Prior to the experiment, the participant lay prone on the bed. One surgeon helped to find and locate the major landmarks on spine and pelvis. In this study, the landmarks included the third and seventh spinous process of the thoracic vertebra (T3, T7), the first, third, and fifth spinous process of lumbar vertebra (L1, L3, L5), the left posterior superior iliac spine, the right posterior superior iliac spine, and the iliac crest. Then, optical markers were placed on these landmarks and captured by Optotrak Certus motion analysis system (Northern Digital Inc., Ontario, Canada) at the rate of 100 Hz throughout the whole experiment. The detailed placements are illustrated in Figure 1A .
Before the trial of level walking and stair climbing, the participants were kept standing in a neutral position for at least five seconds for data collection to obtain baseline. Afterwards, the same surgeon demonstrated the requirements for level walking and stair climbing and then guided the participants to practice the two activities until they felt that they could carry out each activity naturally. The schematics of the level walking and stair climbing are showed inFigure 1B . Participants repeated each activity three times for data collection.
Before the trial of level walking and stair climbing, the participants were kept standing in a neutral position for at least five seconds for data collection to obtain baseline. Afterwards, the same surgeon demonstrated the requirements for level walking and stair climbing and then guided the participants to practice the two activities until they felt that they could carry out each activity naturally. The schematics of the level walking and stair climbing are showed in