You’ve probably seen it.
You finish a run, check your watch, and it proudly tells you: “You spent 12 minutes in Zone 3.”
Cool… but what does that even mean?
Everyone’s obsessed with staying in the right “zone,” like it’s some secret code for fat loss or performance, when in reality, your body doesn’t care about zones. It cares about effort.
Let’s cut through the noise and explain what these zones actually are, when they’re useful, and why you shouldn’t panic if your watch says you’ve “left Zone 2.”
Where Heart-Rate Zones Came From
Heart-rate zones aren’t a smartwatch gimmick, they come from exercise-physiology research in the 1980s-90s.
Scientists noticed two key thresholds in how the body produces energy:
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LT1 / VT1 (Aerobic Threshold): the first small rise in blood lactate, still mainly fat-fuelled, sustainable.
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LT2 / VT2 (Anaerobic Threshold): the sharp rise where lactate and fatigue start limiting performance.
To make this usable outside a lab, coaches mapped those thresholds to percentages of maximum heart rate (HRₘₐₓ).
That’s how we ended up with the 5-zone model most watches use today, a simplified map of a very complex system.
The Simplified Zone Breakdown
| Zone | % of HRₘₐₓ (approx.) | What’s Happening |
|---|---|---|
| Zone 1 | 50 - 60 % | Easy recovery. Aerobic metabolism, low stress. |
| Zone 2 | 60 - 70 % | Aerobic base work. Builds mitochondria, endurance, efficiency. |
| Zone 3 | 70 - 80 % | “Tempo.” Mix of aerobic + anaerobic systems. |
| Zone 4 | 80 - 90 % | Threshold. High lactate tolerance, race-pace territory. |
| Zone 5 | 90 - 100 % | Max effort. Short, explosive, quickly fatiguing. |
Just remember, these aren’t boxes. They’re a continuum.
All three energy systems (aerobic, glycolytic, and ATP-PC) are always contributing, only the mix changes.
What the Research Actually Shows
Achten & Jeukendrup (2003) - Heart rate correlates with oxygen uptake and lactate quite well… until heat, dehydration or fatigue enter the chat.
-Good guide, not gospel.
Coyle et al. (1991) - The best endurance athletes aren’t those with the highest HRₘₐₓ, but those who can work close to threshold for longest.
-Performance lives near your “comfortably uncomfortable” pace.
Seiler & Tønnessen (2009) – Elite endurance athletes train polarised: about 80 % easy, 20 % hard.
-Most of your running should feel easy, not heroic.
San Millán & Brooks (2018) - Lactate isn’t waste; it’s actually recycled as fuel by the aerobic system.
-Even at high intensity, the aerobic tap is still wide open.
Why Watches Get It Wrong
Your watch estimates HRₘₐₓ using formulas like 220 − age.
That can be off by 10-15 bpm, and heart rate itself drifts with heat, caffeine, sleep, hydration, stress… even whether you had an argument that morning.
Heart rate also lags behind effort by 30-60 seconds.
So when you surge up a hill and your wrist says you’ve left Zone 2, it’s not that your physiology suddenly flipped, the data’s just catching up.
And while most watches give decent heart-rate data at rest, meta-analyses show accuracy drops significantly during exercise, especially with wrist-based sensors or movement artefacts (PMC, 2024).
So if your watch says you’re “red-lining” halfway through a steady run, take it with a pinch of sweat.
Individual Differences: Why One Runner’s Zone 2 Is Another’s Zone 4
Here’s where most people get caught out.
Two runners can be side-by-side, same pace, same watch, and their heart rates can be completely different.
That’s not an error, it’s physiology.
Everyone’s aerobic system develops at different rates based on genetics, training background, and muscle efficiency.
Someone with a highly developed aerobic base can run fast while still staying in Zone 2.
Someone newer to training might hit Zone 4 at that same pace, because their aerobic engine isn’t as efficient yet.
