HIIT the zone: How intense interval training can blast fat and improve fitness exponentially

By Editor On May 27, 2025

If you’re serious about burning fat, boosting endurance, and maximising your workout results, understanding heart rate zones and High-Intensity Interval Training (HIIT) is crucial.

In this guide, we’ll break down heart rate training zones, explain how HIIT works, and show you how to use both for efficient fat loss and fitness.

What Are Heart Rate Zones and Why Do They Matter?

Your heart rate (HR) during exercise reflects how hard your body is working and how close you are to your aerobic and anaerobic thresholds—the key to targeting specific fitness goals.

Calculate Your Maximum Heart Rate (MHR) with this simple formula:
220 – your age = Estimated MHR

The 5 Heart Rate Training Zones

Zone 1: 50–60% of MHR (Very Light Effort)

Ideal for: Warm-ups, cool-downs, and active recovery
Main fuel source: Fat stores
Benefit: Enhances blood flow and sets the stage for more intense work.

Zone 2: 60–70% of MHR (Light to Moderate)

Ideal for: General fitness and fat-burning workouts
Fuel source: Mostly fat
Benefit: Strengthens the cardiovascular system and improves endurance.

Zone 3: 70–80% of MHR (Aerobic Zone)

Ideal for: Endurance training and increasing stamina
Fuel source: A balance of fat and carbohydrates
Benefit: Enhances lactate threshold and aerobic capacity.

Zone 4: 80–90% of MHR (Threshold)

Ideal for: Improving VO2 max and high-intensity intervals
Fuel source: Mainly glycogen and ATP-PC system
Benefit: Builds speed and strength, though sustainable only for short bursts.

Zone 5: 90–100% of MHR (Maximum Effort)

Ideal for: HIIT sprints and maximum output
Fuel source: Stored glycogen and ATP
Benefit: Maximizes calorie burn and fitness capacity; only for advanced athletes.

HIIT vs. Fat-Burning Zones: What’s Better for Weight Loss?

While zones 1–2 primarily burn fat, burning more calories overall—not just fat—is the key to losing weight and keeping it off.

HIIT sessions spike your heart rate into zones 4 and 5, which means you burn more calories in less time compared to steady-state cardio.

And the resultant Excess Post-exercise Oxygen Consumption (EPOC) effect keeps your metabolism elevated for hours—even days—after your workout.

What Is High-Intensity Interval Training (HIIT)?

HIIT is a workout strategy that alternates between short bursts of intense exercise and brief periods of active recovery.

Try this HIIT Treadmill Workout:

Warm-up: 5 minutes at 50–60% MHR
Work Interval: 15–20 seconds sprint at 75–90% MHR
Recovery Interval: 30–40 seconds jog or walk at 50–60% MHR
Repeat: For 15–20 minutes
Cool-down: 5 minutes at low intensity

How to Increase HIIT Intensity the Smart Way

To progress safely and effectively, manipulate more than one variable:

Increase speed AND resistance (e.g., treadmill incline, cycling tension)
Extend interval duration
Shorten rest periods gradually

Benefits of HIIT Beyond Fat Loss

Shorter workouts with maximum results
Better muscle preservation than long-duration cardio
Long-lasting calorie burn thanks to EPOC effect
Increased cardiovascular efficiency and VO2 max

HIIT is powerful, but without the right nutrition, results will plateau. To optimise fat loss and recovery, eat enough to fuel your sessions, time your meals around workouts, and maintain a calorie deficit to lose weight, but never starve yourself.

HIIT Guidelines: Your Checklist

Use large muscle groups (legs, glutes, back)
Always warm up and cool down
Train for 15–20 minutes max
Progress by adjusting speed, time, and difficulty
Monitor your heart rate
Don’t train HIIT on consecutive days
Only do HIIT with a solid fitness base
Eat strategically before and after
Consult a medical professional if you have injuries

Ready to Burn Fat Faster and Train Smarter?

Integrate heart rate zone training and HIIT into your routine today to accelerate your fitness goals and redefine your limits. Your best shape starts with science-backed strategy—and it starts now.

Author: Pedro van Gaalen

When he’s not writing about sport or health and fitness, Pedro is probably out training for his next marathon or ultra-marathon. He’s worked as a fitness professional and as a marketing and comms expert. He now combines his passions in his role as managing editor at Fitness magazine.

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