2.3. Experimental procedures
After markers were placed on the subject, the subject was asked to stand naturally while a static neutral standing trial was obtained to create a model. The medial markers, which were used to establish knee and ankle joint centers, were then removed to allow normal gait. Each subject was instructed to walk barefoot along the walkway at a self-selected speed. Practice trials were permitted until the subject was comfortable walking and was able to contact the force plate with only one foot without altering his or her gait. Typically, three or less practice trials were performed. Each subject was asked to walk while maintaining three different postures (Fig. 1). First, each subject walked while maintaining the selfselected natural posture. The natural posture was collected first in order to capture each subject's typical posture prior to any modification. Second, each subject walked while maintaining a swayback posture. To facilitate the swayback posture, the subject was positioned in posterior pelvic tilt with the trunk posterior to the hips, and in hip and knee hyperextension. The subject was instructed to maintain the swayback posture while walking even though it would feel like “leading with the hips”. Third, each subject walked while maintaining a forward flexed posture. In order to facilitate the flexed posture, the subject was positioned in slight hip and knee flexion with the trunk leaned forward so that the trunk was in front of the hips. The subject was instructed to maintain this posture while walking and that it would feel like “leading with the head”. For the swayback and flexed postures, each subject was instructed to walk at the same speed as during the natural trials. To monitor walking speed, the subject was timed using a stop watch while walking 3.045 m. Based on pilot testing, a criterion of ±0.4 s of the subject's average time for the natural trials was used and feedback was given if necessary after each trial to speed up or slow down. In order to be accepted, the subject had to walk within the expected time, contact the force plate with a single foot, and maintain the posture as determined visually. Three acceptable trials were obtained for each limb while maintaining each posture. For this study, only data from the right lower extremity were analyzed.