The physical capacity of wheelchair users is usually evaluated by their power output, which is generally calculated from the mechanical work of the resultant braking force only. This method does not take into account the kinetic energy variations of the subject-and-wheelchair system. The purpose of this study was to verify if these variations could be neglected for estimating the subject's mechanical work. Three young wheelchair users performed a straightforward displacement with a Wireless Wheelchair Ergometer. According to the subjects, their total estimated mechanical work could be 2 to 5 times higher than the mechanical work of the resultant braking force applied on the system. These results showed that the system's kinetic energy variations should not be neglected for estimating the subject's mechanical work in real conditions.