Effects of a Knee Extension Constraint Brace on Selected Lower Extremity Motion Patterns During a Stop-Jump Task

Transcription

1 Journal of Applied Biomechanics, 2008, 24, Human Kinetics, Inc. Effects of a Knee Extension on Selected Lower Extremity Motion Patterns During a Stop-Jump Task Cheng-Feng Lin, 1 Hui Liu, 2 William E. Garrett, 3 and Bing Yu 1 1 University of rth Carolina at Chapel Hill, 2 Beijing University of Sports, and 3 Duke University Medical Center Small knee flexion angle during landing has been proposed as a potential risk factor for sustaining noncontact ACL injury. A that promotes increased knee flexion and decreased posterior ground reaction force during landing may prove to be advantageous for developing prevention strategies. Forty male and forty female recreational athletes were recruited. Three-dimensional videographic and ground reaction force data in a stop-jump task were collected in three conditions. Knee flexion angle at peak posterior ground reaction force, peak posterior ground reaction force, the horizontal velocity of approach run, the vertical velocity at takeoff, and the knee flexion angle at takeoff were compared among conditions: knee extension constraint, nonconstraint, and no. The knee extension constraint significantly increased knee flexion angle at peak posterior ground reaction force. Both knee extension constraint and nonconstraint significantly decreased peak posterior ground reaction force during landing. The and knee extension constraint did not significantly affect the horizontal velocity of approach run, the vertical velocity at takeoff, and the knee flexion angle at takeoff. A knee extension constraint exhibits the ability to modify the knee flexion angle at peak posterior ground reaction force and peak posterior ground reaction force during landing. Lin and Yu are with the Center for Human Movement Science, Division of Physical Therapy, University of rth Carolina, Chapel Hill, NC; Liu is with the Sports Biomechanics Laboratory, College of Sports Human Movement Science, Beijing Sports University, Beijing, China; and Garrett is with the Duke Sports Medicine Center, Duke University Medical Center, Durham, NC. Keywords: anterior cruciate ligament, injury prevention, landing biomechanics Anterior cruciate ligament (ACL) injury is one of the most commonly seen injuries in sports in which landing and cutting tasks are frequently performed (Boden et al., 2000; Kirkendall & Garrett, 2000; McNair et al., 1990). The majority of ACL injuries occur as a result of a noncontact mechanism; that is, there is no direct physical contact to the knee when an injury occurs (Boden et al., 2000; Ferretti et al., 1992; McNair et al., 1990). Female athletes have significantly higher risk of sustaining a noncontact ACL injury than male athletes do, and the elevated risk for noncontact ACL injury among females may result from altered lower extremity motion patterns the female athletes have (Decker et al., 2003; Henry & Kaeding, 2001; Huston et al., 2000; James et al., 2004; Lephart et al., 2002). The literature repeatedly suggests that small knee flexion angle during athletic landing tasks might be a potential risk factor of sustaining noncontact ACL injuries. Analysis of video records of actual ACL injuries by Boden et al. (2000) revealed that most noncontact ACL injuries occurred with the knee close to extension in landings or quick deceleration. Cochrane et al. (2007) reported that 92% of the noncontact ACL injuries occurred at knee flexion angle smaller than 30 degrees. Several in vivo (Cerulli et al., 2003; Lamontagne et al., 2005), in vitro (Markolf et al., 1995), and computer simulation (Pandy and Shelburne, 1997) studies also demonstrated that peak ACL strain and ACL loading occur at small knee flexion angles. Studies further demonstrated that populations at elevated risk for noncontact ACL injuries performed athletic tasks at decreased knee flexion angles (Chappell et al., 2002; Cochrane et al., 2007; Colby et al., 2000; Malinzak et al., 2001; Weinhold et al., 2007). 158

2 A Knee Extension 159 The posterior ground reaction force is responsible to the ACL loading by producing an external knee flexion moment (Yu et al., 2006a). The increased external knee flexion moment increases the internal knee extension moment and therefore the demand of quadriceps forces. Large quadriceps forces at small knee flexion angles may injure the ACL (DeMorat et al., 2004). s have been applied as prevention modalities for knee and ankle injuries. Sitler et al. (1990) and Albright et al. (1994) found that wearing knee s reduced the medial collateral ligament injury rate on the football players compared with the control groups. Feuerbach and Grabiner (1993) and Feuerbach et al. (1994) reported that applying an ankle enhanced the proprioceptive input and decreased postural sway. As an effort to prevent noncontact ACL injury, a knee extension constraint has been recently developed as a potential training tool to assist athletes in increasing knee flexion angle during landing tasks. A previous study (Yu et al., 2004) demonstrated that wearing a knee extension constraint significantly increased knee flexion angle at initial contact