You strap on your watch, hit start, and within minutes it tells you: “Aerobic” or “Anaerobic.” Two words that sound like they belong in a biology textbook, not on your wrist. But your watch throws them at you mid-workout, expecting you to understand what they mean—and whether you should care.

Here’s the thing: you should. Because the difference between aerobic and anaerobic isn’t just vocabulary. It’s the difference between burning fat and building speed, between lasting an hour and surviving two minutes.

The Short Version

Aerobic means “with oxygen.” Anaerobic means “without oxygen.”

When you move at a steady, sustainable pace, your body has time to use oxygen to break down fuel—mostly fat—and keep you going. That’s aerobic.

When you sprint, lift heavy, or go all out, your body needs energy faster than oxygen can arrive. So it switches to a backup system that burns sugar without oxygen. That’s anaerobic. It works, but it creates lactate and it runs out quickly.

What Your Watch Sees

Your smartwatch doesn’t actually measure oxygen in your blood during a workout—at least, most don’t. Instead, it uses heart rate as a proxy.

Aerobic effort usually lives in Zones 2 and 3. Steady breathing, manageable effort. You’re not counting the seconds until you can stop.

Anaerobic effort lives in Zones 4 and 5. Breathing is ragged. Muscles burn. Your watch might buzz to tell you you’ve entered “threshold” or “performance” mode. What it’s really saying is: You’re running on backup fuel now.

Over time, some watches also track your “anaerobic training load”—a number meant to show how much high-intensity work you’ve done. It’s not magic. It’s just math based on how long you stayed in those higher zones.

Why Both Matter

Here’s where people get it wrong: They think aerobic is for beginners and anaerobic is for athletes. Not true.

You need aerobic fitness to go long. A strong heart, efficient lungs, muscles that know how to use fat for fuel. That doesn’t happen by accident. It happens because you spent hours in Zone 2, even when it felt slow.

You need anaerobic fitness to go hard. To sprint past someone, climb a steep hill, or finish a race strong. That doesn’t happen by accident either. It happens because you practiced discomfort.

Most recreational runners spend too much time in between—too hard for aerobic gains, too easy for anaerobic gains. Your watch can help you see that pattern. Whether you change it is up to you.

What Recovery Has to Do With It

Anaerobic work breaks your body down faster. That’s the point. But it also means you need more recovery.

Your watch might suggest rest days after high-intensity workouts. It might show you your “HRV” (heart rate variability) and tell you whether you’re ready to push again. These aren’t just features. They’re the other half of the equation.

Aerobic work builds your engine. Anaerobic work builds your power. Rest is when you actually get faster.

The Long and Short of It

Aerobic training is about duration. Anaerobic training is about intensity. You need both.

Your smartwatch labels your workouts to help you see which one you did today—and which one you’ve been neglecting. It’s not grading you. It’s just keeping track.

Use it to ask better questions: Did I go hard enough to earn that rest day? Did I go easy enough to actually recover? Am I mixing both, or stuck in the middle?

The answer isn’t in the data. It’s in what you do next.