Original Article. CHUNG-HWI YI, PhD, PT 1), SO-YEON PARK, MSc, PT 2), SANG-HEON LEE, MSc, OT 2)

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1 Original Article J. Phys. Ther. Sci. 16: 1 5, 2004 CHUNG-HWI YI, PhD, PT 1), SO-YEON PARK, MSc, PT 2), SANG-HEON LEE, MSc, OT 2) 1) Department of Physical Therapy, College of Health Science, Yonsei University: 234 Maji-li, Hungob-myon, Wonju city, Kangwon-do , Republic of Korea. TEL FAX ych0406@dragon.yonsei.ac.kr 2) Department of Rehabilitation Therapy, The Graduate School, Yonsei University Abstract. There is a risk of injury to the knee during landing. The kinematics involved in different landing strategies may be related to the occurrence of trauma. Several sources suggest that the angle of knee extension on touchdown and impact with the ground determines the magnitude of the impact force and, indirectly, knee loading. This study compared the initial knee angle, maximum knee flexion angle, and angular velocity at the instant of impact on drop-landings between healthy men and women. In this study, 48 participants (25 male, 23 female) dropped from a height of 40 cm onto their right leg. The CMS-HS measuring system was used to analyze the kinematic data. There was no significant difference in the mean knee flexion angle at landing between the two groups. However, the range of knee flexion on landing (43.64 ± 4.63 in men, ± 6.23 in women) and the angular velocity (0.25 ± 0.04 /sec for men, 0.23 ± 0.04 /sec for women) differed significantly (p<0.05). Women land with a straighter knee than men and this might increase the likelihood of a knee injury. Key words: Drop-jump, Kinematic analysis, Knee angle (This article was submitted Aug. 26, 2003, and was accepted Oct. 9, 2003) INTRODUCTION There are many reports that the incidence of anterior cruciate ligament and patello-femoral injuries is greater in female athletes than in their male counterparts in comparable sports, such as basketball, volleyball, and soccer 6, 9, 10, 13, 18). Huston et al. 12) studied 20 young height-matched subjects, 10 males and 10 females, who performed three jumps from three vertical heights. The largest gender difference in knee angle occurred at the highest jump height of 60 cm. Women were found to land with a significantly straighter knee angle than men from heights of 40 and 60 cm, thereby exposing their knee joint to greater forces per unit body weight when landing from a jump than men do. Lafortune 14) compared healthy basketball players with those previously injured while catching rebounds. The non-injured athletes had greater flexion at the hip and knee joints during landing. Interestingly, previously injured athletes had less hip and knee flexion 14). Huston et al. 12) theorized that the smaller knee flexion angle utilized by women during the impact phase of landing would also increase their vertical ground reaction forces and knee joint loads. Landing imposes forces on the body that must be absorbed primarily by the lower extremity 5). If loads become too great for the body to accommodate or if impact absorption fails, an injury is likely 17). These pathomechanics should be considered as a possible cause of the higher rate of knee injuries in

2 2 J. Phys. Ther. Sci. Vol. 16, No. 1, 2004 Table 1. Age, height, and weight of the subjects (n=48) Males Females Variable Mean SD Mean SD Age (yr) Height (cm) Weight (kg) women 12). Several lines of evidence suggest that the angle of knee extension on touchdown and impact with the ground determines the magnitude of the ground impact force, and therefore, indirectly, knee loading. Although drop-landings have been studied extensively, few studies have examined gender differences 14). Therefore, this study compared the initial knee angle, maximum knee flexion angle, and angular velocity at the instant of impact for drop-landings made by healthy men and women. METHODS Fig. 1. Experimental setting. Subjects Twenty-five healthy male (age, 24.6 ± 2.2 yrs; height, ± 5.7 cm; weight, 71.5 ± 8.2 kg) and 23 healthy female (age, 23.0 ± 3.8 yrs; height, ± 4.8 cm; weight, 57.5 ± 10.3 kg) volunteers participated in the study. They were free of injury or other physical impairment in the lower extremity at the time of testing. The age, height, and weight of the subjects are summarized in Table 1. Test Protocol The participants were instructed to perform three unconstrained jumps from a height of 40 cm. A CMS-HS measuring system (Zebris Medizintechnik GmbH) was used to analyze the kinematic data. This system consists of a measuring sensor with a stand, a basic CMS-HS unit, markers, and a cable adaptor with 10 channels that can be used for individual markers. The measuring procedure is based on determining the spatial coordinates of miniature ultrasound transmitters; the sound pulse times between the transmitters and three microphones integrated in the measuring sensor are measured. Active reflective markers were placed on the right side of the body at three sites: the lateral malleolus, a point midway between the greater trochanter and knee joint line, and lateral femoral epicondyle (Fig. 1). 3M double coated tape was applied to the active reflective markers, which were held in place by adhesive tape. Signals were sampled at a rate of 60 Hz for 8 seconds. The data of the kinematic coordinates were imported into the WinData 2.19 program (Zebris Medizintechnik Gmbh) to compute joint kinematics and analyzed. Each participant was instructed to drop, not jump, from the raised platform to the ground, attempting to land with their toes as close as possible to a mark on the floor 40 cm from the platform, in an attempt to standardize the horizontal jumping distance of the participants. After landing, the subject returned to his or her normal standing position. The arms were not constrained during the landings and were generally at the subjects sides or waist for balance. Each subject practiced the maneuver before data collection, to allow the subjects time to become comfortable with the data collection conditions. The subjects were asked to perform three landings each. Statistical analysis The knee joint angular position and velocity were calculated from the kinematic data. Independent t- tests were used to determine whether there was a difference between males and females in the initial knee flexion angle at the moment of impact,

