Why VO2 Testing — What Your Watch Can't Tell You
Heart rate zones are guesses. VO2 testing shows what's actually happening inside your body. Here's why it matters for every endurance athlete.
You train at heart rate 145 and think it's Zone 2. Your watch says so. Your coach says so. But your watch doesn't know if you're burning fat or carbs. Only a gas exchange test can tell you that.
Same heart rate. Different metabolism.
Two athletes on an easy ride. Both at 145 bpm. Completely different results.
Heart rate formulas like "220 minus age" can be off by 10-20 bpm. The only way to know your real zones is to measure your gas exchange. One 20-minute test. Then you know.
RER — your real-time fuel gauge
RER (Respiratory Exchange Ratio) is the ratio of CO2 you exhale to O2 you inhale. It tells you exactly which fuel your body is burning — right now, not estimated.
RER = VCO2 ÷ VO2 — shows which fuel your body is burning right now.
40x more energy in fat. Most athletes can't use it.
~90 minutes. Then you hit the wall.
30+ hours. If your body knows how to use it.
The difference between bonking at kilometer 35 and finishing strong isn't more gels — it's a trained fat metabolism. Athletes who know their FatMax zone train their body to burn fat at higher intensities, sparing glycogen for when it matters.
One FatMax test shows you at which heart rate and wattage your body burns the most fat.
VO2max — the strongest predictor of how long you live
A JAMA study with 122,007 patients showed: improving your fitness from the bottom 25% to above average reduces mortality risk more than quitting smoking. No upper limit of benefit.
VO2max is highly trainable — 15-20% improvement within months. But first, you need to know your number.
What it means for your sport
Running — Your "easy pace" is probably 30 sec/km too fast. A VT1 test reveals your true Zone 2 heart rate — preventing Achilles tendon issues, shin splints, and chronic fatigue.
Triathlon — FatMax zone for the Ironman marathon. The difference between a 3:30 and a 4:30 marathon off the bike is often fat oxidation, not fitness.
Road Cycling — FTP doesn't tell you where you burn fat vs carbs. VO2max + FatMax shows the full picture — and whether your base training actually builds base.
Hyrox / CrossFit — How hard on the SkiErg so you can still push the sled? Precise thresholds prevent blowing up mid-race. RMR testing nails your nutrition.
Trail / Ultra — 8+ hours on your feet. Your fat oxidation rate decides finish or DNF.
How it works with OpenSpiro
- Connect a portable VO2 analyzer via Bluetooth
- Follow a guided protocol — VO2max ramp or FatMax graded test
- OpenSpiro measures O2 and CO2 breath by breath
- Your thresholds (VT1, VT2), FatMax zone, and VO2max appear automatically
- Train with your real zones — not estimates
References & further reading
- Association of Cardiorespiratory Fitness With Long-term Mortality — Mandsager et al., JAMA 2018
- Contextualising Maximal Fat Oxidation During Exercise — Randell et al., Front Physiol 2018
- Optimizing fat oxidation through exercise and diet — Achten & Jeukendrup, Nutrition 2004
- Carbohydrate Dependence During Prolonged Endurance Exercise — Stellingwerff & Cox, Sports Med 2014