Running uphill is tough on the lungs, and running back down is tough on the legs. Either way, hills can derail an 
otherwise pleasant run if you’re not prepared for them. Fortunately, a team of Australian researchers armed with 
the latest technology has come up with some valuable guidance. In a 2010 study, they sent a group of runners out 
on a hilly six-mile course while wired with a portable gas analyzer to measure oxygen consumption, a GPS receiver 
to measure speed and acceleration, a heart rate monitor, and an “activity monitor” to measure stride rate and 
stride length. The results suggest that most runners make two key mistakes: they try to run too fast on the 
uphills, and they don’t run fast enough on the downhills. When you’re running on flat terrain, your speed is 
generally limited by the ability of your heart and lungs to transport oxygen to the muscles in your legs. If you 
try to maintain the same speed while hauling your body up a hill, you’ll quickly notice that you’re breathing 
harder because you’re consuming more oxygen. The problem with this approach is that, once you get to the top of 
the hill, you’ll need time to recover from this extra effort. In the study, runners took an extra 78 seconds on 
average to regain their initial speed after cresting a hill—a delay that wipes out the benefit of pushing hard up 
the hill, lead researcher Andrew Townshend of the Queensland University of Technology says. “Based on our results, 
we suggested that a small decrease in speed on the uphill may be more than compensated for by a quicker return to 
faster running speeds on the subsequent level section,” he says. Surprisingly, the opposite was true on downhill 
sections. Because of the jarring impacts involved in running downhill, most of us simply can’t run fast enough 
downhill to be limited by oxygen. The practical tip: when you get to the bottom of a hill, focus on maintaining 
your momentum (and higher speed) until your breathing forces you to slow down again. The downhill results were 
much less consistent among subjects than the uphill and level sections of the experiment. Some people were able to 
run far closer to their aerobic limits than others, gaining valuable time without getting significantly more tired. 
This suggests that downhill running is a skill you can acquire through practice. Of course, there’s a reason we 
tend to back off when running down hills: it’s hard on the legs and raises injury risk. For that reason, it’s best 
to limit downhill training to short sprints on a fairly gentle grade—a technique that’s also used by sprinters 
and football players to improve their sprint speed. A 2008 study from Marquette University found that a 10 percent 
grade (5.7 degrees) is the ideal gradient to maximize your speed in 40-yard sprints. While these simple tips—slow 
down on the ups, speed up on the downs—should help you distribute your effort more evenly during runs, you’ll need 
to try them out to find the right balance for yourself. “The best I can suggest is that runners should practice 
varying their degree of effort on hills that they frequently use in training, to determine how much they should 
slow down to reap an overall time benefit,” Townshend says. “An experiment of n = 1 for all to try!”

from the book "Which Comes First, Cardio or Weights"