You’ve probably seen the videos of athletes collapsed on gym floors, gasping for air like they’ve just escaped a vacuum. Usually, there’s a caption about "engine building" or "expanding the tank." What they are actually talking about is aerobic capacity. Specifically, they are chasing a higher VO2 max. It’s the gold standard of fitness. Honestly, though, most people approach it all wrong by assuming more pain always equals more gain. That isn't how the human heart works.
VO2 max is essentially the maximum amount of oxygen your body can utilize during intense exercise. It’s a measurement of milliliters of oxygen used in one minute per kilogram of body weight ($ml/kg/min$). Think of it as the size of your car's engine. A bigger engine can move faster and handle more stress before it starts to redline. But here’s the kicker: it’s not just about your lungs. Your heart has to pump the blood, and your muscles have to actually "grab" that oxygen out of the bloodstream to create energy. If any part of that chain is weak, your number stays flat.
If you want to know how to increase VO2 max, you have to stop thinking about "getting tired" and start thinking about physiological adaptations. You are trying to force your left ventricle to get bigger and your mitochondria to multiply. That takes a specific kind of internal pressure.
Why your current "hard" workouts might be failing you
Most people go to the gym or head out for a run and hang out in what coaches call "no man's land." This is that middle-intensity zone where you’re huffing and puffing too much to have a conversation, but you aren't going fast enough to actually trigger a massive VO2 max spike. It’s exhausting, but it’s inefficient. You’re just getting "kind of tired" without giving your heart a reason to change its structure.
To move the needle, you need to push against the ceiling. Dr. Stephen Seiler, a renowned exercise physiologist, famously popularized the "80/20" rule of polarized training. He found that elite endurance athletes—the people with the highest VO2 max scores on the planet—spend about 80% of their time going incredibly slow and only 20% going absolutely hammers-to-the-wall.
Why the slow stuff? Because long, easy efforts increase the stroke volume of your heart. Your heart literally stretches a bit to hold more blood. Then, the high-intensity 20% forces the heart to pump that blood with massive force. If you only do the middle-intensity stuff, you miss out on both the stretching and the high-force pumping. You end up stuck with a mediocre engine.
The Norwegian 4x4: The gold standard for a bigger engine
If you’re looking for the single most effective way to jumpstart your numbers, look toward Scandinavia. Researchers at the Norwegian University of Science and Technology (NTNU) have spent decades refining what is now known as the "Norwegian 4x4 intervals." It’s brutal. It’s simple. And it works better than almost anything else ever studied in a lab setting.
The protocol is basically this:
You warm up for ten minutes. Then, you do four minutes of high-intensity effort. This isn't a full-blown sprint where you die after thirty seconds. You should be aiming for about 90% to 95% of your maximum heart rate. After those four minutes, you take a three-minute active recovery break—walking or very light jogging. Then you do it again. Four times total.
The reason four minutes is the "magic" number is because it takes about two minutes for your oxygen transport system to fully "ramp up" to its peak. If you do 30-second sprints, you finish the interval before your heart even reaches its maximum pumping capacity. By doing four minutes, you spend a solid two minutes in the "red zone" where the actual adaptation happens. Do this twice a week. That’s it. Any more and you’ll likely fry your central nervous system.
Don't ignore the "Peripheral" side of the equation
We talk a lot about the heart, but your muscles are the end-users of that oxygen. You could have the world's most powerful pump, but if your muscles are inefficient, that oxygen just stays in the blood and goes right back to the lungs. This is where "capillarization" comes in.
Capillaries are the tiny blood vessels that wrap around your muscle fibers. The more you have, the more surface area there is for oxygen to jump from your blood into your cells. How do you get more? Zone 2 training. This is the "boring" stuff. We're talking about a pace where you can breathe exclusively through your nose and carry on a full conversation.
I know, it feels like you're doing nothing. But during these long, slow sessions, your body produces a signal called VEGF (Vascular Endothelial Growth Factor). This signal tells your body to grow more "fuel lines" to your muscles. Without these extra lines, your VO2 max will always hit a wall, no matter how many intervals you do.
The role of genetics and the "Non-Responder" myth
Look, we have to be honest here. Genetics play a massive role. Some people are born with a "floor" that is higher than most people's "ceiling." The legendary cross-country skier Joan Benoit Samuelson or cyclist Miguel Indurain had physiological gifts that can't be fully replicated by training. Some studies, like the famous HERITAGE Family Study, showed that while almost everyone improves with training, a small percentage of people are "low responders" who see very little change in their VO2 max despite doing the work.
However, "low responder" doesn't mean "no responder." Often, people who think they can't increase their VO2 max are simply doing the wrong type of volume. They might need more base building or, conversely, they might need to stop being afraid of the "pain cave" during intervals. Also, body composition matters. Since the formula is $ml/kg/min$, losing excess body fat (if you have it) automatically increases your VO2 max because the denominator in that math equation gets smaller. You become a lighter car with the same size engine.
📖 Related: Bear Creek Family Practice: What to Actually Expect From Your Local Clinic
Nutrition and recovery: The invisible variables
You can't build a bigger engine if you don't have the raw materials. Iron is the most overlooked component here. Iron is the core of hemoglobin, the protein in your red blood cells that actually carries the oxygen. If you are even slightly iron deficient—which is common in endurance athletes due to "foot-strike hemolysis" (literally breaking red blood cells by hitting the ground)—your VO2 max will crater. No amount of training can fix a lack of hemoglobin.
Also, stop fasted cardio if your goal is performance. High-intensity intervals require glycogen. If you try to do a 4x4 session on an empty stomach, you won't be able to hit the power outputs required to stress the heart enough. You'll just be tired and slow. Eat some carbs an hour before. Give your body the fuel it needs to actually redline.
Putting it all together into a weekly plan
If you actually want to see a change in your testing numbers over the next 12 weeks, you need a balanced approach. Don't just go out and run hard every day. That’s a recipe for shin splints and a plateau.
👉 See also: The Cream Pie in Sex: Risks, Realities, and Why People Are Obsessed With It
A "pro" style week for a recreational athlete might look something like this:
Monday is a total rest day or a very light walk.
Tuesday is your "High Intensity" day—the Norwegian 4x4s.
Wednesday is 45 to 60 minutes of easy Zone 2.
Thursday is another Zone 2 session, maybe some light strength training to keep your joints healthy.
Friday is your second "Hard" day—maybe shorter 2-minute intervals at a slightly higher pace.
Saturday is your "Long" day—90 minutes to two hours of very easy movement.
Sunday is rest.
This structure provides the two things the body needs to change: high-pressure stimulus and low-pressure volume. The intervals tell the heart to get stronger; the long easy days tell the muscles to get efficient.
Actionable steps to start today
- Find your "true" max heart rate. Don't use the 220-minus-age formula; it's often wrong. Do a hill sprint test or a supervised ramp test to see what your heart can actually do.
- Invest in a chest strap. Wrist-based sensors are notoriously laggy during intervals. If you're doing a 4x4, you need to know exactly when you hit that 90% mark.
- Prioritize the slow stuff. If 80% of your workouts aren't "easy," you aren't training your aerobic base properly. Slow down to go faster later.
- Test, don't guess. Use a Cooper Test (12-minute run for distance) or a 20-minute all-out cycling power test to estimate your VO2 max. Re-test every 8 weeks.
- Check your ferritin levels. Get a blood panel. If your iron is low, your VO2 max has a hard cap that no amount of running will break.