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Heart Rate in Triathlon
Heart rate monitoring is one of the most valuable tools for triathlon pacing. Unlike single-sport events, triathletes must distribute effort across three disciplines and avoid blowing up before the run. Each discipline has different cardiovascular demands due to body position, muscle groups used, and the cooling effects of water and wind.
Swimming heart rates are typically 10-15 bpm lower than running at equivalent effort due to the horizontal position, hydrostatic pressure, and cooling from water immersion. Cycling heart rates fall between swimming and running because of the seated position and wind cooling. Understanding these differences is essential for effective race pacing.
Karvonen Formula
The intensity percentage varies by discipline and race distance. Longer races require lower intensity to sustain performance. Sprint triathlons allow higher intensity (80-90% HRR) while Ironman events typically require 65-75% HRR to avoid bonking on the run.
Discipline-Specific Zones
| Discipline | Sprint %HRR | Olympic %HRR | Half IM %HRR | Ironman %HRR |
|---|---|---|---|---|
| Swim | 75-85% | 70-80% | 65-75% | 60-70% |
| Bike | 80-90% | 75-85% | 70-78% | 65-75% |
| Run | 85-95% | 80-90% | 75-83% | 70-80% |
Pacing Strategy
- Swim: Stay in your target zone to avoid oxygen debt entering T1.
- Bike first half: Stay at the lower end of your zone to conserve glycogen.
- Bike second half: Gradually increase if feeling good, but never exceed zone.
- Run: Start conservatively, allow HR to climb naturally over the run leg.
- Watch for cardiac drift: HR increases over time at the same effort due to dehydration and heat.
Frequently Asked Questions
Why is swim HR lower than run HR?
The horizontal body position during swimming reduces the heart's workload compared to the upright position in running. Additionally, hydrostatic pressure from water assists venous return, water immersion triggers the dive reflex (lowering HR), and water provides cooling. Expect swim HR to be 10-15 bpm lower than running at the same perceived effort.
Should I use HR or power for the bike?
Power meters provide real-time effort data unaffected by cardiac drift, heat, or caffeine. If available, use power for the bike and HR as a secondary check. If you only have HR, be aware that HR may lag behind actual effort by 30-60 seconds and can drift upward during long rides.
How does race distance affect target HR?
Longer races require lower intensity to sustain energy over many hours. A sprint triathlete might race at 85-90% of max HR, while an Ironman athlete must stay at 70-75% to avoid depleting glycogen stores before the marathon run. Going even 5% too high on the bike can cost 15-20 minutes on the run.