Why You Need to Know Your Aerobic Threshold?
Pinpointing your Aerobic Threshold (AeT) heart rate is the first step in setting up your training intensity zones. Your AeT defines the top of Zone 2—the aerobic base-building zone where you should do the vast majority of your endurance training. Without an accurate AeT, you are guessing at your intensities, and most athletes guess too high.
The gold standard for determining AeT is a laboratory Gas Exchange Test or Metabolic Efficiency Test. But you do not need a lab. The nose breathing test is a simple, accessible method that works well for most athletes—roughly 70 percent of the population—and can be performed on a treadmill or outdoors.
For a complete overview of all AeT testing methods and how they compare, see our assessment guide.
How Does the Nose Breathing Test Work?
The test uses a principle from exercise physiology: the intensity at which you can no longer comfortably breathe through your nose or carry on a conversation in full sentences corresponds closely to your Aerobic Threshold. Exercise physiologists call this the First Ventilatory Threshold (VT1).
Below this intensity, fat metabolism provides the majority of your energy. Breathing is slow and controlled. Above it, glycolytic metabolism begins to dominate, CO2 production rises, and your body forces you to breathe through your mouth to keep up with the demand. The transition is surprisingly narrow—often only 2 to 4 beats per minute separate comfortable nose breathing from unsustainable nose breathing.
The test asks you to find that transition point by gradually increasing intensity and noting the heart rate at which nasal breathing becomes difficult. That heart rate is your AeT.
Before You Start
Equipment: You need a heart rate monitor (chest strap or arm band). Wrist-based optical monitors are not accurate enough for zone-setting purposes.
Fueling: Start well-fueled and hydrated for both the treadmill and outdoor versions of this test. Eat your most recent meal at least two hours before testing to allow for digestion, but do not test in a depleted or fasted state. You are testing the capacity of your aerobic system, and it needs adequate fuel to give you an accurate reading.
Timing: Choose a day when you are reasonably rested—not the day after a hard session or a poor night of sleep. Both AeT and AnT fluctuate with your recovery state, and testing when fatigued will give a lower, less representative result.
The Treadmill Test (Recommended)
The treadmill provides the most controlled environment for this test because you can hold speed and grade precisely constant. This eliminates the variables—terrain, wind, footing—that make outdoor testing less precise.
1. Confirm your heart rate monitor is on and recording.
2. Set the treadmill incline to 10 percent.
3. Warm up for at least 15 minutes at a very slow walking pace. This should feel extremely easy for the first 10 minutes. Continue until you break a sweat. You are now warmed up and your aerobic system is fully engaged. Do not skip this step—your aerobic metabolism is slow to respond to increased energy demands, and starting the test without a thorough warm-up will give inaccurate results.
4. After the warm-up, without stopping, begin to increase speed gradually—0.1 to 0.2 miles per hour every 3 minutes.
5. Throughout the test, breathe only through your nose. Monitor whether you can carry on a conversation in full sentences.
6. When nose breathing becomes noisy and uncomfortable, or you can no longer speak in complete sentences, note your heart rate and slow down by 0.2 mph (roughly 2 to 5 beats per minute).
7. Find the speed just below the uncomfortable nose breathing that you can sustain for 15 minutes while breathing only through your nose. There may be only a 2 to 3 bpm difference between very easy breathing and unsustainable breathing. This is normal—the transition is narrow.
8. Your average heart rate during these 15 minutes is your AeT. This heart rate sets the top of your Zone 2 for the next 6 to 10 weeks of training.
The Outdoor Test
An outdoor test is not as controlled as a treadmill test, but it can still set a good baseline for your AeT. The most important element is keeping a steady heart rate for as long as possible. Use a long, gradual hill in the range of 5 to 10 percent grade if available. A flat area can work, but hills give more control over intensity.
1. Confirm your heart rate monitor is working.
2. Warm up for 15 minutes with walking or slow running at very low intensity. Continue until you break a sweat.
3. After the warm-up, without stopping, begin to increase heart rate gradually—5 beats per minute every 3 minutes—while breathing through the nose only.
4. When nose breathing becomes too difficult, back off just enough to find a sustainable nose breathing pace. Hold this pace for 10 to 15 minutes. Note your heart rate. There is usually only a 2 to 4 bpm difference between easy breathing and unsustainable breathing.
5. Your average heart rate during these 10 to 15 minutes is your AeT. This sets the top of your Zone 2 for the next 6 to 10 weeks.
If you have trouble breathing through your nose due to congestion, allergies, or a deviated septum, use the conversational test instead: the AeT intensity corresponds closely to the point at which you can no longer speak in full, normal sentences without needing to catch your breath.
When Does This Test Not Work?
The nose breathing test works well for athletes with a history of aerobic training. But it has an important limitation that you need to be aware of.
Athletes with severe Aerobic Deficiency may be able to nose-breathe well above their actual Aerobic Threshold. If you have spent years doing primarily high-intensity training without a structured aerobic base, or if you have spent a lot of time at high intensity while nose breathing, your ventilatory threshold may not align well with your metabolic threshold. In these cases, the nose breathing test will overestimate your AeT and you will end up training too hard.
If you suspect this applies to you, or if your test results seem surprisingly high, we strongly recommend using the Heart Rate Drift Test instead. The Heart Rate Drift Test has a 95 percent or better correlation with laboratory Metabolic Efficiency Tests and does not depend on ventilatory sensitivity. It is the test we include as the first workout in every training plan we offer.
For athletes who are well trained aerobically, the nose breathing test is a useful quick check that can be performed frequently to monitor improvement. But the Heart Rate Drift Test remains our primary recommendation for accuracy.
What to Expect Over Time
As your aerobic system becomes more powerful through consistent Zone 1 and Zone 2 training, you will notice that both your AeT heart rate and your pace at that heart rate will increase over time. These improvements come in small increments and accumulate over many months of training.
In the long term, your aerobic capacity can continue to improve for years with proper training. Retest every 6 to 10 weeks to track your progress and adjust your training zones accordingly. You should also retest if you have had a significant break from training, an illness, or a major change in life stress, as any of these can shift your AeT temporarily.
A rising AeT—moving faster at a higher heart rate while still breathing comfortably through your nose—is the clearest, most tangible evidence that your aerobic base training is working.
Which Aerobic Threshold Test Should I Use?
If you have access to a treadmill and want a quick, repeatable check on your AeT, the treadmill nose breathing test is a good option. If you prefer to test outdoors, the outdoor version works well on consistent terrain.
If you want the most accurate field-based test and are willing to invest 60 minutes, use the Heart Rate Drift Test. It correlates at 95 percent or better with lab tests, does not depend on ventilatory sensitivity, and works for all athletes regardless of training background.
If you want the gold standard, seek out a Metabolic Efficiency Test at a qualified lab. See our guide to choosing a testing lab for what to look for.
For a complete comparison of all testing methods with ratings, see our assessment guide.