during landing of a stopjump task. Although not significant as a result of small sample size, the knee extension constraint may also decrease peak impact posterior ground reaction force during landing of the stop-jump task (Yu et al., 2004). The previous study, however, failed to demonstrate that the altered lower extremity kinematics and kinetics when wearing knee extension constraint was indeed due to the knee extension constraint or due to the or the combination of knee extension constraint with the. The effects of the knee extension constraint on the lower extremity motion patterns in landing tasks have to be determined to support the knee extension constraint as an important element of for modifying lower extremity motion patterns. The purpose of this study was to determine the effects of the knee extension constraint on the knee flexion angle and peak posterior ground reaction force during a stop-jump task, in which the noncontact ACL injury frequently occurs, and on the overall performance of the stop-jump task. We hypothesized that wearing the knee extension constraint would increase knee flexion angle at peak posterior ground reaction force and decrease peak posterior ground reaction force. We further hypothesized that wearing a nonconstraint would not affect the knee flexion angle at peak posterior ground reaction force and the peak posterior ground reaction force. We finally hypothesized that wearing the two s would not affect the horizontal velocity of approach run, vertical velocity at takeoff, and the knee flexion angle at takeoff of the stop-jump task. subjects for this study (Table 1). A recreational athlete was defined as a person who regularly played sports or exercised at least three times for more than three hours per week, without following a professionally designed training program. The use of human subjects in this study was approved by the Biomedical Institutional Review Board in School of Medicine of the University of rth Carolina at Chapel Hill. Written consent was obtained from each subject before the data collection. Two types of knee s were tested in this study. One was a knee extension constraint, which was made of 6061-T6 aluminum with upright upper thigh and lower calf cuffs (4titude; dj Orthopedics, LLC, Vista, CA, USA; Figure 1). A resistive torque against knee Table 1 Means and Standard Deviations of Subject Characteristics Item Male (N = 40) Female (N = 40) Age (years) 22.4 (3.10) 23.2 (2.70) Body mass (kg) 78.8 (9.40) 60.1 (11.50) Body height (m) 1.78 (0.06) 1.63 (0.07) Methods A total of 40 male and 40 female recreational athletes between 18 and 30 years of age without a known history of lower extremity disorders were recruited as the Figure 1 The knee with knee extension constraint used in this study.

3 160 Lin, Liu, Garrett, and Yu extension motion engaged at 40 of knee flexion and applied a gradually increasing resistance to knee extension motion up to 10 of knee flexion at which a rigid stop prevents further knee extension. The resistive torque was 3 N m at 10 of knee flexion. The other knee was a nonconstraint that had the same appearance as the knee extension constraint, but did not have the knee extension constraint. Both knee s were off-the-shelf versions in five sizes (extra small, small, medium, large, and extra large). Each subject was asked to perform a stop-jump task under three conditions: no, with the nonconstraint, and with the knee extension constraint. The s were applied only to the dominant leg, which was defined as the leg naturally used to perform a singlelegged hop or jump. The order of conditions during data collection was randomized by randomly drawing to eliminate the possible effects of fatigue on the results. Subjects were not informed about what types of they were wearing during data collection. The stop-jump task consisted of an approach run of up to five steps followed by a two-footed landing and takeoff for maximum height (Figure 2). Subjects were instructed to run as fast as they could, land with each foot on a separate force plate, and felt comfortable performing the stop and jump. The stop-jump task was described to each subject before warm-up and practice. The specific technique was not demonstrated in detail to avoid any coaching effect. Each subject had at least 5 min of warm-up before initial data collection and practice of stop-jump task before each condition. The warm-up included stretching, walking, jogging, and jumping and no specific dosage of each warm-up activity was requested. The practice of stop-jump task was similar across the subjects. Subjects were given at least 1 min of rest between trials to prevent fatigue. Reflective markers were placed bilaterally on the anterior superior iliac spines, lateral thighs, anterior superior shanks, anterior inferior shanks, lateral malleoli, Figure 2 Our stop-jump task consists of a running approach of up to five steps, two-footed landing, and a jump for maximum height at takeoff. heels, and first and fifth metatarsophalangeal joints of each subject (Figure 3, left) by the same investigator. Another reflective marker was placed between the 4th and 5th lumbar vertebrae (L4-L5; Figure 3, left). Each subject performed