Exercise Zones Explained: Why Your Fitness Tracker Isn’t Enough

There is so much talk about exercise zones but rarely is there a clear explanation of what they are, how we use them, and why we should care about them in the first place.

So first, the bad news. That watch you’re wearing, the machine you’re exercising on, the ring you wear to bed—it’s probably not giving you an accurate measure of the heart rate zones you actually want to train in. And that equation you’re relying on—“220 minus your age = HRmax, then take 65–75% of that for Zone 2”—isn’t very accurate either.

So let’s break it down. In this article I will cover:

• What the zones are
• How to determine your specific zones
• What that information even means for your performance and long-term health

Heart rate zones are rooted in a key principle of human physiology: our bodies have four primary “gears” for producing energy—similar to how a car or bike shifts into different gears depending on the speed and terrain.

Think of your body as an engine designed to make ATP (adenosine triphosphate), the energy currency that powers every movement you make. The more efficiently you produce ATP, the more energetic, resilient, and healthy you become. Efficient ATP production even supports faster recovery from inflammation and illness.

Inside every cell, your body converts four main fuel sources into ATP: fat, glucose, lactic acid, and creatine. Each of these fuels acts like a different gear. Fat provides the most energy, but it burns slowly. The others—glucose, lactic acid, and creatine—produce ATP more quickly, but with different trade-offs.

Just like driving a car, each gear is suited for a different purpose. Training in specific heart rate zones taps into different fuel systems, placing unique demands on the body and producing distinct fitness adaptations. That’s why exercise physiologists design training programs to target each energy system strategically—so you can reach your specific performance and health goals.

Today, the most popular way to understand this relationship between fuel sources, ATP production, and performance is through the “Zone” system (see Figure 1).

Heart rate zones were developed to help us plan workouts and fueling strategies more effectively, depending on our individual goals. In the U.S., the five-zone model has become the most commonly used framework in athletic training and sports medicine. In Figure 1, you can see the dominant fuel sources associated with each zone. While there's a clear progression in energy systems as you move from Zone 1 to Zone 5, there’s also considerable overlap—especially in the higher-intensity zones (3 through 5). Pro tip: This is exactly where most people need extra support in understanding how their metabolism really works.

Interestingly, not all countries use the same zone system. Some sports and international training protocols may use three, seven, or even more zones. Regardless of the system, they’re all attempting to describe the same underlying physiology: how the body creates ATP during different intensities of exertion. That means "Zone 3" for an Italian cyclist may not correspond exactly to "Zone 3" for an American runner. No matter the labeling, the goal is the same—to understand what’s happening inside your cells when you generate energy.

Now that we’ve covered the importance of heart rate zones, you might be wondering: Can I just use my smartwatch or favorite AI tool to figure out mine? Honestly?

Not really. Here’s why: the generic zone calculators rely on broad assumptions and average formulas that don’t account for individual variation. And when it comes to heart rate, variation is the rule—not the exception.

Take resting heart rate or even maximum heart rate (HRmax)—the highest your heart can beat during intense exercise. I have one patient with an HRmax of 199, while her husband’s is just 150. If your HRmax is miscalculated, your entire zone profile will be inaccurate.

To truly understand how your body produces energy—and tailor your training for optimal results—you need personalized, precise data. Only then can we accurately translate your heart rate into meaningful insight about what’s happening at the cellular level.

As mentioned above, zone training is a powerful tool to help you reach your goals. Here’s how it works:

In these lower zones, your body primarily uses fat as a fuel source. This type of training can be sustained for long periods with minimal wear and tear on the body, making it highly beneficial for overall health and longevity. Benefits include:

Best of all, you can do a lot of this type of training with low impact on the body.

Pro tip 2:  No matter what your fitness goals are, zones 1-2 are essential.

[Here’s a link to a key study connecting fitness, quality of life, and longevity.]

At these higher intensities, your body relies more on carbohydrates, lactate, and creatine for fuel. This kind of training is associated with distinct performance and metabolic benefits, including:

This kind of training is essential for nearly every physical performance goal—from long-distance endurance to explosive sprinting.

But to truly reap the benefits, we need two things:

I’ve worked with so many patients who believed they were training in Zone 2—only to discover through testing that they were actually in Zone 4. That explains the cortisol surges, sugar cravings, poor sleep, and stubborn belly fat.

Whether someone is training for a half marathon or rebuilding fitness after surgery, pregnancy, or illness, training in the right zones can dramatically accelerate progress. And all amateur athletes—from weekend runners to rec league players to Olympic hopefuls—benefit from weekly variety in their training. That diversity ensures all fuel sources are being tapped into and optimized.

With all the time and effort we invest in our workouts and overall health, it’s crucial to understand how to use our body’s “gears,” how those gears correspond to heart rate zones, and which zones we’re actually engaging during exercise.

This is exactly why we use exercise testing in our lab at Jyzen.

We use a graded exercise test with indirect calorimetry because it gives us precise, real-time data about your unique metabolism. Specifically, it tells us:

One recent patient is a marathon runner and we were able to determine precisely how many carbs she needs to consume per hour during the race. Utilizing the information we gathered, her next race was her best ever!

When we review your results together, I’ll show you exactly how hard to work, what heart rate to target, and how to tailor your training to meet your specific goals—whether that's better performance, fat loss, or recovery.  

Then, I’ll have you come back in three to six months, because as you become fitter and healthier, your numbers will change.

Why? Because the heart rate needed to produce energy will shift as your body becomes more efficient and powerful at generating ATP.

Health is a moving target, and that’s why we all need precise, scientific measurements that evolve with us.

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If you are interested in working with our doctors to evaluate and optimize your health on a deeper level, please contact our Patient Care Concierge. You may call 415-729-0449 X 123 or email Concierge@jyzen.com to begin your journey to better health today.