Most runners take a simple view to running hills: it's easier going down than going up. This is generally true, but there's an exception to the rule. While running downhill never burns as many calories as running on a flat plane or running uphill, there's a point at which running downhill starts to burn an increased number of calories.
Classic View
The classic belief about running on an incline compared to a decline comes from a 1963 study published in the "Journal of Applied Physiology." This study examined the calorie burn of runners at various grades, from a 15-percent incline to a 20-percent decline. This study found that -- as many would suspect -- the energy cost of running uphill increases as the grade becomes steeper, and the energy cost of running downhill decreases as the grade becomes steeper.
Limitation
Further research has placed limits on this intuitive model. A 2002 study in the "Journal of Applied Physiology" found that the model works in general, but things change when the decline becomes steeper than 15 percent. At this point the energy expenditure starts to increase, although it never reaches the same level as running on a flat plane or an incline.
Reason for Increased Calorie Burn
On the surface, it doesn't make sense that running would burn more calories as the decline becomes steeper. But the explanation is rooted in the biomechanics of running on a decline. At these steep declines you lose the "pendulum-like" momentum of your legs swinging, forcing you to use more energy as you run. According to the authors of the 2002 study, the biomechanics of running and walking change so much at this decline that it may not be proper to even think of the movements as running or walking; as the authors put it, "We wonder whether it is legitimate to speak of 'walking' or 'running' at the steepest slopes."
Benefits of Decline Running
According to an article by endurance coach Troy Jacobson, on his website, there are multiple benefits to training on downhills regardless of the calorie burn. According to Jacobson, running downhill helps you to practice a quicker leg turnover and foot strike -- a key to running faster. Practicing on downhills can also help you run faster on downhill portions of races, helping you make up for slower uphill portions in a hilly race. According to Jacobson, practicing downhills can also reduce the post-race soreness typically associated with hilly courses.