five successful trials of the stop-jump task with maximum effort for each condition. A successful trial was defined as a trial in which the subject performed the stop-jump task as defined and landed with each foot on a separated force plate as required. Three-dimensional videographic data were collected for each subject during the stop-jump task. A videographic acquisition system with six infrared video cameras was used to record the real-time three-dimensional trajectories of the reflective markers on the subject at a frame rate of 120 frames per second. Two Type 4060A Bertec force plates (Bertec Corporation, Worthington, OH, USA) were used to collect the ground reaction force signals at a sample rate of 1,200 samples per channel per second. The videographic and ground reaction force signals were recorded using the Peak Performance Motus videographic and analog data acquisition system (Peak Performance Technology, Inc., Englewood, CO, USA). The videographic and force plate data collection were time-synchronized to 1,200 frames/second and 1,200 samples/channel/second using a linear interpolation method. A static standing trial with additional six markers was collected after all stop-jump trials were completed. The additional six markers were placed bilaterally on the medial and lateral tibial condyles and medial malleoli (Figure 3, right). The same person performed marker placement for all the subjects. The raw three-dimensional coordinates of the markers in each stop-jump trial were filtered through a Butterworth low-pass digital filter at estimated optimum cutoff frequencies (Yu et al., 1999). The three-dimensional local coordinates of markers on the medial and lateral tibial condyles and medial malleolus in the tibia reference frames were determined from the three-dimensional coordinates of markers on the tibia in the standing trial. The three-dimensional coordinates of the medial and lateral tibial condyles and medial malleolus relative to the laboratory reference frame in stop-jump trials were estimated from the locations and orientations of the tibia reference frames and the local three-dimensional coordinates of the corresponding markers. The threedimensional coordinates of the hip joint centers in stopjump trials were estimated from the three-dimensional coordinates of the markers on the right and left ASISs and L4-L5 joint (Bell et al., 1990). The knee joint center was defined as the middle point between markers on the medial and lateral tibial condyles. The ankle joint center was defined as the middle point between the markers on medial and lateral malleoli. The three-dimensional coordinates of the knee and ankle joint centers and medial and lateral malleolus were used to define the tibia reference frame. The three-dimensional coordinates of the knee and hip joint centers and medial and lateral tibial condyles were used to define the femoral reference frame. Knee

4 A Knee Extension 161 Figure 3 Left: marker placements of the dynamic trials. Right: marker placements of the static trial. Data from the first three successful trials of each subject under each condition were used for analysis. Within-subject standard error was used as a measure of reliability for each dependent variable (Table 2). Analyses of variance with mixed design were performed to compare knee flexion angle at peak posterior ground reaction force, peak posterior ground reaction force, horizontal velocity of approach run, vertical velocity at takeoff, and knee flexion angle at takeoff among conditions and genders. The condition was treated as a repeated factor, and gender was treated as an independent factor. was included in data analysis as an independent variable for determining possible interaction effects between condition and gender on dependent variables. In case of a significant interaction effect between condition and gender on a given dependent variable, one-way analysis of variance with repeated measures was conducted to compare the dependent variable among conditions for each gender. Follow-up t tests were performed to locate significant differences among conditions when a significant condition effect was detected in an analysis of variance for a given dependent variable. A Type I error rate of 0.05 was used as an indication of statistical significance for each analysis of variance. Bonferroni adjustment was made to guarantee the overall Type I error rate for a given analysis of variance. All statistical analyses were performed using SPSS 11.5 computer program (SPSS for Windows, Chicago, IL, USA). joint angles were determined as Euler angles of the tibial reference frame relative to the femoral reference frame with flexion-extension about z-axis as the first rotation, valgus-varus about y-axis as the second rotation, and internal-external rotation about x-axis as the third rotation (Chao, 1980). The stance phase of the stop-jump task was divided into two subphases: a landing phase and a jumping phase (Figure 2). The landing phase was defined as the time period from the initial foot contact with the ground to the maximal knee flexion. The jumping phase was defined as the time period from the maximal knee flexion to the takeoff. The initial foot contact