Gradient of the treadmill is the measure of the slope of the treadmill belt. If the belt length is 100 centimeters, 10 percent gradient would mean that the end of the belt towards the hand rail / console would be 10 centimeter (10 percent of the length) higher from the ground than the other end. This would mean that the person walking on the treadmill will be walking up a slope with a 10 percent gradient. In Bruce protocol for treadmill exercise testing, the gradient in stage 1 is 10 percent. It increases by 2 percent in every stage.
In Modified Bruce protocol, which is used for less fit persons, the gradient in stage 1 is zero, meaning that both ends of the belt are at the same level. Stage 2 of Modified Bruce protocol has a 5 percent gradient while stage 3 has a 10 percent gradient. In effect, the stage 3 of Modified Bruce protocol will be same as stage 1 of standard Bruce protocol.