What’s the Difference Between Aerobic and Anaerobic?

The inside scoop on how these zones can affect your ride and fitness training.

The terms are tossed around easily in indoor cycling classes, as well as other fitness venues—and they’re often met with perplexed looks. And it’s no wonder, given that the words are jargon-y and often misused or used in incorrect contexts (like the instruction to "go anaerobic!"). Simply put, “aerobic” means “with oxygen”, whereas “anaerobic” means “without oxygen”, a distinction that’s confusing since you’re breathing the entire time you're engaged in indoor cycling.

Hopefully, some scientific background will help clarify these issues: During a high-intensity bout of exercise, such as sprinting in an indoor cycling class, your body is working so hard that your cardiovascular system can no longer deliver oxygen to your muscles fast enough, which means you’ve crossed into the anaerobic zone. When you reach the point where your body can no longer meet its demand for oxygen, you’ll cross what's called the anaerobic threshold. You’ll know you’re there because you’ll be breathless, and you’ll be able to sustain that super high intensity for only a short time.

During an aerobic interval, by contrast, you’ll be working at a more manageable intensity. This means your cardiovascular system will be able to continue to supply your muscles with plenty of oxygen, and you’ll be able to maintain your current pace and intensity. Remember, too, that during exercise your body uses stored fat (fatty acids, to be specific) and carbohydrate (in the form of blood sugar or stored glycogen) for energy; you’ll see why this is relevant below.

Here’s a detailed look at how the differences between aerobic and anaerobic intervals stack up:

Aerobic

Heart rate:  60 to 80 percent of your maximum heart rate (MHR)

Duration of the interval: Usually up to 15 minutes at a time, followed by a brief recovery interval

Moves that will get you there: Seated and standing flats (while pedaling at a brisk pace), seated and standing climbs, jumps

Energy sources that are used: At lower intensities, your body uses a combination of stored fat and carbohydrate; at higher aerobic intensities, it switches to stored fat, which is a better long-term source.

Benefits: Builds cardiovascular fitness and endurance, improves your VO2 max (or maximum oxygen uptake), burns lots of calories (which can help you lose weight)

Anaerobic

Heart rate: 80 to 92 percent of your maximum heart rate—an all-out, full-on effort, in other words

Duration of the interval: Up to 30 seconds at a time, followed by a recovery interval

Moves that will get you there: Sprints on a hill or a flat road, time trials, races, high-intensity interval training (HIIT)

Energy sources that are used: Primarily carbohydrate, which is more easily accessible in the short term but can be depleted rapidly, leading to fatigue relatively quickly

Benefits: Builds stronger muscles and endurance, helps you work harder for longer, burns even more calories because of the higher intensity

Keep in mind that many indoor cycling classes include a combination of aerobic and anaerobic components in an interval ride.

So it’s pretty rare that you’re working exclusively in one domain or the other. And that’s actually a good thing because this way you’ll be building speed, stamina, and strength, in both energy zones.

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