<qa>  What is the main reason for the speed increase?However, if the speed is increased linearly it is likely that it will increase at too fast a pace. Therefore the test will become a measure of the subjects ability to move their legs quickly andor efficiently enough, rather than metabolic factors. If a steep gradient is chosen as the constant inclination then a high initial metabolic cost will result, which will limit the test time. Another common method used is to maintain a constant speed while increasing the sine of the angle of inclination. The problem with this method of testing is that if too low a speed is selected then a very steep gradient results before the level of tolerance is reached, and if too high a speed is selected there is a high initial metabolic cost. These problems were addressed in a recent paper by Porszasz et al., 2003 7. They demonstrated that by increasing walking speed linearly and treadmill gradient curvilinearly, a linear increase in work rate occurs with the subjects fatiguing at a moderate speed. If this protocol could be adapted for use with incomplete SCI patients it would provide an accurate means of testing changes in cardiopulmonary function which may result after treadmill training. 