3 3 Table 2. The kinematic data on landing (n=48) Males Females Mean SD Mean SD T P Knee flexion angle on landing (degrees) Maximum knee flexion angle (degree) Knee flexion range* (degrees) Angular velocity (degrees/sec) *: Maximum knee flexion angle - Knee flexion angle on landing. Table 3. ANOVA output for regression analysis: Knee flexion angle and gender Model Sum of Squares DF Mean Square F P Regression * Residual Total *: Predictor, gender. Table 4. Coefficients for regression analysis: Knee flexion angle and gender Unstandardized coefficients Standardized coefficients T P B Standard error Beta Constant Gender maximum knee flexion angle, and angular velocity during vertical drop landing. The level of significance was set at p<0.05. Stepwise multiple regression analysis was used to identify the most informative variable for predicting the range of motion change at the knee joint during landing. A dummy variable for gender was used. RESULTS After a drop from 40 cm, the mean knee flexion angle on landing was 4.05 ± 3.78 and 5.45 ± 4.86 for men and women, respectively, the difference was not significant (p>0.05, t-test) (Table 2). After impact, the knee flexes as the vertical momentum of the body is arrested. The maximum knee flexion angle occurred an average of ± ms after impact ( ± ms in men; ± ms in women). Men and women landed with ± 4.57 and ± 6.47 of maximum knee flexion, The difference was significant (p<0.05) (Table 2). With respect to the knee flexion range during landing, the mean angle change was ± 4.63 in men and ± 6.23 in women, and the difference was significant (p<0.05, t-test) (Table 2). The angular velocity was 0.26 ±.04 /sec for men and 0.23 ±.04 /sec for women, and the difference was significant (p<0.05, t-test) (Table 2). To control for the effects of gender, body weight, and height variables, a stepwise multiple regression for knee flexion range was performed. The regression model was as follows (Tables 3 and 4). Knee flexion range = gender where gender=0 and 1 for male and female, To control for the effects of gender, body weight, and height, a stepwise multiple regression for angular velocity was performed. The following regression model was obtained (Tables 5 and 6). Angular velocity = gender Where, gender=0 and 1 for male and female,

