Ironman Training: What Is Zone 4 / Threshold?

SUMMARY:
Zone 4 sits around 87–93% of max heart rate, 91–105% of FTP and 99–104% of CSS swim speed (RPE 7–8). This is the threshold zone, used to build strength and control near your aerobic limit in Ironman training. It feels hard but sustainable, requiring full focus and consistent effort. Training in Zone 4 improves pacing discipline, helps your body clear lactate more efficiently and develops the ability to stay composed under race-day stress. It is especially valuable during climbs, pace changes and strong finishes.

What Is Zone 4 Training?

If you’re training for an Ironman, you’ve likely spent countless hours in Zone 2 building aerobic endurance. You may even throw in some Zone 3 tempo sessions. But there’s one training zone that often gets overlooked: Zone 4. While Ironman is an endurance event, strategic use of Zone 4 workouts can make a dramatic difference in your speed, lactate threshold and overall efficiency on race day.

Zone 4 training refers to high-intensity efforts around 87–93% of max heart rate, 91–105% of FTP and 99–104% of CSS swim speed (RPE 7–8). This is also known as the lactate threshold zone, where your body is working hard but still able to clear the lactate being produced. These efforts are sustainable in focused intervals of 3 to 10 minutes and help improve your ability to manage intensity, stay efficient and hold strong pace under pressure.

ZONE 4 TRAINING METRICS

• Heart Rate: 87–93% of your max heart rate (HRmax)

• Power (Cycling): 91–105% of your FTP (Functional Threshold Power)

• Perceived Effort: (RPE 7–8)

• CSS swim speed: 99–104%

• FLJUGA Training Calculators. Set your zones and train smart.

Benefits of Zone 4 Training for Ironman Athletes

1. Raise Your Lactate Threshold

A higher threshold means you can race faster without accumulating fatigue. This is especially crucial during the bike and run legs of an Ironman.

2. Improve Your Race Pace Efficiency

Even though your Ironman race pace is typically in Zone 2–3, training in Zone 4 makes those intensities feel easier, allowing you to sustain your target pace longer.

3. Boost VO2 Max & Cardiovascular Strength

Zone 4 efforts strengthen your heart and lungs, helping your body deliver oxygen more efficiently across all race intensities.

4. Develop Mental Toughness

These workouts hurt. But pushing through discomfort in training builds the mental grit needed to handle race-day fatigue.

5. Increase Speed & Power

Even in long-course racing, power matters. Climbing, surging and staying strong through tough patches all require the muscular and neuromuscular gains you get from Zone 4 efforts.

How to Use Zone 4 Training in Ironman Prep

While critical for development, Zone 4 training must be timed and dosed appropriately. Overuse can quickly lead to burnout, fatigue, or even injury.

When to Focus on Zone 4:

• Early & Mid-Season: Use structured intervals to build threshold and power.

• 8–12 Weeks Out from Race Day: Shift focus to race pace work (Zone 2–3), maintaining only occasional Zone 4 sessions.

• Taper & Race-Specific Phases: Keep Zone 4 minimal, just enough to stay sharp without fatiguing the system.

Example Zone 4 Workouts for Triathletes

Bike (Indoor or Outdoor)

• 4×8 min @ Zone 4, with 4 min easy spin between

• 6×5 min @ Zone 4, with 3 min recovery

Run (Track or Treadmill)

• 3×10 min @ Zone 4, with 5 min easy jog

• 6×3 min @ Zone 4, with 2 min jog recovery

Swim (Pool or Open Water)

• 10×100m @ Zone 4, 15–20 sec rest

• 5×200m @ Zone 4, 30 sec rest

Tip: These sessions are best done when you’re well-rested and fresh. Save them for quality days. Always warm up and cool down.

The Zone 4 Training Trap: Don’t Overdo It

While the benefits of Zone 4 training are real, more isn’t always better. Many Ironman athletes fall into the trap of doing too much hard work, leading to:

• Overtraining and injury risk

• Poor recovery between key long sessions

• Neglect of aerobic base development

Follow the widely recommended 80/20 training principle:

• 80% of your training should be low intensity (Zone 1–2)

• 20% should be higher intensity (Zones 3–5, including Zone 4)

This structure ensures you’re building the endurance foundation needed for long-course racing while still getting the benefits of speed and strength development.

Strategic Zone 4 = Ironman Gains

Zone 4 training is a powerful tool in your Ironman arsenal, but it must be used wisely and purposefully.

Incorporate Zone 4 during key phases of your training to:

• Raise your threshold

• Sharpen your mental game

• Improve race-day efficiency

Whether you’re doing threshold intervals on the trainer, hill repeats on the run or sprint sets in the pool, a smart dose of Zone 4 will help you go faster, stronger and longer.

FAQ: Zone 4 Training for Ironman

How often should I include Zone 4 training in a week?
Once or twice per week is usually sufficient, especially during the early to mid phases of training.

Should I do Zone 4 work during race week?
No. During race week, you should prioritise recovery, light movement and short efforts to stay sharp, skip Zone 4 altogether.

What’s the difference between Zone 4 and VO2 max intervals?
Zone 4 targets lactate threshold, while VO2 max intervals (Zone 5) are shorter and more intense, aiming to increase your max oxygen uptake.

Can beginners do Zone 4 training?
Yes, but start conservatively. Focus on short intervals, good pacing and proper recovery to avoid overload.

How long should Zone 4 intervals last?
Most Zone 4 sets range from 3 to 10 minutes per rep.

FURTHER READING: BUILD YOUR IRONMAN BASE

• Ironman Training: What Is Zone 2 / Endurance?

• Ironman Training: What Is Zone 4 / Threshold?

• Ironman Training: What Is Zone 5 / VO2 Max?

• Ironman Brick Training: 10 Threshold Sessions

• Ironman Bike Training: 10 Threshold Sessions

• Ironman Run Training: 10 Threshold Workouts

• Ironman Swim Training: 10 Threshold Workouts

Final Thoughts

Done right, threshold training makes your race pace feel easier, your legs feel stronger and your mindset unbreakable. Use it wisely and you’ll toe the Ironman start line faster, fitter and more prepared than ever. Zone 4 isn’t just about pushing limits, it’s about expanding them. It teaches your body to handle more and stay composed when the race gets hard.

Even just one or two well-placed sessions each week can reshape how your body and brain handle sustained effort. It’s not about doing more. It’s about doing the right work, at the right time, with purpose. Train smart. Train sharp. Zone 4 is where progress gets forged.

You don’t need to live in Zone 4, but visiting regularly can change the game.

Always consult with a medical professional or certified coach before beginning any new training program. The information provided is for educational purposes only and is not a substitute for personalized advice.