with the ground and the takeoff were identified from the ground reaction force data. The knee flexion angles of the dominant leg at peak posterior ground reaction force and the peak posterior ground reaction force on the dominant foot during landing in each trial were identified (Cerulli et al., 2003; Lamontagne et al., 2005; Yu et al., 2006b). The knee flexion angle at takeoff was also identified in each trial. The forward velocity of the mid-point of the straight line connecting hip joint centers immediately before the initial foot contact was estimated to represent the velocity of approach run. The vertical velocity of the same point at takeoff of the jump was estimated to represent vertical velocity at takeoff. All signal processing and data reduction were performed using the MotionSoft three-dimensional biomechanical data reduction program package version 6.5 (MoitonSoft, Inc., Chapel Hill, NC, USA). Results Condition and gender had no significant interaction effect on the knee flexion angle at peak posterior ground reaction force (p =.393; Table 3). Condition and gender had significant main effects on the knee flexion angle at peak posterior ground reaction force (p <.001; Table 3). Subjects significantly increased their knee flexion angle at peak posterior ground reaction force during landing of the stop-jump task when wearing the knee extension constraint in comparison with when wearing the without knee extension constraint and when wearing no (p <.001; Table 3). Male subjects significantly increased their average knee flexion angle at peak posterior ground reaction force from 35 of no condition to 40 of constraint condition, whereas female subjects significantly increased theirs from 30 to 37 (p <.001; Table 3). The nonconstraint did not significantly affect the knee flexion angle at peak posterior ground reaction force (Table 3). Condition and gender had no significant interaction effect on the peak posterior ground reaction force during landing of the stop-jump task (p =.97; Table 4), but both had significant main effects on the peak posterior ground reaction force (p <.001; Table 4). Subjects significantly decreased peak posterior ground reaction force when wearing the (p <.001; Table 4). The peak posterior round reaction force of the male subjects decreased to 0.65 and 0.64 times body weight when wearing the knee

5 162 Lin, Liu, Garrett, and Yu Table 2 Means and Standard Deviations of Within-Subject Standard Errors Variable Male Knee flexion angle at peak PGRF 2.30 (1.24) 2.37 (1.65) 1.92 (1.22) Peak PGRF 0.16 (0.10) 0.17 (0.13) 0.16 (0.11) Horizontal velocity of approach run 0.17 (0.12) 0.17 (0.12) 0.18 (0.16) Vertical velocity at takeoff 0.06 (0.03) 0.09 (0.06) 0.08 (0.09) Knee flexion angle at takeoff 2.88 (2.17) 3.45 (3.18) 2.74 (1.52) Female Knee flexion angle at peak PGRF 2.24 (1.32) 2.08 (2.63) 2.77 (2.37) Peak PGRF 0.14 (0.11) 0.14 (0.10) 0.14 (0.07) Horizontal velocity of approach run 0.10 (0.06) 0.14 (0.10) 0.14 (0.08) Vertical velocity at takeoff 0.05 (0.03) 0.06 (0.04) 0.08 (0.06) Knee flexion angle at takeoff 2.35 (1.48) 2.90 (2.15) 3.20 (2.87) Table 3 Mean and Standard Deviation of the Knee Flexion Angle at the Peak Posterior Ground Reaction Force (in Degrees) Male 35 (9) 35 (10) 40 (9)* Female 30 (7) 31 (9) 37 (9)* *Knee flexion angle at the peak posterior ground reaction force was significantly lower in the constraint condition than the no condition. Table 4 Mean and Standard Deviation of the Peak Posterior Ground Reaction Force (BW) Male 0.75 (0.42) 0.64 (0.40) 0.65 (0.55)* Female 0.91 (0.44) 0.83 (0.40) 0.81 (0.38)* Peak posterior ground reaction force was significantly lower in the nonconstraint condition than the no condition. *Peak posterior ground reaction force was significantly lower in the constraint condition than the no condition. Table 5 Mean and Standard Deviation of the Horizontal Velocity of Approach Run (Meters per Second) Male 2.59 (0.65) 2.36 (0.64) 2.44 (0.64) Female 2.34 (0.50) 2.12 (0.43) 2.18 (0.53) Table 6 Mean and Standard Deviation of the Vertical Velocity at Takeoff (Meters per Second) Male 2.87 (0.30) 2.84 (0.33) 2.90 (0.33) Female 2.36 (0.31) 2.37 (0.24) 2.36 (0.30) Table 7 Mean and Standard Deviation of the Knee Flexion Angle at Takeoff (Degrees) Male 16 (9) 17 (10) 17 (8) Female 15 (7) 13 (11) 16 (9) extension constraint and nonconstraint from 0.75 times body weight when wearing no. The peak posterior ground reaction force in females decreased to 0.81 and 0.83 times body weight when wearing knee extension constraint and nonconstraint from 0.91 times body weight when wearing no (Table 4). There was no significant difference in the peak posterior ground reaction force during landing of the stop-jump task between knee extension constraint and nonconstraint. Condition and gender had no significant interaction effects on the horizontal velocity of approach run, vertical velocity at takeoff, and knee flexion angle at takeoff in the stop-jump task (p =.99, p =.44, p =.24). The knee s did not significantly affect the horizontal velocity of approach run (p =.12), the vertical velocity at takeoff (p =.33), and the knee flexion angle at takeoff (p =.12) in the stop-jump task (Tables 5 7).

