Do you know what the single most common mistake gym-goers make is? Surprisingly, it's not bad form or spending enough time at the gym, but rather the order in which they do their workouts!
Most of the time you see the average gym-goer hit the treadmill, spinning class, or elliptical machine for a Low-Intensity, Steady-State ("LISS") cardio* session before they work out. They'll walk, jog, or run for a solid 20 to 30 minutes before they head to the weight machines, thinking that it's better to get the cardio out of the way. After all, they reason, after lifting, they'll be too tired to jog.
Let's take a quick moment to address how the body creates and uses energy. While there are three energy pathways that your body relies on to produce ATP (adenosine triphosphate), the key molecule driving all physical activity, the two quickest are the phosphagen system and glycolysis (we'll get to the third in a moment).
First, the phosphagen system: this is where creatine phosphate (CP) produces a lot of ATP very quickly and is utilized mostly during short-term, high intensity fitness lasting up to about 10 seconds at a time, such as weight training. But because a very limited amount of CP is stored in the skeletal muscle, it burns out quickly.
Next, glycogen, the second-quickest for your body to convert into energy, via glycolysis, is stored in the muscles, in the liver, and in your bloodstream, although it doesn't produce as much ATP per converted molecule as does CP. Additionally, there are only ~100 grams of glycogen stored in the liver for your body to use.
(Note: a main function of glycogen is to maintain a physiological blood glucose concentration, but only liver glycogen directly contributes to release of glucose into the blood. Skeletal muscles are unable to release glucose--because muscles lack glucose 6-phosphatase--and muscle glycogen is mainly a local energy substrate for exercise, rather than an energy source to maintain blood glucose concentration during fasting)
Eventually, your body needs to burn fat to supplement its energy, but it takes time for your body to activate fatty acids. Now we're getting into the third energy pathway: the aerobic system. While aerobic metabolism is the slowest way to resynthesize ATP, it also happens to produce the most (about 18 times the amount as glycolysis).
So, you see, if you preempt your strength-training with cardio, you've already burned through most of your glycogen by the time you step away from the treadmill/bicycle/elliptical/whatever to go lift weights, and now you have very little "quick-tap" energy left to power the workouts that need it most.
As you lift weights, your muscles call on the CP and glycogen for instant energy. The more you burn, the more your body begins to realize that it needs to produce more energy from its only other available source. It then mobilizes those fatty acids, and you begin to burn fat.
Now when you step onto the treadmill or elliptical machine for your LISS workout, your body has mostly depleted its stores of glycogen, and is now burning almost exclusively fatty acids. For the 20 to 30 minutes of cardio, you're burning all fat!
Weight training first burns up most of the CP and glycogen available to your body, ensuring that you burn fat when you do cardio. You'll have the energy to do both your weight training and cardio workouts, and you can push yourself much harder to shred muscle and burn fat.
*It should be clearly distinguished that we are referring to "cardiorespiratory" fitness, which concerns heart (cardio) and all organs related to breathing (respiratory), rather than the more commonly--and somewhat redundantly--used "cardiovascular", which concerns ONLY the function of blood flow (cardio+vascular = heart+blood vessels). We are also referring to cardio as a focused workout, in and of itself, rather than “cardio-type workouts” that many people use as part of a warm-up/cool-down, such as getting on the treadmill for a few minutes to get the blood flowing or gradually bring the heart rate back down from intense exercise.
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