Heart rate zones are one of the most useful tools in endurance and fitness training — yet most people who own an Apple Watch or a Garmin have never thought deliberately about which zone they're training in. This guide explains exactly what the five zones are, how to find yours, and how to use them to train smarter.
What are heart rate zones?
Heart rate zones (commonly labelled Z1 through Z5) are intensity ranges defined as a percentage of your maximum heart rate (HR max). Each zone produces different physiological responses and training adaptations.
The five-zone model is the most widely used system and the one Underdog uses for its point calculations.
The five heart rate zones
Zone 1 — Active Recovery (50–60% HR max)
Zone 1 is gentle movement: an easy walk, a slow bike ride, or a light cool-down jog. Your breathing is effortless and you could hold a full conversation without pausing. Z1 is used for recovery between hard sessions and for building total movement volume without adding fatigue.
Physiological effect: Improved blood flow and recovery, minimal training stress.
Zone 2 — Aerobic base (60–70% HR max)
Zone 2 is the most important zone for long-term cardiovascular fitness. Often called "conversational pace," you can speak in complete sentences but wouldn't want to hold a long phone call. Nose-breathing is usually possible throughout.
Physiological effect: Maximum mitochondrial density gains, improved fat oxidation, aerobic base building, VO2 max improvement over time. Research from Dr. Iñigo San Millán and others consistently identifies Zone 2 as the sweet spot for metabolic health and endurance development.
How much? Most endurance athletes target 70–80% of their total training time in Zone 2.
Zone 3 — Tempo (70–80% HR max)
Zone 3 is comfortably hard. You can still speak, but only in short sentences. This is a sustainable but meaningful effort — long tempo runs, moderate cycling efforts, and most recreational sport sessions naturally land here.
Physiological effect: Improved lactate clearance, higher aerobic power output, increased stroke volume.
Zone 4 — Lactate threshold (80–90% HR max)
Zone 4 is hard. Full sentences are gone — you're down to a word or two. This is race pace for most runners doing a 5K or 10K. Sustainable for 20–60 minutes depending on fitness level.
Physiological effect: Raises lactate threshold, improves high-intensity sustainable power, builds mental resilience.
Zone 5 — Maximum effort (90–100% HR max)
Zone 5 is all-out. You cannot speak. This is sprint intervals, VO2 max efforts, and the final kick in a race. Sessions are short — typically 30 seconds to 4 minutes at a time — with full recovery between efforts.
Physiological effect: VO2 max improvement, maximum cardiac output, neuromuscular power development.
How to calculate your heart rate zones
Every zone is based on your personal maximum heart rate (HR max). The most accurate way to find it is a proper field test — but for most people, this formula is a reasonable starting point:
HR max = 220 − your age
A 35-year-old would have an estimated HR max of 185 bpm. Their Zone 2 range would be 185 × 0.60 = 111 bpm to 185 × 0.70 = 129 bpm.
Keep in mind this formula has ±10–15 bpm variance. If you're serious about zone training, a proper lactate threshold test or a ramp test on a stationary bike gives a much more accurate HR max. We cover this in depth in our max heart rate guide.
How Underdog uses heart rate zones
Underdog reads your heart rate data automatically from Apple Health after every workout. It calculates how many minutes you spent in each zone and converts that time into points:
- Higher zones earn more points per minute than lower zones
- But every zone counts — including Z1 and Z2
- Every workout type is scored: running, cycling, strength, HIIT, swimming, and more
- Points are tallied weekly and determine your league ranking
The result is a competitive fitness metric that rewards consistent effort — regardless of whether you're a marathon runner or someone coming back from injury.
How to use zones in your training
A well-structured training week typically follows the 80/20 rule: roughly 80% of training time in Zone 1–2, and 20% in Zone 3–5. This distributes stress in a way that maximises adaptation without overtraining.
If you're new to heart rate zone training, start simple: do your easy runs actually easy (Z2), and make your hard days actually hard (Z4–Z5). Avoid the common trap of doing all your training in Zone 3 — too hard to recover quickly from, too easy to drive adaptation. Our Zone 2 guide covers the aerobic base approach in depth.
The bottom line
Heart rate zones turn workout intensity from a vague feeling into a measurable number. The data is already sitting in your Apple Watch — Underdog makes it competitive.