New runners almost always track pace. Experienced runners often graduate to heart rate. Coaches argue about it endlessly. Here's an honest breakdown of when each metric serves you — and why heart rate zones win for long-term fitness development.
The case for training by pace
Pace is simple, immediate, and satisfying. You ran 5 minutes per kilometre. You PR'd your 5K. Numbers go down over time, you're improving. There's a clarity to it that heart rate doesn't offer.
For racing, pace is essential — you need to know if you're on track for your goal time. For structured speed workouts (intervals, tempo runs), pace targets are often more practical than heart rate targets because they're absolute.
Pace is also consistent in controlled conditions: same course, same temperature, similar fatigue. If you run the same route every week, pace tells you something reliable about your fitness.
The problem with pace
Pace breaks down the moment conditions change. Run the same effort on a hot day and your pace will be 15–30 seconds per kilometre slower than in cold weather — with the exact same physiological cost. Run after a poor night's sleep, in altitude, with elevated stress, or during a high training load week, and your "easy" pace will feel like a hard pace.
Chasing pace targets in poor conditions leads to training too hard, accumulating fatigue, and overtraining. This is one of the most common reasons recreational runners plateau or get injured.
The case for heart rate zones
Heart rate responds to your actual physiological state. When it's 32°C and humid, your heart rate at a given pace will be higher than usual — and training to that higher heart rate would be overtraining. But running slower to keep your heart rate in Zone 2? That's appropriate self-regulation.
Heart rate zones account for:
- Daily variation in readiness and fatigue
- Weather and environmental stress
- Cumulative training load
- Illness and hormonal variation
- Differences between individuals of different ages and fitness levels
This last point matters especially for group training and competition. Two people running at the same pace can be in entirely different heart rate zones. A 5:30/km jog might be Zone 2 for a fit 30-year-old and Zone 4 for a beginner. Pace says they're doing the same thing. Heart rate says they're not.
The real-world hybrid
Most effective training programmes use both. A common structure:
- Easy days: train by heart rate zone (Zone 1–2, regardless of pace)
- Hard days: train by pace or power for precise quality work
- Race day: execute by pace and feel
The reason: easy days should be truly easy, and heart rate is the best way to enforce that. Hard days should hit specific physiological targets (lactate threshold, VO2 max), and pace or power is a more precise way to hit them consistently.
Why Underdog uses heart rate zones
Underdog is built on the heart rate zone model because it's the only metric that creates a genuinely fair competition between athletes of different ages, fitness levels, and training backgrounds.
A 55-year-old spending 40 minutes in Zone 3 is working as hard — relative to their capacity — as a 25-year-old at the same zone. Pace would make the comparison meaningless. Zone time makes it fair. That's the foundation of the Underdog league system: your effort, measured against your own physiology, compared to others doing the same.