The aim of the presented research was to determine the impact of the change in the position of the center of gravity of the human-wheelchair system on the load on its axles. The wheelchair user, while performing propulsion phases, moves segments of his body, in particular the upper limbs. Such movement causes a cyclic shift of the center of gravity, which changes the value of the force loading the wheels. This in turn causes a change in the friction conditions as the front and rear wheels of the wheelchair have different frictional parameters. The paper presents the results of calculations from a mathematical model characterizing the biomechanics of movement when driving and propelling a manual wheelchair. The basis for the analysis were experimental studies focused on determining the location of the center of gravity on the plane of motion. The basis for the analysis were experimental studies focused on determining the location of the center of gravity on the plane of motion, carried out on a dedicated test stand.