Wind Resistance

• The primary argument made for using a treadmill at a one- or two-percent incline is to compensate for a lack of wind resistance while running on treadmills. For slower runners, however, wind resistance is not a huge factor and there will not be much difference for them between running on a treadmill with no incline and running outdoors.
  
  

Increased Cardio

• Setting your treadmill on a low incline makes you work harder, which allows you to boost your heart rate without having to jog at a faster speed. This may be beneficial if you are still getting used to treadmill running or if you are nursing an injury and don't want to run at a vigorous pace.
  
  

Hill Training

• Utilizing the incline function on a treadmill is a good way to simulate hill running which can help build muscle power---particularly in the upper leg muscles---without having to worry about running on irregular terrain outside.
  
  

Varying Incline

• One drawback to using incline on a treadmill to simulate real hills is that a button must be pressed to change incline; it is difficult to change incline in a way that properly simulates the gradual changes in elevation experienced in outdoor running.
  
  

Considerations

• Running up hills requires different biomechanics than running on level ground. If you don't vary your training routine and use the hill feature too much, you may build in imbalances that will affect your speed and body mechanics when you run on flat ground. It might be better to use one of your treadmill's preset hill programs, which will vary the size and frequency of hills for you.