6 A Knee Extension 163 Discussion The results of this study showed that the knee extension constraint had significant effects on the knee flexion angle at peak posterior ground reaction force and peak posterior ground reaction force during landing of the stop-jump task. The results of this study also showed that knee extension constraint and did not significantly affect the horizontal velocity of approach run, vertical velocity at takeoff, and knee flexion angle at takeoff. The horizontal velocity of approach run and the vertical velocity at takeoff were two major measures of effort in the stop-jump task. These two measures of effort may also affect the peak ground reaction force in the stopjump task. The consistency of the effort to perform the stop-jump task provides further support that the differences in the selected lower extremity motion patterns under different conditions were indeed the and knee extension constraint effects, instead of the effects of effort. In addition, vertical velocity at takeoff is the only determinant of the jump height. Similar vertical velocity at takeoff across conditions in this study suggests that wearing the two s did not significantly affect the jumping height of the stop-jump task. These results are consistent with the literature showing that wearing the knee s would not affect performances of athletic tasks (Stephens, 1995). Increased knee flexion angle at peak posterior ground reaction force while wearing the knee extension constraint was indeed an effect of the knee extension constraint as we hypothesized. The decreased peak posterior ground reaction force while wearing both s, however, was apparently an effect of bracing, which was not as we hypothesized. The most likely explanation for this effect of bracing on the peak posterior ground reaction force is that wearing knee s tested in this study increased hip flexion angular velocity at the initial foot contact with the ground and the peak posterior ground round reaction force. As Yu et al. (2006b) showed, the greater the hip flexion angular velocity at initial foot contact with the ground, the lower the peak posterior ground reaction force during landing of the stop-jump task. Small knee flexion angle is likely to be a risk factor of sustaining a noncontact ACL injury. As previously mentioned, clinical studies repeatedly demonstrated that noncontact ACL injuries occur at small knee flexion angles (Boden et al., 2000; Cochrane et al., 2007; Griffin et al., 2000). Scientific studies also repeatedly demonstrated that decreasing knee flexion angle would increase ACL loading (Li et al., 1999; Markolf et al., 1995; Pandy & Shelburne; 1997). Markolf et al. (1995) found that the in vitro ACL loading increased as the knee flexion angle decreased with the same anterior shear force applied at the knee. Li et al. (1999) found that the in situ ACL loading increased as the knee flexion angle decreased and peaked at 15. Pandy and Shelburne (1997) demonstrated that the ACL loading increased as the knee flexion angle decreased during isometric contraction of the quadriceps muscle in a computer simulation study. DeMorat et al. (2004) demonstrated that a quadriceps force of 4,500 N can result in ACL injuries in cadaver knees at a knee flexion angle of 10. Studies also demonstrated that individuals with elevated risk for noncontact ACL injury had small knee flexion angles during landing of a variety of athletic tasks (Chappell et al., 2002; James et al., 2004; Malinzak et al., 2001). These studies together demonstrate that small knee flexion angle during landing tasks my predispose individuals to noncontact ACL injury. Peak posterior ground reaction force is an important indicator of peak ACL loading (Cerulli et al., 2003; Lamontagne et al., 2005). The posterior ground reaction force is responsible to the ACL loading by producing an external knee flexion moment. The increased external knee flexion moment increases the internal knee extension moment, which increases quadriceps forces. Large quadriceps forces at small knee flexion angles were demonstrated to injure the ACL (DeMorat et al., 2004). Sell et al. (2007) also demonstrated that the external knee flexion moment and knee flexion angle at peak posterior ground reaction force had significant correlation with the proximal anterior shear force in a vertical stop-jump task. Based on the relationship among the posterior ground reaction force, quadriceps force, and ACL injury, peak posterior ground reaction force and the corresponding knee flexion angle should be important concerns of the noncontact ACL injuries. Increased ACL loading as knee flexion angle decreases is mainly due to effects of the knee flexion angle on the patellar tendon tibial shaft angle (Nunley et al., 2003) and ACL elevation angle (Li et al., 2005). Nunley et al. (2003) demonstrated that the patellar tendon