If you are trying to develop a leaner and more muscular physique, then you are probably engaging in both, weight training and cardio training. Weight training improves muscle force production capacity and increases lean muscle mass. However, cardio training improves heart and lung function by inducing better oxygen transport and utilization, not to mention that it helps you burn fat.

Both weight and cardio training have been incorporated simultaneously into training routines in many sports and physical activities. The combination of strength training and cardio training is sometimes referred to as concurrent training.

So that begs the question: “When I go into the gym to train, should I do my weights first or my cardio first?”

In a perfect world you would have two separate workouts in one day; you would have your weight training workout; then a separate cardio workout at another time. However, most people don’t have time to do two separate workouts each day. Therefore, you’re faced with the question: is my cardio first or is my weight training first?

I believe you should do your weight training first. There are some good reasons for this.

During weight training, your body primarily uses ATP and glycogen stores to power your muscles. Weight training is an anaerobic exercise, meaning that the muscles perform in the absence of oxygen. A weight training set, is an exercise of short duration, typically lasting under one minute. Cardio, or aerobic exercise, on the other hand, is a lower intensity activity performed for longer periods of time. It requires a great deal of oxygen to generate the energy needed for this type of exercise. At the onset of cardio, your body may burn some glycogen, as it gets started. But prolonged moderate-level aerobic exercise at 65% of maximum results in the maximum contribution of fat to the total energy expenditure. Once you get going, in other words, fat may contribute as much as 40 to 60% of the total calories expended during the cardio exercise.

Coming back to the topic of which exercise to do first. It makes sense to do your weight training first, as your glycogen stores will be fresh and at their maximum level. That way you can put all of the necessary energy into the weight training. As your glycogen stores become more exhausted towards the end of your weight training workout, then you begin your cardio session. At this point, your body is primed to switch over to burning more fat, which is the predominant fuel used in cardio exercise.

So far, I have focused only on prioritizing weights and cardio by fuel source (glycogen vs. stored fat.) But there’s another reason to do weight training before cardio. Studies suggest that strength endurance performance decreases when preceded by cardio. One 2015 study published in the Journal of Strength Conditioning and Resistance found that strength endurance performance decreased when preceded by high-intensity intermittent running or cycling exercise. Results indicated that prior aerobic endurance exercise induced a significant level of fatigue that remained during subsequent resistance exercise protocols. It seems that the science supports doing weight training BEFORE cardio, if you must do it on the same day.