Backpack carriage has been the most common approach of load transfer in the daily. However, backpack load has been proven to be a potential risk factor for accidental falls during level walking. Compared to level walking, stair walking requires more cognitive and physical demands, as evidenced by a higher lower-limb joint range of motion and torques (Costigan et al., 2002, Nadeau et al., 2003). Therefore, stair has been suggested as one of the most dangerous locations in the school and at home (Cayless, 2001). Whilst it is well known that stair descent requires more balance compensation than stair ascent. Including the more excellent trunk motion relative to the pelvis (Krebs et al., 1992) and the head-in-space flexion and angular excursions (Cromwell, 2001). Consequently, stair descent with backpack load has raised concerns among teachers and parents.

Previous biomechanical studies on stair descent with backpack load were limited to investigations of joint movements and forces. With the increase of the backpack load, the ankle dorsiflexion angle, knee flexion angle (Chen and Qu, 2018), hip flexion angle (Kinoshita, 1985), and external knee adduction torque (Hall et al., 2013) increased, respectively. Further work has confirmed the lateral spinal tilt to the support limb increased when carrying the 15 % and 20 % BW load (Hong et al., 2011) and the risk of stair contact (stair ascent) or over-stepping (stair descent) also increased when passing over a step in the high load condition (Chen and Qu, 2018). However, the information about balance control and muscle activation during stair descent has not been investigated in the adolescents with backpack load. A better understanding of how backpack load perturbs gait stability during stair descent is essential. The motion of the center of mass (COM) and its relative position to the center of pressure (COP) of the supporting limb has been used to assess the gait stability (Lee and Chou, 2007), which may also be important in the gait assessment during stair descent.

This study aims to reveal the effect of increasing load on adolescents' gait during stair descent. The data of kinematics, kinetics, and EMG were collected synchronously. Gait parameters, especially indicators of balance control, were analyzed. Based on previous studies, we hypothesized that carrying a backpack may affect the balance during stair descent by increasing the COM-COP IA in the sagittal plane and the trunk anterior tilt angle.