How to Find Your Aerobic Threshold (AeT) for Ultramarathon Training
Understanding your aerobic and anaerobic thresholds is the foundation of effective endurance training. Here's how to test yourself and what the results mean.
Why Thresholds Matter
Most ultramarathon training plans prescribe workouts by “zone”—but those zones are meaningless if they’re based on generic formulas rather than your actual physiology.
The Training for the Uphill Athlete methodology anchors your entire training system to two metabolic markers:
- Aerobic Threshold (AeT): The top of Zone 2—the highest intensity you can sustain using primarily aerobic metabolism
- Anaerobic Threshold (AnT): The top of Zone 3—often called lactate threshold, the point where lactate accumulates faster than you can clear it
Knowing these two points allows you to train at the right intensities for the adaptations you’re seeking.
The Zone System
| Zone | Name | Intensity | What’s Happening |
|---|---|---|---|
| 1 | Recovery | Below AeT | Very easy, conversational, active recovery |
| 2 | Aerobic Base | At/just below AeT | Primary endurance building zone |
| 3 | Tempo | Between AeT and AnT | ”Comfortably hard,” muscular endurance |
| 4 | Threshold/VO2 | At/above AnT | Hard, sustainable for 20-60 min max |
| 5 | Anaerobic | Well above AnT | Very hard, short intervals only |
For ultramarathon training, you’ll spend the vast majority of time in Zones 1-2, with strategic doses of Zone 3-4 as fitness develops.
Finding Your Aerobic Threshold (AeT)
The Heart Rate Drift Test is a simple, free, self-administered test to identify your aerobic threshold.
Heart Rate Drift Test Protocol
Equipment needed:
- Heart rate monitor (chest strap strongly recommended—wrist-based monitors are less accurate)
- GPS watch or app that records heart rate over time
- Flat to gently rolling terrain (treadmill works well)
Protocol:
-
Warm up for 15 minutes at an easy pace
-
Run for 60 minutes at a steady pace, keeping your heart rate as constant as possible
- Choose a heart rate you suspect is near your AeT (start conservatively)
- Maintain consistent effort throughout
- Flat or treadmill is ideal to control variables
-
Compare heart rates from the first and second half:
- Calculate average heart rate for minutes 0-30
- Calculate average heart rate for minutes 31-60
- Calculate the “drift”: (Second Half HR - First Half HR) / First Half HR × 100
Interpreting Results:
| Heart Rate Drift | What It Means |
|---|---|
| < 5% | You’re at or below your AeT—this is your Zone 2 ceiling |
| 5-10% | Slightly above AeT, test again at lower HR |
| > 10% | Significantly above AeT, test again at lower HR |
Example
- First 30 min average HR: 142 bpm
- Second 30 min average HR: 149 bpm
- Drift: (149 - 142) / 142 × 100 = 4.9% ✓
This heart rate (142) is at or just below the aerobic threshold.
Tips for Accurate Testing
- Be well-rested—don’t test after hard training days
- Stay hydrated—dehydration elevates heart rate
- Control temperature—heat increases drift artificially
- Same conditions each test—test on similar terrain in similar weather for comparison
- Chest strap > wrist monitor—wrist-based HR is too variable for threshold testing
Finding Your Anaerobic Threshold (AnT)
The anaerobic threshold (also called lactate threshold) can be estimated with a 30-Minute Time Trial.
30-Minute Time Trial Protocol
- Warm up for 15-20 minutes, including a few short pickups
- Run as hard as you can sustain for 30 minutes
- This should be a true max effort for 30 min—not comfortable
- Flat course or treadmill preferred
- Record your average heart rate for the final 20 minutes
- Discard the first 10 minutes (HR is still rising to steady state)
That average heart rate from the final 20 minutes is approximately your AnT.
Alternative: Lab Testing
If you have access to a sports performance lab, a blood lactate test will give you precise AeT and AnT values. This involves running at progressively harder efforts while blood samples measure lactate concentration.
