You’re huffing. You’re puffing. Your chest feels like a drum set in the middle of a solo. You glance down at your Apple Watch or Garmin, and it says 165. Is that good? Is it dangerous? Most people just guess. They look at the little colorful zones on the screen and hope for the best. But honestly, knowing what should your heart rate be during exercise is way more nuanced than just subtracting your age from 220.
That old formula? It’s called the Fox formula. It was basically an observation made in 1970 by Dr. William Haskell and Dr. Samuel Fox. They weren't even trying to create a global fitness standard; they were just looking at some data for a heart disease study. Yet, here we are decades later, still using it like it’s gospel. It’s often wrong. Sometimes by a lot.
If you’re 40, the math says your max is 180. But what if you’ve been running marathons for a decade? Or what if you’re just starting out and your heart is still getting used to the stairs? Your "real" number might be 195 or 170. This matters because if you're training in the wrong zone, you’re either wasting time or burning out your central nervous system.
The Myth of the Perfect Number
We love precision. We want a single digit to tell us we’re healthy. Unfortunately, your heart is a living organ, not a calculator. Your heart rate on a Tuesday might be five beats higher than on a Wednesday just because you had an extra espresso or didn't sleep well.
When people ask what should your heart rate be during exercise, they usually want to know how hard they should push. The American Heart Association (AHA) generally recommends staying between 50% and 85% of your maximum heart rate. That’s a massive window. It’s like saying "drive somewhere between 20 and 80 miles per hour." It doesn't help you much when you're actually on the road.
Why Your Max Heart Rate is Probably a Lie
The 220-age rule is a blunt instrument. Dr. Martha Gulati and her team at Northwestern University found that for women, the formula actually overestimates the max heart rate, potentially leading to overtraining. They suggested a different math: $206 - (0.88 \times \text{age})$. If you’re a woman in your 50s, that’s a significant shift in your target zones.
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Then there’s the Tanaka formula: $208 - (0.7 \times \text{age})$. It’s widely considered more accurate for healthy adults, but even then, it’s just an estimate. The only way to truly know your max is a supervised stress test in a lab where they push you until you literally cannot go anymore. Most of us aren't doing that on a Tuesday morning.
Breaking Down the Intensity Zones
You’ve probably seen the "Fat Burning Zone" on a treadmill. It sounds magical. Like you’re melting away calories while barely breaking a sweat. It’s kinda true, but also a bit of a marketing gimmick.
- Zone 1: Very Light (50-60% Max HR)
This is your "I’m walking the dog" pace. You can talk about your weekend plans without gasping. It’s great for recovery and metabolic health, but it won’t make you a faster runner. - Zone 2: Light (60-70% Max HR)
This is the holy grail for endurance. Most elite athletes spend about 80% of their time here. It builds mitochondria. It teaches your body to use fat as fuel. You can still talk, but you’d rather not. - Zone 3: Moderate (70-80% Max HR)
The "grey zone." You’re working hard. You’re sweating. But it’s not quite hard enough to trigger massive aerobic gains, and it’s too hard to be considered "easy." Most casual gym-goers live here exclusively, which often leads to plateaus. - Zone 4: Hard (80-90% Max HR)
Now it hurts. You’re breathing through your mouth. Your legs feel heavy. This is where you build speed and power. You can only stay here for 10 or 20 minutes before your body screams for a break. - Zone 5: Maximum (90-100% Max HR)
Sprints. All-out efforts. Your heart is red-lining. This is for performance, not for general health.
What Really Happens to Your Heart During a Workout
When you start moving, your sympathetic nervous system kicks in. It tells your heart to beat faster to move oxygen-rich blood to your quadriceps, your glutes, your lungs. Your stroke volume—the amount of blood pumped per beat—actually plateaus at a certain point. After that, the only way to get more oxygen is to increase the frequency of the beats.
But here’s the kicker: Cardiac Drift.
You’re 30 minutes into a run. Your pace hasn’t changed. But suddenly, your heart rate starts climbing. 150... 155... 162. Why? You’re getting dehydrated. Your blood is getting slightly thicker, and your body is redirecting blood to your skin to cool you down. Your heart has to work harder just to maintain the same output. This is why strictly following a heart rate number can be frustrating. You might feel like you're failing when you're actually just thirsty.
