This study presents a method for measuring lower extremity kinematics during level walking using a simple external marker system and algorithms. The system, implemented on a computer-aided video motion analysis system (VICON), defines three-dimensional joint angle motion based on embedded axes and Euler rotation angles. Gait analysis was performed on 40 normal young adults, with data collected over three different test days. The study quantified the errors in joint angle estimation due to uncertainties in defining the embedded axes and demonstrated the limitations of the model and marker system in evaluating pathologic gait. The marker system, which uses a minimal number of body surface markers, is easy to implement for routine clinical gait evaluations. The results provide a normative database for comparing and sharing gait data between clinical centers. The study also discusses the sensitivity of joint angle measurements to errors in defining the embedded axes and the impact of these errors on the accuracy of the measurements.This study presents a method for measuring lower extremity kinematics during level walking using a simple external marker system and algorithms. The system, implemented on a computer-aided video motion analysis system (VICON), defines three-dimensional joint angle motion based on embedded axes and Euler rotation angles. Gait analysis was performed on 40 normal young adults, with data collected over three different test days. The study quantified the errors in joint angle estimation due to uncertainties in defining the embedded axes and demonstrated the limitations of the model and marker system in evaluating pathologic gait. The marker system, which uses a minimal number of body surface markers, is easy to implement for routine clinical gait evaluations. The results provide a normative database for comparing and sharing gait data between clinical centers. The study also discusses the sensitivity of joint angle measurements to errors in defining the embedded axes and the impact of these errors on the accuracy of the measurements.