What Happens in the Body When You Train Endurance

🏃 Why Endurance Training Matters — More Than You Think

The treadmill is one of the most-used machines in any gym — and for good reason. Whether we walk or run, indoors or out, we turn to it to improve our endurance and overall health.

But have you ever asked:
👉 Why is endurance training so good for the body?
👉 What’s the most effective way to boost your cardiovascular fitness and VO₂ max?

Let’s break it down.

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🫁 What Is Endurance Training?

Endurance training is all about your muscles’ ability to perform over time. When you train for endurance, your muscles use energy — primarily from carbohydrates and fat — with the help of oxygen. That oxygen supply is the key factor that determines how long your muscles can keep going.

Aerobic vs. Anaerobic Endurance

• If your muscles receive enough oxygen, you stay in the aerobic zone, which delays fatigue and keeps lactic acid at bay and allowing you to work out for much longer.
• If they don’t get enough oxygen, lactic acid builds up, causing muscle stiffness and fatigue — this is anaerobic endurance.

The difference between the two? Your heart’s ability to deliver oxygen efficiently.

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❤️ The Heart of Endurance: Oxygen and Stroke Volume

“The entire purpose of endurance training is to increase the size and elasticity of the heart and blood vessels. This, in turn, increases your body’s maximum oxygen uptake.”
— Professor Jan Hoff, NTNU

The heart is what determines how much oxygen your muscles receive. In very untrained individuals, aerobic limitations often stem from the muscles themselves — especially a reduced capillary network (the smallest blood vessels) that impairs oxygen delivery to the muscle cells. But for most people, the bottleneck is the heart’s ability to pump enough blood.

• For the average 20-year-old, the heart can pump about 20 liters of blood per minute.
• In an elite endurance athlete, this can double — the equivalent of four kitchen taps running full blast.
• With training, the heart becomes stronger and more elastic, increasing stroke volume — the amount of blood pumped per beat.

"Our heart, no bigger than a fist, is a remarkably efficient pump"

— Professor Jan Hoff, NTNU

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🧬 Maximum Heart Rate vs. Stroke Volume: What Changes?

• Maximum heart rate (HR max) is genetically determined — it won’t change with training (though it gradually declines with age).
• What does improve is stroke volume, or the amount of blood the heart pumps with each beat.

"As the heart becomes stronger and more elastic through training that challenges its volume, the stroke volume increases"

— Professor Jan Hoff, NTNU

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Myth Busted:

It was once believed that stroke volume plateaued out at 70% of your HR max.
✅ Recent research has shown that stroke volume continues to increase right up to the intensity level that corresponds with your maximum oxygen uptake. Going beyond that point, however, leads to lactic acid buildup — which actually reduces stroke volume.

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🔥 What’s the Most Effective Way to Improve Endurance?

According to Professors Jan Hoff and Jan Helgerud, the answer is clear:
4x4 high-intensity interval training (HIIT)

Endurance training isn’t just for marathon runners or competitive athletes. According to Hoff, it’s one of the most powerful tools we have for preventing lifestyle diseases like obesity, high blood pressure, high cholesterol, poor circulation, and type 2 diabetes.

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🧠 What Is 4x4 Interval Training?

The 4x4 method is scientifically proven exercise protocol to improve VO₂ max, heart strength, and overall endurance. Professors Jan Hoff and Jan Helgerud are recognized as the originators of this method, also known internationally as the Norwegian Method. It can be performed by walking or running uphill, either outdoors or on a treadmill, and is equally effective with skiing, rowing, or cycling — as long as you're standing on the pedals during the intervals.

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✅ Structure of a 4x4 Session:

1. 6-minute warm-up (moderate pace)
2. 4 intervals of 4 minutes each at 85–95% of your max heart rate
3. 3-minute active recovery between each interval (around 70% HR max)
4. 5-minute cool-down

You can perform it walking or running uphill, skiing, cycling (standing), or rowing.

“You’ll know you’re at the right intensity when you’re breathing heavily after two minutes, but not in pain or discomfort,”
— Jan Hoff

📈 How to Estimate Your Max Heart Rate Without a Monitor

Since your maximum heart rate is genetically determined and varies individually, general estimates often fall short. A better approach is to connect your wearable device to the Myworkout GO app, complete a 4x4 interval session, and receive both a personalized VO₂ max estimate and an accurate maximum heart rate.

If you don’t have a heart rate monitor, there are simple ways to find the right intensity:

• After 4 minutes of hard effort, you should feel like you could continue for one more minute.
• After all four intervals, you should feel like you could manage one more.
• If you can carry on a conversation during the intervals, you’re going too easy.

The other option is to do a full interval session and take your pulse at the end of the final interval. Then add 15 beats to that number to estimate your maximum heart rate.

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🧪 How Often Should You Do 4x4 Training?

• One session per week is enough to maintain your current endurance level.
• Two sessions per week will increase your VO₂ max and help maintain a biological age of 20 — even into your 70s.

“The bigger your engine, the further you can go. The key to more life lies in increased capacity.”
— Professor Jan Helgerud

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🩺 Health Benefits of Endurance Training

✅ Reduces risk of lifestyle diseases:

• Obesity
• High blood pressure
• Type 2 diabetes
• Poor circulation
• High cholesterol

✅ Improves:

• Heart function
• Lung capacity
• Mobility
• Energy levels
• Biological age

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📱 Track Your VO₂ Max with Scientific Accuracy

💡 Tip: Use the Myworkout GO app for your next 4x4 interval session.
Developed with Professors Helgerud and Hoff, it offers guided interval training and accurate VO₂ max measurement — backed by science.

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✅ Summary: Why You Should Train Endurance

• Your heart adapts to the demands of training — and it’s the bottleneck for performance.
• 4x4 intervals offer the most efficient way to increase VO₂ max and heart strength.
• Just two sessions per week can extend your healthspan by decades.

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📲 Start Now:

Download Myworkout GO on iOS or Android
& take your first step toward younger, stronger, longer-lasting health.