“Should I do aerobic training before or after a weights session?” This is a frequently asked question and one over which a wide spectrum of opinion exists even though it may seem like trivia if you're new to weight training. Yet, as with many issues in the exercise sciences, answers to complex questions can be blurred by qualifications and exceptions and tempered by the exercise goals you have set -- weight loss, muscle, strength, sporting prowess, appearance and so on.
The following attempts to clarify the issue and provide some clear direction.
Cardio and Aerobics
Aerobic exercise, often called ‘cardio’ for short, is any exercise at an intensity at which oxygen can be sustainably supplied to large muscle groups over time and which places consistent demands on the heart and lung system, the cardiorespiratory system.
Cardio is something you do at a sustained pace over a longer period of time rather than in short bursts of energy such as in interval running or lifting weights. Cardio is walking, jogging, distance running, swimming and cycling; and using treadmill, stepper, cross trainer and rowing machines in the gym. Blood glucose and stored glucose and fats are the main fuels used in aerobics.
Training with Weights
In contrast, lifting weights is an activity practiced in short bursts of anaerobic (without oxygen) activity. In effect, ‘anaerobic’ doesn’t mean that we stop using oxygen, it just means that the activity is of such an intensity that the muscle's requirement for oxygen is exceeded, resulting in metabolic products such as lactate and an eventual inability to continue at that intensity. Stored muscle glucose and phosphocreatine are the main fuels used in strength training.
Now that you're clear on the essential difference between aerobics and weight training, let’s consider this in the context of doing cardio before or after a weights session. I’ll assume that a ‘session’ is one visit to a gym for the purposes of a workout. Let’s examine the scenarios I propose.
Scenario 1 - Cardio after Weights
You walk into the gym and do a warmup on the treadmill for 10 minutes, but you don't want to do too much cardio because you reckon you need the energy to max out your weights session. Anyway, you heard that you'll burn more fat if you do it after the weights.
Saves energy for weight lifting. This may seem to be good logic; however, doing 40 minutes of cardio at moderate pace is not going to deplete enough energy to prevent you from lifting well. As long as you've replaced your carbohydrate glucose stores after any previous exercise session with proper eating, the body will have stored up to 500 grams, or a pound of glycogen.
A jogging or running treadmill session of 40 minutes may use about 600 kcalories of energy, depending on your size and pace. Of this, some fuel will be fat, some will be stored glucose and some blood glucose. A reasonable estimate is that you would use around 80 to 100 grams (3 or 4 ounces) of stored glucose out of, say, 400 grams that you have available. You can see that you have plenty left in reserve for strength training.
What's more, if you replace some of this used fuel with a sports drink or energy bar before you start the weights, you’ll only be a little depleted from when you walked in the door.
Burn more fat. Now this one really sounds attractive, the idea being that if you deplete some carbohydrate stores, particularly blood glucose, with an initial weights session, you'll be in fat burning mode. Theoretically this makes some sense but as we saw in my article, So You Want to Burn More Fat, the fat burning zone is a mythical construct and what really matters is how much energy you expend overall.
Score for Scenario 1: sounds good, but in reality only 2 points out of 5.