Whether you are optimizing your exercise regimen for performance, or to live longer, Zone 2 heart rate training deserves your attention. This article will concentrate on the importance of Zone 2 training, the science behind it,  and the potential perils and pitfalls that await you due to overtraining if it’s ignored.  
“Train slow to run fast”.  The training regimens of some of our greatest athletes can teach us a lot. The science behind Zone 2 training illuminates why this is true.  Zone 2 or low HR training is also one of the best tools we have to achieve metabolic health and longevity.  Low, zone 2 heart rate cycling, swimming, rowing, and running are also key to improving performance and minimizing the risk of developing stress-related injuries.
Not only will Zone 2 heart rate training boost your performance, it just might save your life.  After all, your heart is just a muscle.  
Wendy was a stellar runner and cyclist.  On the trails every Sunday she always merged with the group out front.  I might have been able to catch up to them, but I wouldn’t have been able to speak.  On the road, she powered up hills like they were non-existent.  She pushed 300Watts on her bike like I pushed 200.  She was also a competitive triathlete.  Then came February…  
Running and Overtraining
Peloton! Your new bike arrives.  It looks sleek. You put your water bottles in the holders and jump on.  You choose a class and pound out a 30 min heart-thumping ride.  You repeat this a few times a week.  Certainly, this is good for your health, right? Yes, there are benefits to HIIT-style training, but…  The importance of base training or zone 2 heart rate (HR) training has fallen by the wayside for far too many of us.  Our society loves quick, hard challenges. We gamify everything… you watch the leaderboard and want to move up further.  Zone 2 HR training is hard… but the payback is worth it. 
Keep reading… we are going to explore a very exciting way to identify your zone 2 heart rate during your ride or run!
Did you purchase a bike or start running to optimize your lifestyle for longevity? In this post, we are going to dive deep into why Zone 2 HR training is a powerful tool to promote longevity and to keep you on the road for years to come.  We are also going to talk about the downsides of training too hard, or too long.  Your heart really doesn’t like either. 
To understand why Zone 2 training and its impact on our metabolic health, we need to learn about mitochondria!
Metabolic health and zone 2 heart rate training on a bike

Read: How does your metabolic health affect your body and joints? 


Mitochondria… the key to a healthier life. 

Mitochondria.  We are taught that they are the powerhouse of our cells. Mitochondria are so much more, but we’ll get to that later.  We assume mitochondria are like an engine, give them fuel, and they just work.  Well, they are, sort of.  However, like a fine race car engine, they require your attention to work in peak condition.   The healthier your mitochondria are, the healthier you will be.  Poorly functioning mitochondria, or “mitochondrial dysfunction” is seen in people with heart disease, dementia, Type 2 diabetes, Metabolic Syndrome, cancer, and so on.  In pandemic terms… the mitochondria in your immune system will dictate how well your immune system will perform.  
Mitochondrial dysfunction is also a root cause element of insulin resistance.  Since nearly 50% of people have insulin resistance, this is a subject matter we need to pay attention to.  
Scientists are now finding evidence of mitochondrial dysfunction nearly 5-10 years before those changes will manifest as something like Type 2 Diabetes that your doctor can find on your blood tests.  When it comes to mitchondria, the issues we care about are: 
  1. the number of mitochondria you have.
  2. the metabolic flexibility of your mitochondria- in many diseases, the mitochondria can only process glucose, and not fat.  This inflexibility leads to significant downstream effects. 
  3. Mitochondrial efficiency.  How well do your mitochondria process the various substrates– glucose, fat, and lactate.  

Wendy hit the trails on a bright chilly Sunday in February.  But this day was different.  She stayed near the back of the pack.  Hills made her far more short of breath than usual.  Perhaps it was just one of those days.  


Zone 2 Heart Rate Training…

Sometimes I rail against Peloton and other platforms that push us harder and harder each day. I want to stress… I am not against HIIT training.  Let’s call it a “concern” about how frequently you do it. By working out too hard, too often— overtraining is a very real possibility, and a difficult problem to contend with once it’s present.  Along with overtraining comes an increased risk of injury. Recovery from injury takes a toll: emotionally, physically, and physiologically. I discussed how profoundly a long recovery can affect us here… it is something to be avoided, if possible. 


Goals, Longevity, and Zone 2 Training. 

