Anaerobic means “without oxygen,” operating at a level where you’re breathing hard, going all out. I remember running through LaGuardia Airport years ago, heavy briefcase in hand, trying to catch a flight to Chicago. I made it but couldn’t talk when I got to the gate. That’s anaerobic. Another would be the feeling after shoveling heavy snow — I lived in Chicago — for ten minutes or more, panting and out of breath. Remember running to the point of being out of breath as a kid? The shorter and higher the intensity, the greater the use of anaerobic energy.
All of our muscle fibers can be divided into two categories: slow-twitch and fast-twitch. We use slow-twitch for endurance, like jogging, walking or cycling long distances at a steady pace. Fast-twitch muscles kick in when we “go anaerobic” or work at a high energy level. Going from a jog to a sprint, or cycling as hard as possible, will do it. Lifting heavy weights is another example of using fast-twitch. We’ll talk more about strength training and muscle fibers in the next chapter.
Another big plus to doing anaerobic exercise is that it increases your metabolism and more calories continue to be burned after exercise; your RMR (resting metabolic rate) may run higher for as long as twenty-four hours. RMR is the amount of energy consumed at rest.
Aerobic exercise uses more calories during the activity because it goes on longer. Your recovery will be faster and enhanced metabolism will be less than during a high intensity, anaerobic workout. You’ll work for a shorter time at a high intensity, but the longer recovery period means that total energy expenditure and fat use as an energy source will be similar to an aerobic activity.
The increase in RMR can be a real benefit for those who do lots of exercise; they’re burning more calories per unit of body weight on a continuous basis than those who aren’t fit. Fit individuals have more muscle that’s metabolically active, so they have a higher RMR and use more fat energy 24/7 than their sedentary peers.