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External Factors That Mess With Your Stats
If you're obsessing over what should your heart rate be during exercise, you have to account for the "noise."
- Heat and Humidity: Your heart rate can jump 10–20 beats per minute just because it’s 85 degrees outside. Your body is fighting two battles: moving your muscles and keeping you from overheating.
- Caffeine: That pre-workout supplement? It’s a stimulant. It can artificially inflate your heart rate, making you think you’re working harder than you actually are.
- Stress: If you just had a brutal meeting with your boss, your resting heart rate is already elevated. When you hit the gym, you’ll hit your "target" faster, but not for the right reasons.
- Altitude: If you're hiking in the Rockies, there's less oxygen. Your heart has to pump faster to compensate for the thin air.
The Talk Test: The Low-Tech Winner
Before we had chest straps and wrist sensors, people used their lungs. It’s called the Talk Test.
If you can sing a song, you’re in Zone 1.
If you can speak in full sentences, you’re in Zone 2.
If you can only manage short, three-word bursts, you’re in Zone 4.
If you can only grunt or wheeze, you’ve hit Zone 5.
Research, including studies published in the Journal of Strength and Conditioning Research, consistently shows that the Talk Test correlates remarkably well with actual ventilatory thresholds. Sometimes, your body knows more than your watch.
Medication and Your Heart Rate
This is a big one. Beta-blockers, often prescribed for high blood pressure, are designed to keep your heart rate down. If you’re on these, you will never hit your "target" heart rate. You could be working at an 11/10 intensity level, and your watch might only show 110 bpm. If you fall into this category, you must use a "Rate of Perceived Exertion" (RPE) scale instead of raw numbers.
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RPE is a scale from 1 to 10.
- 1 is sitting on the couch.
- 10 is running away from a literal bear.
- Most exercise should happen between 4 and 8.
When To Be Concerned
Exercise is stress, but it shouldn't feel like a medical emergency. If your heart rate stays high (above 100 bpm) for a long time after you’ve stopped moving, that’s a sign of poor recovery or overtraining.
Palpitations are another red flag. If you feel like your heart is skipping beats, fluttering like a bird, or "thumping" unevenly, stop. It’s usually nothing—often just too much caffeine or a lack of electrolytes—but it’s not something to push through.
And then there’s the "Maximum Heart Rate" fear. Can your heart explode? Generally, no. Healthy hearts have a built-in "rev limiter." Your body will usually make you vomit or faint before you can actually damage the muscle from pure intensity. However, if you have undiagnosed underlying conditions, like hypertrophic cardiomyopathy, that's a different story. This is why a quick check-up is smart before you start a high-intensity interval training (HIIT) program.
Actionable Steps for Your Next Workout
Don't just stare at the screen. Use these steps to dial in your training without getting lost in the math.
- Find Your True Resting Heart Rate: Measure it the second you wake up, before you even get out of bed. Use your index and middle finger on your neck or wrist. Do this for three days and take the average. A lower number usually means a more efficient cardiovascular system.
- Ignore the 220-Age Rule: Use the Tanaka formula ($208 - (0.7 \times \text{age})$) for a more realistic baseline.
- Calibrate Your Zones: Go for a run or a bike ride at a pace where you can just barely maintain a conversation. Check your heart rate. That is your individual Zone 2 ceiling. Use that as your anchor.
- Track Recovery, Not Just Output: Watch how fast your heart rate drops in the first two minutes after you stop exercising. A drop of 30–50 beats in the first minute is a sign of a very healthy heart.
- Watch for Trends: If your heart rate is 10 beats higher than usual for the same pace, take a rest day. You’re likely fighting off a cold or you’re under-recovered.
Knowing what should your heart rate be during exercise isn't about hitting a specific number every single time. It's about understanding the language your body speaks. Some days your heart is a Ferrari; other days it’s a tractor. Both are fine, as long as you aren't trying to drive the tractor like it’s on a Formula 1 track.
Instead of obsessing over the exact digits, focus on the range. If you can stay consistent, keep your "easy" days easy, and save your "hard" days for when you actually feel energized, you’ll see way more progress than you ever would by following a 50-year-old math equation. High-quality training is about the intersection of data and intuition. Use the monitor, but listen to your breath.