Heart rate
zones worth
fighting for.
Underdog turns every minute in Zone 2, Zone 3, and beyond into points. Compete in weekly leagues against friends. Your effort is the only metric that matters.
How it works
Three steps to compete.
Connect Apple Health
Grant read access to Apple HealthKit. Heart rate and workout data syncs automatically — from Apple Watch, chest strap, or any connected sensor. No manual logging, ever.
Earn zone points
Every minute in Z1–Z5 earns points. Zone 2 builds your aerobic base. Zone 3 and above score higher. Running, cycling, strength, HIIT — every workout type counts equally.
Compete in leagues
Start in Bronze. Climb to Diamond. Each week points reset — perform and you rise, slack off and you drop. Create private leagues with a shareable invite code.
The fair fight
A 70-year-old and a 25-year-old can score exactly the same.
Underdog measures relative effort— your heart rate as a percentage of your personal maximum. That's the only fair way to compare humans with different bodies, ages, and fitness levels.
No pace bias. No gear advantage. No ability cliff. Whether you ran a sub-4 mile yesterday or you're coming back from injury — if you spent 30 minutes in Zone 3, you earned those points.
Features
Built to compete.
League system
Bronze, Silver, Gold, Platinum, Diamond. Weekly promotion and relegation based purely on your zone points. No pay-to-win, no shortcuts — just consistent effort.
Zone tracking
Full Z1–Z5 breakdown for every workout. See exactly where you spent your effort.
Apple Watch sync
Wear it and compete. Heart rate data syncs from Apple Health automatically after every workout.
Weekly insights
Zone breakdown, step trends, intensity timeline. Know exactly when and how hard you pushed.
Private leagues
Invite code. Friends only. See exactly where you rank in real time.
Privacy-first
Health data stays on device. Never sold, never used for ads.
Zone guide
What are heart
rate zones?
Heart rate zones are intensity ranges based on a percentage of your maximum heart rate. Each zone produces different physiological adaptations. Consistent Zone 2 training is the most research-backed way to build aerobic base and improve VO2 max long-term — and Underdog tracks every minute of it.
Underdog awards more points for higher zones — but every zone counts toward your weekly total. Full zone training guide →
Apple Watch
Heart rate zones on Apple Watch — automatically.
Underdog reads your heart rate data directly from Apple Health. Every workout your Apple Watch records — running, cycling, HIIT, strength — is analysed by zone with zero manual input.
Zone 2 and Zone 3 training are the most effective ways to build your aerobic base and improve VO2 max. Underdog tracks every minute you spend there and converts it into a score your entire league competes against.
Download free
Training guides
From the lab.
Heart Rate Zones Explained: Z1 to Z5
What each zone means, how to calculate your zones from your max heart rate, and exactly how much time to spend in each one.
7 min readZone 2 Training: The Science of Slow Cardio
Why exercising at 60–70% of your max heart rate builds the most durable aerobic base — and how Underdog keeps you accountable.
9 min readApple Watch Heart Rate Zones: The Complete Setup Guide
How Apple Watch measures heart rate, how zones are calculated, and how Underdog turns that data into a competitive score.
6 min readStart competing
Your zones.
Your score.
Your league.
Every minute in a heart rate zone earns points. The leaderboard doesn't lie.