You check your watch after a HIIT session and see a barcode of colours — grey, green, a lot of orange, some red, then back to green. It looks like a traffic light having an identity crisis.
Those colours map to your heart rate zones — ranges of heart rate expressed as percentages of your maximum. Each zone reflects a different intensity level and trains a different energy system. During intervals, your heart rate bounces rapidly between zones, which is exactly why HIIT is so effective.
Your heart rate takes 10-15 seconds to catch up to your actual effort level. During a 20-second Tabata sprint, your watch might show Zone 3 (green) when your body is working at Zone 5 (red). The colours on your watch during short intervals are always running behind reality. 📄 Buchheit & Laursen 2013 — Sports Medicine
The 5 Zones Explained Simply
Research consistently shows that spending time at or above Zone 4 (80%+ of max HR) is the primary driver of VO2max improvement. Zone 2 builds your aerobic base; Zone 4-5 raises the ceiling. 📄 MacInnis & Gibala 2017 — J. Physiology
Why HIIT Creates a Unique Zone Pattern
During steady-state exercise, your heart rate sits in one zone for the entire workout — maybe Zone 3 for a 30-minute jog. Your watch shows a single colour band.
During HIIT, your heart rate oscillates rapidly: Zone 4-5 during work, dropping to Zone 3-4 during rest (but rarely back to Zone 2 during short rest periods). This creates the characteristic zigzag pattern on your workout summary. 📄 Buchheit & Laursen 2013 — Sports Medicine
The key insight: during HIIT, you spend cumulative minutes in Zone 4-5 that you'd never reach in a continuous workout. A 20-minute jog might keep you in Zone 3 the entire time. A 15-minute HIIT session might accumulate 8-10 minutes in Zone 4-5 — even though no single interval lasts more than 30 seconds. That accumulated time at high zones is what drives superior cardiovascular adaptation. 📄 Milanović et al. 2015 — Sports Medicine
Estimating Your Max Heart Rate
To use zones, you need your maximum heart rate. The classic formula is 220 minus your age. It's a rough estimate — individual variation can be ±10-15 bpm — but it's good enough for setting training zones without a lab test. 📄 Tanaka et al. 2001 — J. Am. Coll. Cardiology
What This Means for Your Workout
During HIIT, don't obsess over which zone the watch shows in real-time — it's lagging behind your actual effort. Instead, use zones in your post-workout summary to check whether you're hitting the right intensity. Aim for 60-70% of your total HIIT time in Zone 4 or above. If most of your session stays in Zone 3, push harder or shorten your rest.
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SOURCES (5 peer-reviewed studies)
- Buchheit M, Laursen PB. High-intensity interval training, solutions to the programming puzzle. Sports Medicine. 2013;43(5):313-338.
- MacInnis MJ, Gibala MJ. Physiological adaptations to interval training and the role of exercise intensity. The Journal of Physiology. 2017;595(9):2915-2930.
- Milanović Z, Sporiš G, Weston M. Effectiveness of HIIT and continuous endurance training for VO2max improvements. Sports Medicine. 2015;45(10):1469-1481.
- Tanaka H, Monahan KD, Seals DR. Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology. 2001;37(1):153-156.
- Stahl SE, et al. How accurate are the wrist-based heart rate monitors during walking and running activities? BMJ Open Sport & Exercise Medicine. 2016;2(1):e000106.
