Purpose: The aim of this research is to establish whether, and to what extent, the tilt angle, gear ratio of the propulsion system and propulsion frequency of a wheelchair influence the position of the centre of gravity. Furthermore, it verifies the usefulness of such research using an original test stand.
Materials and methods: The article presents the effects of three operational parameters of a wheelchair on the position of the centre of gravity of the human body. The study included 27 wheelchair propulsion tests of a wheelchair with pushrim propulsion using the following variable parameters: gear ratio of the propulsion system, propulsion frequency and wheelchair tilt angle. The position of the centre of gravity of the human body was measured in dynamic conditions at 100 Hz. The results were represented with ellipses defining the region of variability of the position of the centre of gravity of the human body. The coordinates of the centre of gravity were measured in relation to the reference system, with the start point at the centre of the axis of rotation of the rear wheelchair wheels. The measurements were taken in a horizontal plane in relation to the base on which the test stand was positioned.
Results: The research carried out shows that the inclination angle of the wheelchair has the greatest influence on position of the ellipse describing the position of the centre of gravity of the human body. By controlling the change in the inclination angle value in the range from 0° to 5.4°, the standard deviation of the length of the horizontal half-axis of the ellipse (SD a) equal to 31.2 mm was obtained. For comparison, by changing the frequency of pushes (40 to 50 pushes per minute) of the wheelchair at a constant inclination angle, the standard deviation of the horizontal half-axis length (SD a) equal to 8 mm was recorded. The results of the study show a change in the position of the centre of gravity of the human body in dynamic conditions. They are relative to the contact points of the wheelchair wheels with the ground. Using the dimensions of the plotted ellipses, one can determine the values of pressure that affect the wheelchair's individual wheels. Conclusions-Increasing the value of each aforementioned parameter resulted in the increase of strength required by the operator to propel the wheelchair. It directly influenced the position of the centre of gravity during the test.