Heart Rate Zones, Explained Like You're Human
You've seen the colored bars on your watch. Zone 2, Zone 4, threshold. Here's what they actually mean — and why running in the right one matters more than running fast.
Your watch says you spent 40 minutes in Zone 2 and 8 minutes in Zone 4. Cool. But what does that mean? And should you care?
Yes. You should. Because heart rate zones are probably the most useful training tool you’re ignoring.
Five zones, one idea
Heart rate zones divide your effort into five levels. Each zone trains a different energy system. The key insight: most of your running should be easy, and the hard stuff should be properly hard.
| Zone | Name | How it feels | What it trains |
|---|---|---|---|
| Zone 1 | Recovery | Walking pace, could talk all day | Active recovery |
| Zone 2 | Easy aerobic | Comfortable, conversational | Fat burning, base fitness |
| Zone 3 | Tempo | Bit harder, sentences get shorter | Lactate clearance |
| Zone 4 | Threshold | Hard, can manage a few words | Race pace, VO2max |
| Zone 5 | Max effort | Can’t talk, want to stop | Speed, peak power |
Don't have a heart rate monitor? Use the talk test. Zone 2: you can hold a full conversation. Zone 3: you can speak in sentences but prefer not to. Zone 4+: you can't finish a sentence. It's surprisingly accurate.
The Zone 2 problem
Here’s where most recreational runners go wrong: they run too hard on easy days and too easy on hard days. Everything ends up in Zone 3 — that uncomfortable middle ground where you’re working hard enough to be tired, but not hard enough to trigger real adaptation.
Zone 3 feels productive. You’re sweating, you’re breathing hard, you finished faster than last time. But it’s a trap. You’re accumulating fatigue without the aerobic benefit of Zone 2 or the speed benefit of Zone 4.
I can see when every run is in Zone 3. Your heart rate average is always 145-155, your pace is always the same, and your fitness isn't moving. That's not training. That's just running.
Polarized training: the 80/20 rule
Elite runners spend about 80% of their training in Zone 1-2 and 20% in Zone 4-5. Almost nothing in Zone 3.
That sounds backwards. How do you get faster by running slow? Because Zone 2 builds your aerobic engine — the foundation that everything else sits on. More mitochondria, better fat oxidation, stronger heart. You can run Zone 2 daily without breaking down. You can’t do that with Zone 4.
The hard days? Those go properly hard. Intervals at Zone 4-5, with real recovery between reps. That’s where speed comes from.
Flotti tracks your zone distribution across the week. If you're spending too much time in Zone 3, it'll flag it — and suggest slowing down your easy runs or adding a proper interval session instead.
Finding your zones
Your zones are based on your maximum heart rate or lactate threshold heart rate. The old formula (220 minus your age) is famously unreliable — it can be off by 10-15 beats.
Better options:
- A hard race effort — your max HR from a 5K all-out is close to your true max
- A threshold test — 30 minutes at the hardest sustainable pace, average HR of the last 20 minutes
- Your watch’s estimate — surprisingly decent on modern Garmin/Polar devices, especially after a few hard sessions
Optical wrist sensors can lag during intervals, spike during strength work, and struggle with darker skin tones. For zone-based training, a chest strap is still the gold standard. It doesn't have to be expensive — a basic Garmin HRM is enough.
What Flott does with your zones
Flott pulls your heart rate zone configuration from intervals.icu. When Flotti analyzes a run, it doesn’t just say “nice run” — it tells you where you spent your time and whether that matches the session’s intent.
Your easy run yesterday averaged 138 bpm — right in the sweet spot of Zone 2. That's exactly where it should be. Fitness is building without stacking fatigue. Keep this up.
That "easy jog" averaged 152 bpm. That's Zone 3 for you. Slow it down — seriously. Easy means easy. Your aerobic base will thank you in six weeks.
Heart rate zones tell you how to train, not just how much. The magic formula: most runs easy (Zone 2), a few runs hard (Zone 4+), almost nothing in between. Your watch already tracks this. Flott makes sure you actually pay attention to it.
The Flott Blog
Training smarts, dev stories, and Flotti opinions.