Graduation date: 2007Rapid change-of-direction tasks have been associated with non-contact anterior cruciate ligament (ACL) injuries and females are more likely to suffer this injury compared to males. The purpose of this investigation was to evaluate the biomechanical behavior of running and three rapid change-of-direction tasks and determine whether sex differences exist across tasks. The biomechanical factors included knee and hip kinematics and kinetics, in addition to static lower extremity alignment and dynamic hip strength. Data were collected in a controlled laboratory setting on healthy collegiate female and male basketball and soccer players (N = 21). Three-dimensional kinematics and kinetics were recorded in conditions of running, lateral false step, sidestep cut, and a combination of a lateral false step with a sidestep cut. Static lower extremity alignment was represented by measurement of standing Q-angle. Hip strength was determined by measuring isokinetic eccentric hip abduction strength. Three queries were addressed: 1.) sex differences in the kinetics and kinematics of the knee and hip during running and rapid change-of-direction tasks, 2.) the effect of sex on relationships of standing Q-angle and hip strength to frontal plane knee biomechanics, and 3.) description of the patterning of knee and hip biomechanics across tasks and sex. The results of this study suggest knee abduction and internal moments of knee adduction and hip abduction increase when athletes step laterally in combination with a sidestep cut compared to all other tasks. Females demonstrate differences in hip position and loading compared to males across the four tasks. The data did not support evidence of an effect of sex on standing Q-angle or hip abduction strength. Additionally, no relationships between sex, standing Q-angle, or hip abduction strength to frontal plane knee biomechanics were found. Four patterns of knee and hip kinematics and kinetics were found to describe lower extremity biomechancis during running and rapid change-of direction tasks. In summary, the addition of frontal plane motion and loading during change-of-direction tasks significantly affected knee position in a manner associated with ACL injury. Differences in lower extremity kinematics and kinetics between females and males appear to be specific to hip position and loading.