That’s why HR zones are personal, not universal.You can’t just copy another runner’s numbers, your body’s thresholds sit where your physiology puts them.
Recent research confirms huge individual variability in where athletes hit aerobic and anaerobic thresholds, even under identical protocols (Springer, 2024).
It takes time, testing, and understanding to figure out what’s true or close to true for you.
Once you’ve logged enough sessions and start noticing how HR, pace, and effort link together, that’s when heart-rate monitoring becomes genuinely useful.
Quick Ways to Estimate Your Zones
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Best option: lab test (VO₂ or lactate). Expensive, but spot-on.
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Good option: field test, warm up, then run hard for 30 min and take your average HR from the final 20 min as your threshold (≈ LT2).
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Simplest: find your true HRₘₐₓ with short hill sprints or an all-out 3-min effort; don’t rely on 220 − age.
Use those numbers to build personalised zones — not what your watch guesses.
When Zones Actually Help
Used properly, heart-rate zones can be brilliant tools:
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Zone 2 base work: improves fat use, capillary density, and aerobic capacity.
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Zone 4–5 intervals: sharpen speed and lactate tolerance.
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Recovery control: keeps easy days truly easy.
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Training balance: helps distribute intensity (the famous 80/20 rule).
Just don’t micromanage every run, use zones to guide, not dictate.
When They Don’t
Common myths worth killing off:
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“If I go above Zone 2, I stop burning fat.” ❌ You still are, just using more carbs too.
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“Zone 5 means 100 % anaerobic.” ❌ Your aerobic system’s still working flat out.
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“No Zone 4 = wasted session.” ❌ Adaptation comes from consistency, not punishment.
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“Zones never change.” ❌ They shift with fitness, fatigue, heat and stress.
The “Three Taps” Analogy
Think of your energy systems as three taps filling one tank:
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Aerobic - oxygen-based, efficient, the main supply.
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Glycolytic - carbs-based, faster but limited.
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ATP-PC - explosive, short-lived.
All three are always on.
Jogging? Aerobic tap wide open, others dripping.
Sprinting? Anaerobic taps turned up, but aerobic still running full blast to help clean up the mess.
Heart-rate zones are simply a rough estimate of how much each tap is open, not which ones are on or off.
How to Train Smarter
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Use HR as feedback, not a leash.
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Do most of your running easy enough to recover, and make your hard days truly hard.
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Focus on progress and performance, not perfect numbers.
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Remember: your watch is a tool, not a coach.
The Bottom Line
Heart-rate zones are a great way to understand intensity, but they’re not magic, and your body doesn’t care which colour your watch flashes.
The goal isn’t to stay in a zone; it’s to train across all of them, building strength, speed, and endurance from every system working together.
So next time your watch beeps that you’ve “left Zone 2,” smile.
Because your body already knows what it’s doing.
Ready to train smarter, not just harder?
At The Lab Liverpool, we help people understand the “why” behind their training, whether it’s conditioning, strength, or performance.
Check out our Small Group Personal Training or 1-2-1 Personal Training
References:
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Coyle, E.F. et al. (1991). Physiological determinants of endurance performance as a function of age and training status. Journal of Applied Physiology, 70(2), 682–693.
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Achten, J. & Jeukendrup, A.E. (2003). Heart rate monitoring: applications and limitations. Sports Medicine, 33(7), 517–538.
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Seiler, S. & Tønnessen, E. (2009). Intervals, thresholds, and long slow distance: the role of intensity and duration in endurance training. Sports Science, 13, 32–53.
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San Millán, I. & Brooks, G.A. (2018). Re-examining lactate: an old friend newly rediscovered. Journal of Physiology, 596(24), 5595–5608.
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Springer (2024). Individual variability in exercise intensity and threshold determination. Sports Medicine, 54, Article 02159-1.
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PMC (2024). Accuracy of wearable heart-rate devices during exercise: a systematic review and meta-analysis. Frontiers in Physiology, 13, 11560992.
