The Aerobic Threshold: Building Your Engine

Have you ever had to grind your way through a brutal workout or a 60 hour week?

If you’re regularly struggling to get beyond 3pm without an energy crash, you don’t need more caffeine. You have an engine issue.

Simply put, your gas tank isn’t large enough.

Your Aerobic Threshold is the the invisible ceiling that needs to be raised to increase the capacity of your gas tank…thereby giving you more energy.

The visual I’ll describe next will explain it and then I’ll discuss how you can improve it.

The Two Bar Concept

Picture a single bar standing straight up and down made of two stacked segments.

The bottom segment, the green part, is your aerobic ceiling. This is how fast (or how intense) you can go while your body still has enough oxygen to produce energy cleanly. Think of this as your cruising speed. You can hold this speed for a long time without falling apart.

The top segment, the red part, is your anaerobic reserve. This is your afterburner. It’s the extra gear you tap into when the aerobic system can’t keep up. It’s powerful, but it’s depleted quickly.

Strength coach Joel Jamieson uses this exact picture to explain why two people can have the same top speed but completely different endurance. Imagine two runners who can both sprint 10 mph. The first one has a tiny green bar and a huge red one. To hold 10 mph, he’s living almost entirely in the red. He burns out fast. The second runner has a big green bar, so 10 mph is barely above his cruising speed. He’s sipping from the red zone instead of chugging it. He holds that pace far longer.

Same top speed. Wildly different staying power. The difference is the size of the green bar.

This is where so many people get it backwards. They assume that if they keep training at a higher intensity (the red bar), eventually their heart rate at that intensity level will come down. That’s not how conditioning works.

You cannot improve the green bar by training in the red…this can only be accomplished by improving your capacity training in the green.

Efficiency is built at lower heart rates, where you aerobic system can actually improve. A well functioning aerobic system (gas tank) is needed to replenish the afterburner (the red bar) when you step on your proverbial gas pedal.

The Aerobic Threshold Is Where the Green Bar Ends

The point where your body starts dipping into that red reserve has a name. As the demand for energy climbs, your system eventually can’t supply enough oxygen to keep up, and it starts producing energy anaerobically instead. Joel Jamieson explains that the more you rely on anaerobic energy, the faster you fatigue.

That’s not a motivation problem you can push through. It’s biology.

Here’s the part that matters for you: the goal isn’t to get better at suffering in the red. The goal is to raise the green bar so you spend less time there in the first place.

When you improve your aerobic fitness, the exact same workload requires less energy from the anaerobic side. The pace that used to redline you becomes your comfortable cruise. You’ve made the engine bigger, so daily life stops tapping the afterburner.

This Is What Zone 2 Actually Builds

Zone 2 is the training intensity that lives right around the bottom of that green bar. It’s steady, conversational effort. You can talk in full sentences. It feels almost too easy, which is exactly why most driven people skip it.

But this is where the magic happens. Zone 2 training boosts your body’s ability to use fat for fuel and clear lactate more effectively, building the cellular machinery that makes the whole aerobic system bigger and more efficient. Notably, even elite endurance athletes spend the majority of their training time here, not hammering intervals.

You don’t improve conditioning by just working harder until you’re tired. You improve it by building capacity first. Zone 2 is the capacity. It’s the gas tank, not the gas pedal.

Why You Probably Don’t Need More Red

Here’s the trade-off nobody mentions. You can’t max out both bars at once. The most explosive athletes on earth don’t have the best endurance, and the best endurance athletes can’t generate much top-end power. You build one at the cost of the other, and you choose based on what you actually need.

So ask yourself honestly: do you need to be more explosive? Or do you need to get through your day without redlining?

For most busy professionals, the answer is the green bar. You don’t need another HIIT class that leaves you wrecked for two days. You need a bigger aerobic engine so that work, life, and your harder training sessions stop pushing you into the red.

You need to raise your Aerobic Threshold.

The afterburner is nice to have. But you can’t run your whole life on the afterburner. Build the engine that lets you cruise.

And if the thought of steady-state cardio makes you want to chew your own arm off, you’re not alone. Check out this blog post which was written for exactly that reason.

Stop redlining your way through life. At No Limits Fitness, our Small Group Personal Training is built around making you more capable, not just more exhausted. We’ll help you build the aerobic engine that gives you energy for everything else that matters.

Ready to raise your green bar? Let’s talk.