6 Mistakes That Can Slow Your Metabolism

Why That's a Problem: You know the more lean muscle you have, the more energy your body uses at rest, and that using weights is an efficient way to build that muscle. Recent research from Iowa State University reports that fitness trackers aren't so great at measuring strength training, though—the four popular models tested were all off on their energy expenditure estimates by at least 25 percent when it came to resistance work. That means you may end up with an inflated sense of how hard you're pushing yourself in the weight room.

The Fix: Listen to your body instead. A good guideline: For any given set of any given exercise, you should feel like you could do 2 more reps when you stop. So if you're doing 12 reps of bicep curls, you'll know you're using the correct weight if you feel like you could make it to 14 reps but not more than that.