The Art and Science of Zone 2 Training
After 19 years of running marathons and ultras, I thought I knew my zone 2 ranges. I decided to get a lactate test in a lab, figuring it would mostly confirm what I already knew. I was way off.
The Confusion Around Zone 2
Zone 2 training is one of the most prevalent topics in endurance athletics. It is also one of the most confusing and filled with misinformation. You will hear Zone 2 described as easy, all-day pace, nose-breathing, and conversational pace. Then you have elite athletes like Tadej Pogacar, whose Zone 2 would have most people in a puddle on the floor.
You can find countless definitions of your Zone 2 heart rates:
60-70% of max heart rate
180 minus your age
Max HR – resting HR, then take 70% of that number and add resting HR
These definitions are all wrong.
So What is Zone 2?
The top end of Zone 2 is your aerobic threshold and is defined by the first lactate threshold. There are two lactate thresholds, measured by the concentration of lactate in your blood.
The first lactate threshold is 2mMol / L. Great, so what does that mean? For trained athletes, if you are below 2mMol / L, you can clear lactate as fast as your body produces it as a byproduct of the cellular metabolism that drives your output. If you stay below aerobic threshold, lactate will not be your limiting factor.
When you exceed 2mMol / L, your body cannot clear lactate as fast as it is produced, and it will steadily increase during activity at this intensity. I like to describe this as the “ticking time bomb.”
At the second lactate threshold, 4mMol / L, lactate will build faster. The fuse on the ticking time bomb is shorter, a maximum of one hour for very well-trained athletes.
While the second lactate threshold, often referred to only as lactate threshold, is helpful to know, it is the first lactate or aerobic threshold that is most important for endurance athletes to understand.
How Do I Find My Aerobic Threshold?
The gold standard is a blood lactate test in a lab. This is a progressive test where you steadily dial up effort with lactate tests taken every 3-5 minutes. The results produce a curve that determines both your first and second lactate threshold.
While less accurate, you can also approximate aerobic threshold with a do-it-yourself test based on heart rate drift, your grade-adjusted-pace relative to your heart rate. After warming up, run for one hour at what feels steady and sustainable, but just barely so. Aim to hold the same heart rate throughout once you settle in.
If your heart rate drift is 3-5%, you are likely around your aerobic threshold. If above 5%, you were likely above aerobic threshold. If below 3%, you can retest 5bpm higher and see if that hits the range.
The easiest way to get your heart rate drift is with Training Peaks, which automatically calculates the metric “Pa:Hr”.
Zone 1 vs. Zone 2
There is a massive distribution in pace, effort, and heart rate across aerobic zones.
The border between Zone 1 and Zone 2 does not have a precise, metabolic definition. I roughly estimate it as 88-90% of your aerobic threshold heart rate.
If your aerobic threshold is 150, you could loosely say that everything 132 and below is Z1 and 133-150 in within the range of Z2.
How to Optimize Zone 2 Training
For athletes who are new to training, the primary goal with Zone 2 training is to avoid going above aerobic threshold. You need to build your aerobic base and aerobic fitness, and this won’t happen if you are exceeding aerobic threshold during workouts. Despite training hard, you will struggle to improve, as you will be aerobically deficient.
But for experienced athletes, Zone 2 is a different story. This is where the popular narrative is wrong. Zone 2 is not easy. The bulk of your easy training should be well below your aerobic threshold. The fitter you get, the more this is the case.
What is the purpose of training in the upper end of Zone 2, or aerobic threshold (AeT)? There are significant benefits from AeT training:
Improved cardiovascular efficiency
Enhanced lactate clearing
Increased endurance at AeT
Increased speed at AeT
Improved running efficiency and economy
For athletes racing marathons or ultras, AeT will be the most specific system to train and will have outsized impact on race performances.
We want to accumulate volume at AeT, but this needs to be done in a careful controlled way. AeT training generates fatigue and requires sufficient recovery. There is also high biomechanical stress, which increases as athletes get fitter.
Many have heard of the 80/20 rule with running. 80% of your volume should be “easy” or aerobic with 20% at higher zones and intensities. However, the distribution of training within that 80% matters and most of that 80% should be well below aerobic threshold.
example training structure
The distribution of zones will vary based on the athlete’s experience, responses to training, and the phase of training. Here is an example of how I would structure aerobic volume for an experienced athlete in a specific training phase for a long event:
Z1 to Low Z2: 40% of aerobic volume
Warm ups and cool downs for all workouts
Recovery workouts (i.e. the day after intervals or key long runs)
Low to Mid Z2: 40% of aerobic volume
Endurance runs
Significant portions of long runs
Aerobic Threshold: 20% of aerobic volume
Structured efforts during long runs
Structured efforts during key weekly workouts
Keep in mind that the AeT portion could be even lower than this. If an athlete is further out from a goal race and focusing more on threshold or Vo2Max, the aerobic training volume may not have any AeT work.
To simplify all of this, your “easy” running should actually be Z1 to low Z2. Aerobic threshold efforts are hard workouts that should be used intentionally and for specific durations.
my experience with zone 2
I estimated my aerobic threshold was around 145 heart rate and mid to upper 7 minutes miles on flat terrain. My easy / aerobic training days were almost always below 140 HR.
After almost two decades of running marathons and ultras, I did a blood lactate test in a lab, and I was shocked with the results. My aerobic threshold was actually 160 bpm and around 6:30 pace. Through years of consistent training, I had pushed my aerobic threshold close to my second lactate threshold (170bpm and 6:10 pace).
I was regularly doing high intensity training sessions above 160bpm into Zone 3 and Zone 4, but I had almost no volume between 140 and 160bpm, and my aerobic training was Z1 to low Z2 at best.
My reward from getting the lactate test was knowing that at least some of my endurance / aerobic training needed to be harder. As the legendary cyclist Greg LeMond said, “It never gets easier, you just get faster.”
“It never gets easier, you just get faster.” – Greg LeMond
I started to push more of my aerobic training into mid / high z2. However, given my experience, fitness, and training volume, I knew I shouldn’t do that all the time. If I run 75 miles in a given week and 80% is in aerobic zones, I can’t go out and run 60 miles each week at 6:30s. That would be a one-way ticket to a quick injury.
Uphills are a great time to put out more effort given there is much lower biomechanical impact. I pushed harder on uphill portions of long runs and endurance runs. I will also run doubles in some training blocks, and uphill treadmill sessions are a great time to push higher output with low impact. I made sure to keep recovery efforts, especially the day after long runs or high intensity sessions, in Z1 or low Z2.
I focus now on strategically accumulating volume at high Z2 to low Z3 through aerobic threshold and steady state efforts. This is similar to the “Norwegian” style training that is getting more attention.
I believe this has been the single biggest driver of my improved performance in recent years, and I credit these sessions with setting the course record at the Devil Dog 100K and landing on the podium at Rocky Raccoon 100.
Through proper Zone 2 training, you can make significant fitness gains and hold a higher output for longer during your races.