The Importance Of Intensity
Intensity and duration have an even greater effect than mode. Our study concluded that long-term fat loss is actually lowest with moderate intensity, long-duration cardio. The greatest fat loss actually occurred with the shortest duration and highest-intensity activities, like sprinting. We also saw that the longer you do cardio per day, the greater the losses are in muscle mass. However, we found very small decreases in muscle and strength when cardio was kept to less than 20 minutes per day.
“But what about the ‘fat-burning zone?'” you shout from the treadmill! By this, you refer to a study conducted in the early 1990s by Dr. Romijn, who concluded that we use the most fat during exercise when performing moderate intensity (65 percent heart rate max), long-duration (45-60 minutes) cardio.9 This study’s conclusions are reflected in the “fat-burning” programs on nearly every cardio machine at the gym. However, a major problem with the study is that what happens during exercise does not always reflect what will occur long term. I want to hammer this point home.
My lab recently conducted another experiment where we compared low-intensity, long-duration cardio of 60 minutes with 4-10 sets of 10-30-second all-out sprints. As expected, we found that long duration cardio decreased muscle size. But get this: Sprinting actually increased size.10 This suggests that sprinting can actually be anabolic and get you shredded at the same time!