Ironman 70.3: What Is Zone 4 / Threshold Training?
SUMMARY:
Zone 4 — around 87–93% of max heart rate, 91–105% of FTP, and 99–104% of CSS swim speed (RPE 7–8) — is the threshold zone that sharpens race efficiency for Ironman 70.3 athletes. It feels hard, focused, and right at the edge of sustainable. Training in this zone improves lactate clearance, muscular resilience, and pacing control — helping you hold strong efforts in the final stages of swim, bike, or run without fading.
What Is Zone 4 Training?
Speed alone won’t carry you through an Ironman 70.3 — but if you can’t hold your goal pace, endurance won’t save you either. That’s where Zone 4 training comes in. Often called the threshold zone, Zone 4 is the intensity where performance changes happen fast — when used right. It boosts your lactate threshold, increases power, and prepares your body to hold strong efforts for longer, deep into the race. Zone 4 is the intensity zone just below your maximum sustainable effort. It’s often referred to as the lactate threshold zone—where your body produces lactate faster than it can clear it. Training here improves your ability to manage and delay fatigue at race pace.
Zone 4 Training Metrics:
Heart Rate: 87–93% of Max HR
Power (Bike): 91–105% of Functional Threshold Power (FTP)
Effort Level: Hard but controlled—tough to sustain, but possible (RPE 7–8)
CSS swim speed: 99–104%
Use FLJUGA’s free calculators to find your pace, power, and heart rate zones.
Unlike Zones 2 and 3, Zone 4 is not sustainable for hours—but it’s essential for sharpening speed and durability in the final 6–12 weeks before your Half Ironman.
Why Zone 4 Training Matters for Ironman 70.3
1. Raises Your Lactate Threshold
Training in Zone 4 helps you run, ride, and swim faster without tiring as quickly. It builds the strength and control needed to hold a harder pace for longer — perfect for racing stronger and finishing with confidence.
2. Builds Strength & Power
Zone 4 efforts improve muscular endurance and help develop the strength needed to power through headwinds, hills, and the back half of long races.
3. Trains Mental Toughness
Threshold work forces you to stay focused when things get uncomfortable. That mental grit translates directly to race-day resilience.
4. Improves Efficiency at Speed
Repeated Zone 4 sessions refine neuromuscular coordination and movement economy, helping you move faster with less effort.
When and How to Use Zone 4 Training for 70.3 Racing
Zone 4 training is a high-impact tool—but it needs to be handled with precision.
Here’s how to use it effectively:
1. Incorporate 1–2 Sessions Per Week
Threshold work is demanding. Limit it to once or twice per week to avoid burnout.
2. Focus on Controlled Intervals (3–10 Minutes)
Zone 4 isn’t about all-out sprints. Build sessions around longer, steady intervals that train you to hold race-like intensities.
3. Use It in the Build Phase (6–12 Weeks Out)
Zone 4 training is most effective during your peak prep period, when you’re converting base endurance into race-ready strength and speed.
4. Pair It with Endurance Volume
Layering Zone 4 efforts into longer workouts helps mimic race fatigue and improves your ability to hold pace when tired.
5. Prioritize Recovery
Always follow hard threshold days with Zone 1–2 recovery work or rest. This ensures adaptation, not overload.
Sample Zone 4 Workouts for Ironman 70.3 Prep
Swim: Threshold Intervals
Warm-Up: 400 easy, 4×50 build
Main Set: 5×200m at Zone 4 pace, 30 sec rest
Cool-Down: 200 pull, 100 easy swim
Total: ~2,200m
Bike: Threshold Power Blocks
Warm-Up: 15–20 min easy spin
Main Set: 3×10 min @ 90–95% FTP, 5 min recovery between
Cool-Down: 15 min easy
Total: 90–120 min
Run: Zone 4 Tempo Intervals
Warm-Up: 10–15 min easy jog
Main Set: 3×8 min Zone 4 pace, 3 min jog recovery
Cool-Down: 10 min jog + strides
Total: ~60–75 min
How Zone 4 Fits Into Your 70.3 Race Prep
Zone 4 isn’t a standalone solution—it’s a sharpening tool that works best when:
Used sparingly during the build and peak phases
Combined with Zone 2 aerobic endurance work
Supported by adequate recovery to avoid overtraining
By targeting this zone with precision, you’ll develop the strength and speed to hold your goal race pace when it matters most.
Mini FAQ:
Q: When should I start Zone 4 training for my 70.3?
A: Ideally 6–12 weeks before race day during your build phase. That’s when you’re converting endurance into race-specific fitness.
Q: How many times a week should I do Zone 4 sessions?
A: Once or twice max per week. More than that can lead to fatigue without added benefit.
Q: Is Zone 4 training the same as threshold training?
A: Yes. Zone 4 and threshold training both refer to training near your lactate threshold—the intensity just below redline.
Final Thoughts:
Zone 4 training isn’t about going all-out. It’s about going strong and smart—teaching your body and mind to thrive just below your redline. Don’t overdo it. Don’t skip it either. Dial it in. Time it right. Then watch your 70.3 pace feel smoother than ever.
Ready to hold your pace when it counts?
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 personalised advice.