I work out a CrossFit gym north of Denver. My workouts have been really challenging, which surprised me because I do a lot of running and biking, and I would expect the cardio workouts to be a lot easier. I asked my trainer about it, and he said that they weren't really cardio. I know that I'm completely out of breath, so it can't just be strength. Why am I having such a hard time?
B.A., Thornton, CO
People tend to think of workouts as either being strength-based or cardio-based, when in fact there is another primary energy system in play in short, intense workouts like you find in many CrossFit workouts.
To give a quick background, any muscular movement, from blinking your eye to deadlifting a maximum weight, to each and every contraction in your leg during a marathon is actually fueled by a molecule called ATP (adenosine triphosphate). When we talk about energy systems, what we're really talking about is what fuel is being used to regenerate the body's stores of ATP.
For a strength workout, we rely on current stores of ATP, or on the phosphagen cycle, which is the quickest route to replace ATP. This is good for exercises or sets that last less than a minute, which is perfect for that set of bench presses, or a 50 meter sprint.
For longer workouts, say running a 10k or hiking a 14er, we're using primarily the oxidative energy system. This is what's truly meant by "cardio". This is the energy system that we use when we're operating at the same intensity level for periods of 20 minutes or more.
In between those, however, is the glycolytic energy system. This is where a lot of the magic of CrossFit (and other workouts that utilize shorter, high-intensity energy-systems work) happens.
This energy system was disregarded for decades, considered to be not-particularly-beneficial, while cardio was considered the best exercise for fat-burning and for overall health benefits. The reason for this is that during exercise, the primary fuel being used the glycolytic system is sugar (glycogen - the source of the name). While sprinters, fighters, and football players understood the benefits, it wasn't until programs like CrossFit brought it to the mainstream that researchers started seeing the benefits: working in the glycolytic energy system may burn less total fat during the workout, but would continue to burn fat and improve health for hours after the workout was over.
What you're experiencing during your workouts is the breathlessness that comes from working intensely in these glycolytic bursts; one of the primary effects is called "oxygen debt", which is the result of your cells working hard enough that the oxygen coming into your system isn't enough to fuel them. This results in them moving into the glycolytic system, burning glycogen and producing lactic acid in the process, and having to catch up on oxygen (and flush the lactic acid) after the workout is over. If you've ever felt the effects of taking several minutes after the workout ends to catch your breath, that is the result of your cells trying to catch up - trying to "repay" the oxygen debt.
The best test to find out if you're doing "cardio" or working in a higher-intensity energy system is to ask yourself if you'd be able to continue working at that intensity for 20 minutes or more. In many CrossFit workouts, the answer would be a resounding "no". Even in workouts that may take longer than 20 minutes ("Murph", anyone?), you're not working at a steady state, at least not for all of it. Instead, you may do a lower-intensity part, such as jogging during a mile run, but the higher-intensity exercises such as pullups require that you work in bursts, doing a few reps, stopping to let your muscles recover (regenerating ATP), and going again.
This is all to say that, while you may have some great cardiovascular abilities, once you get into the higher-intensity that characterizes the other energy systems, all bets are off. The improvements will come quickly, but you'll have to work for them!
While working in the oxidative, or cardiovascular, energy system has its benefits, it is hardly the be-all and end-all of fat-burning or health-generating workouts. The metabolic conditioning aspects of the glycolytic energy system cannot be overstated! Get some intensity going, and feel the burn!