Should You Do Cardio Before or After Weights?

Cardio is maligned by so called "serious lifters" but done exclusively by 60 percent of those who set foot in the gym after 5 p.m. But who's right, and what is the best recipe for results?

Is there a magic ratio of weight training to cardio that should be done? Should cardio be cast aside for a heavier dose of strength training? Does cardio before the workout destroy your gains?

If your goal in training is to lose weight and look better naked, then on a molecular level, your goal is to burn through all your muscle glycogen in order to start burning fat as fuel.

Here's the science.

ATP (adenosine triphosphate) is the fuel that your muscles need in order to contract, but there is only a limited amount contained within each muscle, and when it's gone your body needs to produce more somehow.

There are three main energy systems at work in your body. Here, I will only be addressing two of them.

Name: Anaerobic system (lactic acid system):

What it does: Breaks down glycogen using a complicated chemical reaction that leaves lactic acid as a by-product, which is responsible for the intense burn you get after hiking up a long hill or doing a long set of biceps curls.

Used During: Short duration high intensity activities like sprinting and strength training.

Anaerobic System Conversion : 1 molecule glycogen = 3 ATP.

Name: Aerobic System (oxygen system)

What it does: Uses an even more complex chemical reaction to break down glycogen into ATP using oxygen (remember the KREB cycle from 10th grade?). This is why you breathe harder during cardio than strength training, because your body needs more and more oxygen to sustain your workout.

Used during: Long duration medium intensity activities like jogging or rowing. This system turns on after 8-10 minutes, which is why jogging feels much easier after the first mile.

Each molecule of glycogen can be broken down into 32 molecules of ATP in this cycle, which is great for sustaining longer term exercise.

Aerobic System Conversion : 1 molecule glycogen = 32 ATP.

This is a much more efficient conversion, and can provide more sustained energy than the anaerobic system.

Depending on the activity, it can take up to an hour to use up all the glycogen stored in your cells and liver, at which point the body has no choice but to start burning fat as fuel. Which is the goal for most of us who are trying to shed our winter weight and fit into last year's swim suit.

This hour marker mentioned above is highly dependent on diet as well, which is why a low carb diet can work extremely well for fat loss especially when coupled with exercise. The more carbohydrates that are eaten before a workout, the more glycogen remains in your system to burn off before fat can be used as fuel.

Cardio before a workout. Energy system trained: Aerobic.

Cardio after the workout. Energy System trained: Anaerobic.

Pros and Cons

Cardio First

Cardio before a workout is great because it gives you the opportunity to burn more calories over the course of the training session by elevating your heart rate initially. This increases your internal temperature and elevates the metabolic demands placed on your body.

This ensures your heart rate will remain elevated, and thus overall caloric burn will be increased for the entire workout.

The downside is that you'll be more tired after doing cardio, and won't have as much energy to spend on resistance training, which is better for making lasting physique changes.

Cardio Last

Since the aerobic system is much more efficient in terms of generating ATP, weight training first is great because it allows you to get to the fat burning portion of the workout faster than if you had done cardio first.

Focusing the majority of your energy on making improvements in the weight room, will result in better strength and physique. Then after all the glycogen is depleted, doing cardio will result in a much higher percentage of fat being burned.

The downside to this technique is that it can be difficult to work hard at resistance training, and then push yourself through a cardio session immediately after.

So Which is Better?

Think about burning fat like digging for gold. You have to get through layers of dirt and rock (muscle glycogen) before you can get to the gold.

Doing cardio first is like digging for gold with a shovel. Getting through a single layer requires 32 scoops to be removed. You'll eventually see gold, but it will take a while.

Doing resistance training first is like showing up to the same dig site with a backhoe. Now only 3 scoops are required to get through one layer thanks to your diesel fueled machinery.

So if you're short on time, or just want to maximize your workout so you can get back to your family, pets or World of Warcraft character, hit the weights first and save the cardio for the end.

Shoot for a 45-minute workout, followed by 20-30 minutes of cardio intervals 3 times per week in order to get the maximum benefit.