There are two ways to make a treadmill workout more challenging: increase the speed or raise the incline. While all treadmills have variable speeds, not all allow height adjustments. Yet the benefits of running at a slight incline are undeniable. Just a one-percent grade better approximates the resistance found outside on level ground. Uphill running also has less impact on your knees, making the incline treadmill an all-weather workout that is easy on your joints and can be fine-tuned to individual fitness goals. If you plan to add a treadmill to your home workout routine, keep these buying tips in mind.