A system for motion analysis includes a sensing device positioned at a knee joint of a bicycle rider, and an electronic device in communication with the sensing device. The electronic device is configured to receive, from the sensing device, a series of pieces of sensed data representing orientations of the sensing device at different times, to determine a motion trajectory of a knee of the rider based on the received pieces of sensed data, and to generate an estimation result regarding correctness of a riding posture of the rider based on the motion trajectory.