In gyms across the country — and probably the world — there’s one machine that gets more use than most: the treadmill. Whether we’re walking or running, indoors or out, we do it to improve our fitness. But do you actually know why endurance training is so good for your body? And do you know the most effective way to increase your fitness? Let’s start at the beginning.
Endurance is your muscles’ ability to perform work over a period of 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.
When your muscles don’t get enough oxygen, lactic acid builds up, causing stiffness and fatigue — this is known as anaerobic endurance. On the other hand, if your muscles receive enough oxygen, you stay in the aerobic zone, keeping lactic acid at bay and allowing you to work out for much longer.
So what determines how much oxygen your muscles get? The heart.
“The entire purpose of endurance training is to increase the size and elasticity of the heart and blood vessels,” explains Jan Hoff, Professor of Medicine. “This, in turn, increases your body’s maximum oxygen uptake.”
For the average 20-year-old man, the heart can pump about 20 liters of blood per minute. In a top-level endurance athlete, that number can double — the equivalent of four kitchen taps running at full blast. “Our heart, no bigger than a fist, is a remarkably efficient pump,” Hoff adds.
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.
Your maximum heart rate — the fastest your heart can beat — is genetically determined and doesn’t change with training, although it does gradually decline with age. What does improve with training 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,” says Hoff.
For a long time, it was believed that stroke volume plateaued at about 70% of your maximum heart rate. But more 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.
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.
So what’s the most effective way to train endurance?
Hoff, along with fellow NTNU professor Jan Helgerud, developed a simple but powerful method: 4x4 intervals — four intervals of four minutes each, performed at high intensity.
This can be done walking or running uphill, either outdoors or on a treadmill. It also works with skiing, rowing, or cycling (while standing on the pedals during the intervals).
Start each session with a 6-minute warm-up at moderate intensity. Then perform four 4-minute intervals, with 3-minute active recovery breaks in between. Finish with a 5-minute cool-down.
To get the full benefit, you should reach 85–95% of your maximum heart rate by the end of the first interval.
“You’ll know you’re at the right intensity when you’re breathing heavily after two minutes but don’t feel any pain or stiffness,” Hoff explains. During the recovery breaks, aim to stay active at around 70% of your maximum heart rate.
If you don’t have a heart rate monitor or a fitness tracker, don’t worry — there are simple ways to estimate your maximum heart rate and train at the right intensity.
Since your max heart rate is genetically determined and varies from person to person, one 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.
Alternatively, you can listen to your body:
If you do just one 4x4 session per week, you can increase your endurance by about 0.5%. Train twice a week, and you can maintain the biological age of a 20-year-old until you're 70.
So what are you waiting for?
Test your maximum oxygen uptake and biological age with Myworkout GO — available for free on iOS and Android.
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