4 4 J. Phys. Ther. Sci. Vol. 16, No. 1, 2004 Table 5. ANOVA output for the regression analysis: Angular velocity and gender Sum of Squares DF Mean Square F P Regression * Residual Total *: Predictor, gender. Table 6. Coefficients for the regression analysis: Angular velocity and gender Unstandardized coefficients Standardized coefficients T P B Standard error Beta Constant Gender 2.40E DISCUSSION There are many theories on the etiology of the greater incidence of anterior cruciate ligament and patello-femoral injuries in females playing the same sports as males. Both extrinsic and intrinsic factors are involved. Extrinsic factors include contact vs. non-contact mechanisms, muscle strength, conditioning, prior athletic experience, and shoesurface interface. Intrinsic factors include joint laxity, anatomic limb variation, Q-angle, and the dimensions of the intercondylar notch 2, 10). So far, no one factor has been proven to be the leading cause of the difference. We examined the sex differences in kinematics in response to a single leg jump in non-athletic individuals. Past studies have quantified the maximum knee flexion angle on impact, rather than the initial angle on touchdown, which is known to determine the maximum ground reaction force 16). The maximum knee angle that we recorded is similar to the result of Dufek et al. 8), who reported a maximum knee flexion angle of 77 in a drop from a height of 59 cm, whereas we measured an average maximum knee flexion angle of However, Dufek et al. 8) examined only three male participants. Hewett 11) reported that the maximum knee flexion angles of high school volleyball players ranged from Amoroso 1) proposed that there is a gender difference in body configuration at the time of landing from a free-fall jump. However, body configuration is unlikely to affect the initial ground reaction force impulse, which lasts for only 30 ms, because in this short time, the pelvis and superincumbent body have not started to decelerate. DeVita et al. 7) demonstrated a change in the relative contribution of the hip, knee, and ankle joints to energy absorption between soft and stiff landing styles, and determined that the hip, knee, and ankle absorbed 25, 37, and 37% of the total energy during soft landings, When the subjects consciously reduced the range of joint motion to land stiffly, the relative contribution of the lower extremity joints to the total energy absorption changed to 20, 31, and 50%, Although we did not measure energy absorption, we measured the changes in the maximum angular velocity of knee flexion during landing. The maximum knee flexion angular velocity averaged 0.26 /sec. A reduction in angular velocity might reflect a decrease in energy absorption based on the work-energy relationship. On landing, the angular velocity of the joints is reduced to zero at the end of the descent phase. A higher peak angular velocity reflects the greater angular kinetic energy of the rotating body. More negative work, in the form of torque applied opposite to the direction of rotation, must be done to reduce the angular kinetic energy to zero. The source of the torque needed to decrease the angular velocity at the knee is eccentric activity in the quadriceps. This study provides evidence that the ground reaction force is likely greater in women per unit body weight than in men, because women land with a straighter knee, and the anterior cruciate strain is greatest when the lower extremity is loaded in full knee extension, and decreases with knee flexion. Consequently, since women land with straighter

5 5 knees than men, the likelihood of knee injury increases. However, support for this interpretation requires the application of an inverse dynamics analysis to measure the energy absorption during landings with the knee extended and flexed. Given these findings, a review of the incidence and causes of knee injuries among female athletes is in order, particularly in basketball, soccer, and volleyball. Such a review might suggest areas for further study that could help to prevent anterior cruciate ligament injuries more effectively in females. CONCLUSION We concluded that women land with straighter knees than men, which might increase the likelihood of a knee injury. REFERENCES 1) Amoroso PJ, Bell NS, Jones BH: Injury among female and male army parachutists. Aviat Space Environ Med, 1997, 68: ) Arendt EA, Agel J, Dick R: Anterior cruciate ligament injury patterns among collegiate men and women. J Athletic Training, 1999, 34: ) Baker MM: Anterior cruciate ligament injuries in the female athlete. J Women s Health, 1998, 7: ) Beim G, Stone DA: Issues in the female athlete. Orthop Clin North Am, 1995, 26: ) Caster BL, Bates BT: The assessment of mechanical and neuromuscular response strategies during landing. Med Sci Sports Exerc, 1995, 27: ) DeHaven KE, Lintner DM: Athletic injuries: comparison by age, sport, and gender. Am J Sports Med, 1986, 14: ) DeVita P, Skelly WA: Effect of landing stiffness on joint kinetics and energies in the lower extremity. Med Sci Sports Exerc, 1992, 24: ) Dufek JS, Bates BT: Biomechanical factors associated with injury during landing in jump sports. Sports Med, 1991, 12: ) Engstrom B, Forssblad M, Tornkvist H: Soccer injuries among elite female players. Am J Sports Med, 1991, 19: ) Feretti A, Papandrea P, Conteduca F, et al.: Knee ligament injuries in volleyball players. Am J Sports Med, 1992, 20: ) Hewett TE, Stroupe AL, Nance TA, et al.: Plyometric training in female athletes: decreased impact forces and increased hamstring torques. Am J Sports Med, 1996, 24: ) Huston LJ, Vibert B, Ashton-Miller JA, et al.: Gender differences in knee angle when landing from a dropjump. Am J Knee Surg, 2001, 14: ) Kaiser DK: The incidence of ACL injury in female intercollegiate soccer players. Supplement to Journal of Athletic Training, 1998, 7: ) Lafortune M: Jumping mechanics and jumper s knee. Sports Science and Medicine Quarterly, 1985, 2: ) McNitt-Gray JL: Kinetics of the lower extremities during drop landings from three heights. J Biomech, 1993, 26: ) Nigg BM: Modeling. In: Nigg BM, Herzog W (eds.), Biomechanics of the musculoskeletal system. Chichester, UK: Wiley Inc., 1994, pp ) Simpson KJ, Kanter L: Jump distance of dance landings influencing internal joint forces. I: Axial forces. Med Sci Sports Exerc, 1997, 29: ) Woodland L, Francis R: Parameters and comparisons of the quadriceps angle of college-aged men and women in the supine and standing positions. Am J Sports Med, 1992, 20: