Abstract: BACKGROUND: Stair walking is an external factor that causes elderly people to fall, and the decline in posture control ability caused by dual-task interference is an internal factor that causes elderly people to fall. Tai Chi has been recognized for its ability to improve single-task posture control and prevent falls, but its effect on the ability to control stair-walking posture under dual-task interference is still unknown.  
OBJECTIVE: To investigate the effects of walking up and down stairs with dual-task interference, the type of people (between groups) and the type of task (within groups) on body stability, lower limb joint motion and mechanical characteristics.
METHODS:  Totally 20 women in Tai Chi Group, 20 women in brisk walking group, and 20 healthy elderly women in control group were included. They were tested on walking up and down stairs, including single task, cognitive task, manual task, and combined task. The kinematic and dynamic data of walking were collected simultaneously by Vicon motion capture system and Kistler force measurement system.  
RESULTS AND CONCLUSION: (1) Regarding physical stability indicators, compared with single task, the distance between the center of mass and the center of pressure and the distance between the center of mass and the center of pressure were significantly increased (P=0.002, P=0.021), and the height of lifting foot (P=0.018) was significantly decreased in the control group. In the Tai Chi group, only the distance of climbing stairs and lifting feet decreased significantly (P=0.034); and in the Tai Chi group, the distance between the front and back of the centroid and the inside and outside of the centroid was significantly smaller than that in the control group (P=0.041, P=0.006); the foot-lifting distance of walking down stairs was significantly greater than that of the control group (P < 
0.001). (2) In terms of lower limb, hip, knee and ankle joint kinematics, in the movement and combination task conditions, in the control group, ankle joint straddle angle increased significantly during ascent (P < 0.001); the ankle joint movement amplitude decreased significantly (P < 0.001), and the straddle angle increased significantly during descent (P < 0.001). The range of motion of ankle was significantly increased during ascent in the Tai Chi group (P < 0.001). In the task paradigm, the range of motion of ankle in the Tai Chi group was significantly larger than that in the control group (P=0.005, P=0.012), and the angle of stride was significantly smaller than that in the control group (P=0.033, P=0.021). (3) Compared with the single task, the peak values of hip extension moment and knee extension moment were significantly decreased (P < 0.001), and the peak value of knee extension moment was significantly increased during ascent (P < 0.001); and the joint torque of lower limb was significantly decreased during descent (P < 0.001). The peak value of knee extension moment in the Taiji group was significantly higher than that in the control group (P < 0.001). (4) It is indicated that long-term Tai Chi exercises can help elderly women strengthen neuromuscular control of lower limbs and increase the height of stepping over steps, thereby enhancing body stability control, resisting dual-task interference, and reducing the risk of falls.

Key words: Tai Chi, stair walking, dual-task, body stability, motor control