The deeper you get into the fitness world, the more science-y terms you’ve probably come across. Take anaerobic exercise, for instance. You’ve probably heard that phrase thrown around, but what does it really mean?
Just from the word “anaerobic,” we can deduce that anaerobic exercise is not aerobic. This we know. But after that, for most of us, things can get a little fuzzy. Oxygen comes to mind. And high-intensity interval training (HIIT) is anaerobic, right?
To help clear up the confusion—and help you take advantage of anaerobic exercise for boosting your fitness program—we talked to exercise physiologists and combed through the science. The result: your go-to guide on all things anaerobic.
What does “anaerobic” exercise really mean?
In the simplest terms, anaerobic exercise is any exercise that doesn’t use oxygen as energy. Of course, that might seem off to you since if your body doesn’t use oxygen on the whole, it definitely can’t exercise. (In fact, it can’t…live.)
That’s true, but with anaerobic activity, oxygen isn’t a player in how your body stays fueled for exercise, Katie Lawton, M.Ed., an exercise physiologist at the Cleveland Clinic, tells SELF.
To help break this down, it helps first to understand how your body gets the fuel it needs to power through those workout sessions. It does this through your body’s metabolic energy systems, which helps you convert food to energy. The two main energy systems are aerobic and anaerobic metabolism.
Throughout the entire day, your body breaks down compounds to release energy and keep things running along. It does this by breaking down a molecule called adenosine triphosphate (ATP) to produce energy. You can then use that energy to do anything from digesting food to climbing Everest, Lawton says.
While different energy systems rely on different fuel sources, ATP is at the center of all of them, and of every metabolic reaction in your body. It sounds complicated, but your body will know which system it needs to tap into based on what activity you’re doing. (More on that later!)
Your body will then funnel the resulting energy to whatever needs it. When you’re exercising, you need that energy to help your heart, lungs, and muscles keep up with the task at hand.
What’s the difference between aerobic exercise and anaerobic exercise?
Both aerobic exercise and anaerobic exercise need that energy, but they have different ways of generating it. During aerobic metabolism, your body uses oxygen to break down ATP for energy. But anaerobic exercise doesn’t rely on oxygen to access that energy. Instead, it breaks down fast-acting compounds (more on that below!) to get the fuel it needs from that ATP.
Aerobic metabolism is great at supplying the body with a lot of energy, but it does it at a pretty slow pace. Aerobic metabolism fuels low-intensity exercise and endurance workouts, like your long runs, walking, jogging, or restorative yoga. It generates energy by burning either fat or carbs:
- Fat: Aerobic exercise that’s low enough in intensity that you can do it easily for more than a few minutes at a time is mostly fueled by fat. Fat is an energy-rich nutrient (providing nine calories per gram, compared to four per gram for carbs or protein), which is why it can provide energy for so long. It just takes a long time for your body to chew through.
- Carbohydrates: As your intensity increases (but before it becomes hard), carbs come into play as energy. Your aerobic metabolism can break down glucose in your bloodstream and glycogen—stored carbs—in your liver or muscles, through a process called slow glycolysis. This can occur when you’re transitioning to a greater intensity, like when you’re just starting to ramp up your swimming pace.
Unlike aerobic metabolic, anaerobic metabolism tuckers out fast. But when it’s going, it’s going, and it releases a lot of energy. It’s the powerhouse behind sprints, plyometric workouts, true high-intensity training, and heavy weightlifting.
Anaerobic metabolism is such a heavy hitter because it uses fast-acting energy pathways called the phosphagen system—which is the most powerful and easiest to fatigue—and the anaerobic (or fast) glycolytic system, which has more staying power (but less than oxygen-based aerobic glycolysis). Both have different sources of fuel:
- Creatine phosphate (CP): At the very beginning of your workout, your anaerobic metabolism’s phosphagen system uses this chemical to jumpstart your energy until it can “figure out” what it’s doing and exactly what it needs (say, if another energy system should take over if it’s easy or low-intensity exercise). You also use CP during all-out, max-level exercise, Lawton says—think seconds, not minutes—and once it runs out, it takes a long time for your body to completely restock its stores. Your body naturally contains this chemical, but you can increase your stores by eating creatine-containing foods such as red meat or taking creatine supplements. (Of course, always check with your doctor before taking any supplements.)
