VT1 and VT2: The Two Thresholds That Set Your Zones

Most heart rate zones start from a formula. Subtract your age from 220, take percentages, and you have five zones. The problem is that the formula describes an average person, and it can miss a real person's maximum by 10 beats or more in either direction. It also says nothing about where that person's body changes gears. Two thresholds do.

What happens at each threshold

At an easy pace, your muscles burn a mix of fat and carbohydrate. They produce some lactate, and the body clears it as fast as it is made. Breathing is steady and you can talk in full sentences.

As the pace climbs, the muscles lean harder on carbohydrate and lactate production rises. To keep the blood from turning acidic, the body buffers that lactate, and the buffering releases extra carbon dioxide. You have to breathe more to get rid of it. The first point where breathing rises out of proportion to the oxygen you are using is the first ventilatory threshold, VT1. Below it, you are in the zone most people call zone 2. You could hold it for hours.

Keep speeding up and you reach a second bend. Lactate now accumulates faster than the body can clear it, the buffers fall behind, and breathing rises faster than even the carbon dioxide output as the body works to hold off the acid building in the blood. You can manage a few words at a time, not a sentence. This is VT2, close to what runners call lactate threshold. Most trained people can hold it for roughly 30 to 60 minutes before they have to slow down.

Above VT2 is interval territory. Between the thresholds is tempo work. Below VT1 is the easy volume that builds the aerobic base.

How the test finds them

During a VO2 max test, you wear a mask with a built in analyzer that measures oxygen and ventilation breath by breath and sends the data wirelessly to a screen. Speed and incline step up in stages until you stop. On the graph, we watch how ventilation rises against oxygen use and carbon dioxide output. The two bends in those lines mark VT1 and VT2, and the heart rate at each bend becomes the edge of a zone.

Why this beats the formula

Two 45 year old men can share the same maximum heart rate and have very different thresholds. A sedentary one might cross VT1 at 120 beats a minute. A trained one might not cross it until 150. The age formula gives them the same zones. The test gives each of them his own.

The thresholds also move. Maximum heart rate barely changes with training, but VT1 and VT2 shift to higher paces and higher heart rates as fitness builds. Retesting after a few months of training shows that shift directly, which is the clearest sign a plan is working.

That's why we build zones from the test instead of a formula.

This article is educational and is not medical advice.

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