camera and software that allows the coaching staff to upload and view the athletes' movements. Many sport organizations already own this equipment, which can be accessed at the coaches' request. The small investment of time and money into the prevention of injuries could benefit the team significantly in the long term.
We thank Dr. M.M. Porter for reviewing and offering advice on the manuscript and the participating coaches and athletes for their time and cooperation. Funding for this work was provided by a Manitoba Health Research Council Studentship.
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Keywords:Copyright © 2012 by the National Strength & Conditioning Association.
anterior cruciate ligament (ACL) injury; kinematics; motor learning; injury prevention