- Adenosine monophosphate (AMP) and adenosine diphosphate (ADP): While it breaks down CP for energy, your anaerobic phosphagen system also smushes together AMP (adenosine plus one phosphate molecule) and ADP (adenosine plus two phosphates) to make adenosine triphosphate, according to the National Strength and Conditioning Association. (That’s adenosine and three phosphate molecules—and yep, it’s the ATP we mentioned above.) Once your body has ATP, it then can break it back down again for more anaerobic energy. The energy you get from this process only lasts up to about 30 seconds.
- Carbohydrates: Yes, your body can use carbs for fuel during aerobic glycolysis, but if you’re working out at a high intensity, only anaerobic glycolysis, or fast glycolysis can keep up. “Anaerobic glycolysis is an effective means of energy production during short, intense exercise, providing energy for a period ranging from about 10 seconds to two minutes,” Kevin M. Pennington, ATC, manager at Northwestern Medicine Athletic Training & sports Performance Clinic, tells SELF. After that, anaerobic metabolism gives way and aerobic metabolism takes over.
The take-home message? Your anaerobic system fuels your body during exercise that’s so intense you can’t keep it up for more than a couple minutes at a time.
That said, your metabolism doesn’t work on a toggle. You don’t switch from 100% using one energy system to 100% using another one. In your body, aerobic and anaerobic metabolism are both humming along at all times. Your body just emphasizes one over the other depending on how much energy you need to keep with whatever it is you’re doing, Lawton explains.
What are the benefits of anaerobic exercise?
The many benefits of using anaerobic metabolism to power your workouts come down to one word: intensity.
Working at a high intensity during exercise is a big part of improving your cardiorespiratory fitness, allowing you to become faster, more powerful, and stronger. It’s the principle behind true HIIT, in which you intersperse short periods of hard, near-max work with longer, easier periods of recovery, Lawton says.
Research shows that high-intensity, anaerobic-metabolism-dependent exercise is effective for building muscle, getting faster, or becoming more powerful and explosive. This can help you crush PRs in the gym and beat your opponent to the soccer ball, Pennington says. Plus, it’s time efficient. With interval training, you can gain many of the benefits of long, slow workouts in less time.
Another, not-so-known benefit? Working each system helps the other get better too. By performing anaerobic exercise, you actually train your aerobic metabolism. When you build strength, you are also boosting your endurance—meaning, more time before you get wiped out by your cardio. And the fitter your aerobic system gets, the harder you can work without your anaerobic metabolism giving out. So if you add things like sprint intervals to your routine, you may find your long and slow runs don’t feel quite so difficult, Pennington explains.
“Regular anaerobic exercise also improves your energy levels,” he says. “It increases your body’s ability to store glycogen, giving you more energy during intense physical activity.” You can work out and play harder, longer—and that only adds to the benefits of any exercise.
How to use anaerobic exercise in your workout routine
All these anaerobic exercise benefits don’t mean, though, that your workout routine should consist primarily, or even mostly, of it. There’s a fine line between pushing and pushing too hard. To get the greatest benefits of high-intensity anaerobic workouts, you need to rest both during and between your workouts.
If the human body can only maintain anaerobic exercise for two or so minutes at a time (and max max efforts even less than that), high-intensity intervals can’t be any longer than that at a time and still focus on your anaerobic system, Pennington says. And your body can’t fuel another true anaerobic interval unless you’ve rested for at least triple (but sometimes even more) the amount of time you worked. So if you want to put that practice to work—after a solid warm-up, of course!—work as hard as you possibly can for 10 seconds, then rest for 30. Repeat.
Similarly, between your high-intensity anaerobic workouts, your body needs rest to refuel and recover. The more intense your exercise sessions, the more rest you’ll need. In general, perform high-intensity on nonconsecutive days—meaning, at least 48 hours between HIIT routines. Between your workouts, turn to straight recovery and light aerobic activities like walking, jogging, or low-intensity strength sessions, Pennington suggests. That way, your body will be ready to go all in for your next max-out workout.