The AeT-AnT Gap: Aerobic Deficiency Syndrome
Once you know both thresholds, calculate the gap between them:
Gap = (AnT - AeT) / AeT × 100
| Gap | Aerobic Status | Training Implication |
|---|---|---|
| < 10% | Well-developed aerobic base | Can benefit from higher-intensity work |
| 10-20% | Moderate aerobic deficiency | Prioritize Zone 1-2 training |
| > 20% | Significant aerobic deficiency (ADS) | Focus almost exclusively on Zone 1-2 |
Example
- AeT: 138 bpm
- AnT: 168 bpm
- Gap: (168 - 138) / 138 × 100 = 21.7%
This athlete has Aerobic Deficiency Syndrome and should spend the vast majority of training time below 138 bpm until the gap closes.
What is Aerobic Deficiency Syndrome (ADS)?
ADS occurs when your aerobic system is underdeveloped relative to your anaerobic capacity. Common causes:
- Too much high-intensity training
- Not enough easy volume
- Inconsistent training history
- Coming from anaerobic sports (CrossFit, HIIT, team sports)
Symptoms of ADS:
- Feeling like you have to go painfully slow to keep heart rate down
- Heart rate spikes quickly on any incline
- Difficulty holding conversation at “easy” pace
- Large gap (>10%) between AeT and AnT
The fix: Patient, disciplined Zone 1-2 training. It’s frustrating—you’ll feel like you’re going too slow—but it works. Over months, your AeT will rise, the gap will close, and your easy pace will get faster.
How Often to Retest
| Test | Frequency |
|---|---|
| AeT (drift test) | Every 6-8 weeks during base building |
| AnT (time trial) | Every 8-12 weeks, or after intensity blocks |
As fitness improves, your AeT should rise (you can run faster while staying aerobic). The gap between AeT and AnT should shrink.
Applying Zones to Training
Once you know your thresholds, here’s how training intensity should distribute for ultramarathon preparation:
Base/Capacity Phase
- 85-90% Zone 1-2 (below AeT)
- 10-15% Zone 3 (only if AeT-AnT gap is <10%)
- 0-5% Zone 4-5
Build/Peak Phase
- 75-80% Zone 1-2
- 15-20% Zone 3
- 5% Zone 4
Why So Much Easy Running?
For events lasting 10+ hours (like a 100-miler), nearly the entire race is run below lactate threshold. Your Zone 2 IS your race pace. Building a massive aerobic base:
- Increases mitochondrial density
- Improves fat oxidation (critical for ultras)
- Builds capillary networks in muscles
- Develops cardiac efficiency
- Reduces injury risk (lower impact than fast running)
High-intensity training has its place, but it cannot substitute for aerobic volume in ultra preparation.
Common Mistakes
1. Using formula-based zones
- “220 minus age” or percentage-of-max formulas don’t account for individual variation
- Your AeT could be 20+ beats different from what formulas predict
2. Running “easy” runs too fast
- If you can’t hold a conversation, you’re probably above AeT
- Slow down—it’s harder than it sounds
3. Adding intensity too soon
- Fix aerobic deficiency before adding Zone 3-4 work
- Intensity on a weak aerobic base yields limited returns
4. Testing in poor conditions
- Heat, dehydration, fatigue, and altitude all affect heart rate
- Control variables for accurate testing
5. Ignoring the data
- If the test says your AeT is 135, train at 135—even if it feels embarrassingly slow
- Trust the process
Related Pages
- Training for the Uphill Athlete: Core Principles
- Crazy Mountain 100 Training Plan
- Sleep Optimization for Ultramarathon Training
Resources
- Training for the Uphill Athlete by Steve House, Scott Johnston, and Kilian Jornet
- Uphill Athlete Heart Rate Drift Test Guide
- Uphill Athlete AeT/AnT Self-Assessment