Have you ever been on a run and felt like you needed to slow down, and upon doing so, felt more tired than you were before? I’ve definitely had this feeling. In fact, I used to feel this way on all of my runs, finding myself getting to a 10-minute pace, or perhaps even slower than that. Going up hills, I thought it might even be faster if I was walking. I felt so incredibly fatigued that picking up the pace didn’t seem like a plausible idea, but it’s actually what I should have done in those situations. Why?

It all has to do with heart-rate. You’ve probably heard about the “fat burning” zone of your heart rate, which is lower than the “cardio” zone. If a runner is going out on a pace that is relatively easy and puts them in the “fat burning” zone, nearly half of the calories burned will come from fat. When your body is using fat calories, it is doing a great deal of work to get you the energy you need, which is why you might start to feel surprisingly tired going at a slow pace, and might want to slow down even more. The reality is, if you pick up the pace, you will feel better. Once your heart-rate is in the “cardio” zone, your body will draw less than 40% of its energy from fat calories. I didn’t believe this at first, but after I started to pick up the pace on my runs, I actually found that the runs felt easier!

One might ask, “Well, wait, what if I want to burn fat calories?” I asked that question too. It turns out that, in workouts of identical length, even though one is burning a lesser percentage of fat calories in the “cardio” zone, one will still burn more fat calories if he or she maintains a heart-rate in the “cardio” zone. One will also burn more calories in general, which is the important factor for weight loss; where they come from is basically irrelevant.

Next time you’re feeling tired on a run, pick up the pace and see how you feel.