I work at a fitness center and my manager is also the personal trainer at our gym. She and I were talking about how I haven’t been lifting as much as I was when I was pregnant (8+ months ago). She is an advocate for lifting. I am opposite. I like lifting, but I would much rather run! My time to workout is limited because of my little man, so I try to get my runs in rather than take time to lift. Plus by the time I’m done running I’m too tired to lift.
She advised me that I’m doing it all backwards. As long as I can remember I have always done cardio first then weights. But her argument makes sense, in a scientific/biological point of view. Let’s see if I can keep it simple but still get the point across...
Cardiorespiratory activity (which includes running, biking, swimming, walking, etc.) is an aerobic activity. You use oxygen, and lots of it, to produce energy and burn calories. Weight lifting, along with short sprints, are considered anaerobic activity. This kind of activity doesn’t require oxygen to produce energy.
Anaerobic activity (weights) breaks down, or burns, the glycogen (sugar) in your body to produce energy. Aerobic activity (cardio) breaks down glycogen first, then because of the oxygen provided, they metabolism can continue and will move on to fat once the glycogen stores are all used. With that knowledge, it makes sense to lift first, then move on to the cardio. If you lift first, you break down more of the glycogen in your body, then when you move on to the cardio you are that much closer to breaking down fat to produce energy.
[A side note to this discussion, “as the duration of exercise lengthens, the contribution of fats as an energy source becomes greater. The opposite is true for high-intensity exercise. As intensity increases, the body relies increasingly on carbohydrates as its fuel source.” (Krause’s Food & Nutrition Therapy. Mahan, L. Kathleen. Edition 12, pg. 589]
Correct me if I’m wrong. All information provided is from my own knowledge and supplemented by wikipedia.com & Krause’s Food & Nutrition Therapy, ed. 12.