tibial shaft angle increases as the knee flexion angle decreases. Increasing patellar tendon tibial shaft angle will increase the anterior shear force applied on the proximal end of the tibia by the patellar tendon when the resultant patellar tendon force remains constant. The increased tibial anterior shear force results in increased ACL loading (McLean et al., 2004; Shelburne et al., 2004a; Yu et al., 2006a). Li et al. (2005) demonstrated that the ACL elevation angle increases as the knee flexion angle decreases. Increasing the ACL elevation angle increases the ACL loading when the loading parallel to the tibial plateau born by the ACL remains constant. Nunley et al. (2003) further demonstrated that, with the same patella tendon force at 25 knee flexion, a 5 increase in knee flexion angle would reduce the anterior shear force at the knee by 8% for males, whereas a 7 increase in knee flexion angle would reduce anterior shear force at the knee by 10% for females. The cadaver testing data reported by Markolf et al. (1995) showed that, at 25 knee flexion, 5 and 7 increases in knee flexion angle would reduce ACL loading by 7% and 9%, respectively. Large posterior ground reaction force during a landing task is also likely to be a risk factor of sustaining a noncontact ACL injury. Yu et al. (2006a) demonstrated that posterior ground reaction force was a major contributor to the external knee flexion moment in the stop-jump task. Increasing posterior ground reaction force would

7 164 Lin, Liu, Garrett, and Yu increase external knee flexion moment, and therefore the quadriceps force (Hsieh & Draganich, 1998; Yu et al., 2006a). The increased quadriceps force increases the anterior shear force through the patellar tendon (Nisell, 1985; Nisell & Ekholm, 1985; Shelburne et al., 2004a) and thus increases the ACL loading (Yu et al., 2006a). Computer simulation studies demonstrated that peak ACL loading, peak anterior shear force, peak patellar tendon force, and peak quadriceps force during level walking occurred at the same time (Shelburne et al., 2004a, b). In vivo studies reported that peak ACL strain occurred at the same time as the initial peak vertical ground reaction force occurred during landing of a sudden stop task (Cerulli et al., 2003; Lamontagne et al., 2005), whereas peak vertical and posterior ground reaction forces, peak proximal anterior tibial shear force, and peak knee extension moment occurred at essentially the same time (the difference is less than s; Yu et al., 2006b). Although knee extension moment was not compared in this study because of the unknown flexion-extension moment added to the knee by the knee, the results of this study combined with the literature indicate that the reduced peak posterior ground reaction force due to the effect of bracing should reduce knee extension moment and ACL loading. The current study investigated only the immediate effects of the knee extension constraint on the lower extremity motion patterns. Further studies are needed to determine the long-term training effects of the knee extension constraint on the lower extremity motion patterns, in which subjects should be trained with the knee extension constraint and evaluated for the changes in knee flexion angle, peak posterior ground reaction force, and peak knee extension moment without the. Future studies should determine the long-term training effects of the not only on lower extremity motion patterns, but also on the risk of sustaining a noncontact ACL injury. In conclusion, the knee extension constraint significantly increased knee flexion angle at peak posterior ground reaction force and significantly decreased peak posterior ground reaction force during landing of the stopjump task. The increased knee flexion angle was an effect of the knee extension constraint, whereas the decreased peak posterior ground reaction force was an effect of the bracing. The knee extension constraint has the potential to be a training tool for prevention of noncontact ACL injuries if future studies can demonstrate its long term training effects on lower extremity kinematics and kinetics and the risk of noncontact ACL injury. Acknowledgment The authors would like to thank dj Orthopedics, LLC, for providing the specially designed knee s that were used in this study. References Albright, J.P., Powell, J.W., Smith, W., Martindale, A., Crowley, E., Monroe, J., et al. (1994). Medial collateral ligament knee sprains in college football. Effectiveness of preventive s. American Journal of Sports Medicine, 22(1), Bell, A.L., Pedersen, D.R., & Brand, R.A. (1990). A comparison of the accuracy of several hip center location prediction methods. Journal of Biomechanics, 23, Boden, B.P., Dean, G.S., Feagin, J.A., Jr., & Garrett, W.E., Jr. (2000). Mechanisms of anterior cruciate ligament injury. 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