Ironman 70.3 Training: What Is Zone 5 / VO2 Max?
SUMMARY:
Zone 5 (93–100% of max heart rate, 106–120% of FTP and above 105% of CSS swim speed at RPE 9–10) is the VO2 max zone used sparingly in 70.3 training to build top-end power. It feels extremely hard, with breathing at maximum and muscles working near full capacity. These sessions improve oxygen delivery, sharpen speed under fatigue and make race pace feel more controlled. Small doses lead to big returns.
What Is Zone 5 Training?
Most athletes training for an Ironman 70.3 focus on building aerobic endurance and they should. But if you want to unlock the top layer of performance, you need Zone 5. This is the highest intensity training zone, where your body reaches its maximum oxygen uptake and pushes close to its absolute physical limit.
Zone 5 efforts are short and demanding. They are performed at high heart rates and generate significant lactate. The goal is not to sustain them, but to condition your body to tolerate extreme effort, improve the delivery and use of oxygen and develop high-end power that supports every other zone.
Zone 5 Training Metrics:
Heart Rate: 93–100% of Max HR
Cycling Power: 106–120% of FTP
Effort Level: 9–10 (very hard to maximal)
CSS swim speed: above 105%
Use FLJUGA’s free pace, power and heart rate calculators.
This zone demands a lot—but it gives back even more when used right.
Why Use Zone 5 in Ironman 70.3 Training?
1. Increases VO2 Max
Zone 5 pushes your cardiovascular system to its max, improving your body’s ability to deliver and utilise oxygen. That means you can go faster at lower perceived effort.
2. Boosts Speed and Power Output
High-intensity intervals improve neuromuscular coordination and explosive strength, making your bike and run more efficient and powerful.
3. Enhances Lactate Tolerance
Training at this level teaches your body to tolerate and buffer high lactate loads, which helps delay fatigue during tough segments or surges on race day.
4. Develops Top-End Capacity for Stronger Training in All Zones
Zone 5 work improves your ability to generate peak effort, which elevates your performance across every other training zone. It builds the upper limit of your engine so sustainable efforts feel more manageable.
How to Use Zone 5 Training in Your 70.3 Prep
Zone 5 is powerful, but it’s also demanding. Use it strategically, not excessively.
Here’s how:
1. Limit to 1–2 Sessions Per Week
Too much Zone 5 can lead to burnout. Schedule 1–2 high-intensity sessions per week during key phases.
2. Time It Right in the Season
Base Phase (Early): Focus on aerobic zones. Save Zone 5 for later.
Build Phase (Mid): Introduce 1–2 Zone 5 sessions weekly to sharpen speed.
Peak Phase (4–6 Weeks Out): Reduce volume but keep intensity for race sharpness.
Taper (Final 1–2 Weeks): Avoid Zone 5; shift to race pace and recovery.
3. Combine with Endurance Work
Insert short bursts of Zone 5 within endurance workouts to mimic race fatigue and improve aerobic/anaerobic crossover.
4. Prioritize Recovery
Zone 5 creates deep fatigue. Allow ample recovery (Zone 1–2 or rest) between efforts and sessions to adapt without over-reaching.
Zone 5 Workout Examples for Ironman 70.3
Cycling Workouts
VO2 Max Intervals: 4–6 x 2–3 minutes @ 110–120% FTP, 2–3 min recovery
Hill Repeats: 6–8 x 30–60s uphill @ Zone 5, 2–3 min spin down
Tabata Intervals: 8–10 x 20s all-out / 10s rest (advanced)
Running Workouts
Hill Sprints: 8–12 x 30s hard uphill, walk or jog down
400m Repeats: 6–10 x 400m @ Zone 5, equal time recovery
Fartlek Sets: 5 x 2 min hard / 3 min easy jog
Swim Workouts
Sprint Sets: 10–15 x 50m Zone 5, 30s rest
Threshold Repeats: 6–8 x 100m hard, 1:1 work:rest
Broken 400s: 4 x 100m with 15s rest between reps
When to Avoid Zone 5
Zone 5 training is powerful, but only when your body is prepared for the load.
Hold off on VO2 max work if:
You’re still in the early stages of base-building
You’re recovering from injury or illness
Your aerobic conditioning isn’t yet solid
You’re not sleeping well or under high stress
This is a high-impact training tool. Use it strategically to build fitness, not to chase exhaustion.
Where Zone 5 Fits in a Balanced Week
Think of Zone 5 as a peak, not a pillar. It’s there to sharpen, not carry the week.
Try this simple rhythm:
Use Tuesday or Thursday for Zone 5 efforts run/swim or bike, depending on your block
Follow with a low-intensity day or recovery swim
Keep endurance and bricks for the weekend
The key: place your Zone 5 sessions when you're fresh, focused and ready to go all in.
Mini FAQ: What Is Zone 5
Is Zone 5 training necessary for Ironman 70.3?
It’s not essential, but it’s highly effective when used strategically. It boosts speed, VO2 max and race-day strength.
When should I start adding Zone 5?
Introduce it during the build phase, about 12–6 weeks before race day. Limit to 1–2 sessions per week.
Can Zone 5 training make me faster at race pace?
Yes. By increasing your VO2 max and improving how efficiently you clear lactate, Zone 5 training raises your aerobic ceiling. This makes your race pace feel easier and more sustainable over time.
FURTHER READING: BUILD YOUR IRONMAN 70.3
Ironman 70.3 Training: What Is Zone 3 / Tempo?
Ironman 70.3 Training: What Is Zone 4 / Threshold?
Ironman 70.3 Training: What Is Zone 5 / VO2 Max?
Ironman 70.3 Swim Training: 10 Essential Sessions
Ironman 70.3 Bike Training: 10 Essential Sessions
Ironman 70.3 Run Training: 10 Essential Sessions
Ironman 70.3 Training: Recovery Week
Ironman 70.3: Beginner’s Guide
Final Thoughts: High Effort, High Return
Zone 5 training pushes your physical and mental boundaries to the absolute limit, challenging you in ways that build exceptional speed, power and resilience. This level of intensity develops qualities that truly stay with you, especially when racing becomes tough and highly demanding. By incorporating this intense training thoughtfully, you can significantly sharpen your overall performance. Apply it wisely and strategically to maximise your gains while carefully minimising the risk of injury or burnout, ensuring sustainable progress over time.
Ready to unlock your speed and power?
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.
