Heart rate is the most accessible window into how hard your body is working. Faster than a lab test, more honest than perceived effort, and now sitting on most runners' wrists in real time. The trick is knowing which zones to train in, and how to figure out where your zones actually are.
There are three frameworks worth knowing. They overlap in places and disagree in others, and most modern coaching pulls from all three.
First, you need your maximum heart rate
Every zone system is a percentage of something, and that something starts with your maximum heart rate. The most-cited formula is 220 minus your age, which is famous because it is simple, and unreliable because individual variation is large. A 35-year-old's true max can range from 175 to 200, and the formula puts everyone at 185.
Two better options:
- The Tanaka formula: 208 minus 0.7 × age. Published by Tanaka et al. in 2001, validated more rigorously than the 220-minus-age formula.
- An actual maximum effort test. The gold standard. Sprint a 5-minute hill until you can barely move, look at your watch's max HR for the run. That number is your real ceiling.
Almost any modern smartwatch with optical HR will record your max during a hard race or interval session. If you have raced a 5K all-out in the last year, your watch's recorded peak HR is probably close to your real max.
Karvonen: the heart-rate-reserve method
Finnish exercise physiologist Martti Karvonen proposed the heart rate reserve concept in 1957. Instead of using a percentage of max HR directly, Karvonen used the range between resting and max HR, which accounts for the fact that two runners with the same max but different fitness levels have different working ranges.
The formula:
Target HR = ((Max HR - Resting HR) × Intensity %) + Resting HR
For a 35-year-old with a max HR of 188 and resting HR of 55, working at 70 percent intensity:
((188 - 55) × 0.70) + 55 = 148 bpm
Karvonen's five zones, by intensity percentage of heart rate reserve:
| Zone | Intensity | What it trains |
|---|---|---|
| Z1 Recovery | 50-60% | Active recovery, easiest warm-ups |
| Z2 Aerobic | 60-70% | Aerobic base, mitochondrial density, fat oxidation |
| Z3 Tempo | 70-80% | Marathon pace, sustained aerobic |
| Z4 Threshold | 80-90% | Lactate threshold, sustained hard effort |
| Z5 Maximum | 90-100% | VO2max intervals, short hard reps |
Karvonen is the system most fitness watches and apps default to because it personalizes based on resting HR. If you only know one HR zone system, this is the one.
Daniels: 5 intensities, defined by purpose
Coach Jack Daniels (the running one, not the whiskey) built his system around physiological adaptation rather than just percentage targets. Each zone has a specific purpose and a specific dosing rule:
- E (Easy): 65 to 79 percent of max HR. The base of everything. The bulk of weekly volume.
- M (Marathon pace): 80 to 89 percent of max HR. The pace you could hold for the full marathon today.
- T (Threshold): 88 to 92 percent of max HR. Comfortably hard. Held for 20 to 60 minutes in segments.
- I (Interval): 97.5 to 100 percent of max HR. VO2max work. Reps of 3 to 5 minutes.
- R (Repetition): Faster than VO2max-velocity, mostly anaerobic. Short fast reps for economy and speed.
Daniels' dosing rules are more interesting than his zone definitions. He caps quality work by zone: T-pace at no more than 10 percent of weekly volume, I-pace at no more than 8 percent (or about 10 km per session, whichever is less), and R-pace at no more than 5 percent. These caps are the rule that keeps amateur runners healthy. Stacking past them is the most common cause of overtraining injuries in marathon programs.
For more on how Daniels' VDOT number sets these zones from a single race result, see the VDOT guide.
Maffetone: one number, focus on aerobic
Phil Maffetone's MAF (Maximum Aerobic Function) method skips the zone hierarchy entirely. Instead of five zones, you have one ceiling, and you stay below it for almost all of your easy mileage.
The formula:
MAF heart rate = 180 - your age
With four optional adjustments (see Maffetone's The Big Book of Endurance Training for details):
- Subtract 10 if you have been sick or are recovering from injury
- Subtract 5 if you have not trained consistently, or have asthma, allergies, or are overweight
- Add 0 if you have trained consistently for two or more years without setbacks
- Add 5 if you are an experienced competitive runner aged over 65 in great health
A 35-year-old healthy consistent runner would have a MAF HR of 145. The Maffetone rule: all easy training, including most of your weekly volume, stays at or below 145. No exceptions.
The argument for Maffetone is that easy days end up actually easy. A common mistake among beginners is creeping their easy pace up until "easy" is really tempo. Maffetone enforces discipline. The argument against is that pure aerobic training without faster work can leave you flat at race pace; most coaches treat MAF as a base-phase tool, not a full training philosophy.
The 80/20 rule
Whatever system you pick, the volume distribution that consistently shows up in research is the same: about 80 percent of weekly running should be in the easy zones (Z1 to Z2 in Karvonen, E pace in Daniels, at or below MAF in Maffetone), and about 20 percent in moderate-to-hard zones.
This ratio was popularized by Stephen Seiler's research on elite endurance athletes, who almost universally do far less hard work than amateurs assume. The temptation for an amateur is to do too much in the "no man's land" of moderately hard, which produces fatigue without the specific adaptation of either true easy or true threshold work.
How Smart Runner uses HR zones
Smart Runner uses the Karvonen method by default (because it personalizes), with Daniels' dosing rule on top (no more than 10 percent threshold-or-harder per week). You can override your zones in Settings if you have lab-tested values, or if you prefer Maffetone for your base phase.
The app shows your live HR zone on the Apple Watch during every run. If you start drifting into Zone 3 on an easy day, the watch nudges you back. This is more useful than it sounds; the easy-day creep is the single most common training error in amateur marathon plans.
If you only remember one thing
Most amateur marathoners do not run easy enough on easy days, which compromises their hard days. Pick a method, find your ceiling for "easy," and respect it for the next four months. The fitness will come faster than you think.