Many of us want to improve our athletic fitness, endurance, and performance. Face it, we also want to live longer, and with fewer health problems. That’s where Zone 2 training comes into play. Zone 2 training involves training at a lower intensity for a longer period of time.  We’ll get into how to determine when you are in Zone 2 later on.  For now, we are going to explore the benefits of proper training and the downsides of ignoring Z2 training.  
Zone 2 training is also referred to as base training. All elite athletes spend months base training. At least 75-80% of their active training is also in Zone 2. Let’s not assume we know more than those who train these world-class athletes.  
Zone 2 heart rate training and longevity


Benefits of Zone 2 Heart Rate Training. 

  • Increase in the number of mitochondria
  • increase in mitochondrial efficiency
  • increase in “metabolic flexibility” 
  • lower resting heart rate
  • a decrease in blood pressure
  • lower risk of injury 
  • improves insulin resistance


During Zone 2 training you will increase your number of mitochondria, mitochondrial efficiency,  and increase your metabolic flexibility.   Metabolic flexibility refers to the ability of your mitochondria to utilize fat and glucose as an energy source (substrate). At low heart rates, your main source of fuel should be fat… not glucose.  Poorly functioning mitochondria, which is likely to be found in ~ 75% of people will result in metabolic inflexibility.. or the inability to utilize fat versus glucose.   
People who are poorly trained, sedentary, or those with insulin resistance, Type 2 diabetes, and Metabolic Syndrome (hypertension, abdominal fat, insulin resistance [high A1c]) are very poor at using fat and often go straight to glucose as a fuel source very quickly.  This is the picture of metabolic inflexibility. This produces excess lactate, and due to poor mitochondrial function their clearance of the lactate is poor, and the lactate builds rapidly.  Accompanying lactate is a hydrogen ion.  That hydrogen changes the pH of its local environment, in this case, skeletal muscle, and that produces weakness and exhaustion. 
lactate induced muscle fatigue
Zone 2 heart rate training enables you to use fat as an energy source for longer, and more efficiently. Thus you preserve your glycogen stores for longer. Glycogen gets broken down into glucose which serves as your fuel source for higher intensity efforts. One end product of using glucose is lactate. Now lactate isn’t as bad as you think it is. In trained individuals, they can shuttle the lactate back into the cell and use it as fuel! That requires an active transporter… MCT-1.  The more Zone 2 exercise you perform, the more of that transport protein you will make. That improves your ability to clear the lactate.  The mitochondria will take the lactate in and use it as fuel.  
Perhaps even more important.  Mitochondrial health is critical to longevity. Many diseases that affect our lifespan are considered to be due to metabolic dysfunction.  Mitochondrial dysfunction is relevant to cancer growth, immune system function, dementia, heart disease, type 2 diabetes, and much more.

Zone 2 heart rate training optimizes your muscles’ mitochondrial function.  

When functioning well, our mitochondria use fat, glucose, and lactate as fuels. In Zone 2 you are using fat oxidation as your primary source of fuel for energy production. In our muscles, we have Type 1 or slow-twitch muscles and Type 2 or fast-twitch muscles. Type 1 fibers have plentiful mitochondria and prefer fat as their source of energy. Type 2 fibers are glycolytic… meaning that they burn glucose. Burning glucose produces lactate. Lactate can be used as fuel if you are well trained.  In others, lactate, and an accompanying hydrogen ion build-up. It’s the hydrogen that’s the problem… lactate is actually fuel.
The hydrogen ion will change the microenvironment around the muscle cell and make it far less powerful.  You can lose more than 50% of your muscle power as hydrogen builds up. That is the hallmark of fatigue. Now, fatigue is more complicated, because there is central, or brain-related fatigue and muscle fatigue, but that’s s topic for another article.
In Zone 2 training we want to be firing all our Type 1 fibers and not fire or use our type 2 fibers. We do not want lactate to build up. There is always some glucose being burned… so lactate will rise a bit in zone 2, but it should rise to a point and stay there.  This equates to you feeling like you can maintain your efforts for a long time.  Once you recruit your Type 2 muscle fibers, you will start to fatigue.  The rate at which you fatigue is variable.  That rate will depend on how well trained you are, and how well your mitochondria clear the lactate.  

How do you know when you are in Zone 2?

Until recently athletes needed to go to a performance lab, or stick themselves to measure lactic acid to try and determine where they transitioned from aerobic (fat oxidation) to Zone 3 and above.  That transition point goes by many names.  We’ll call it the first lactic acid threshold or LT1.  Anything below your LT1 is considered aerobic. You are mainly using fat oxidation for energy production in this zone. 

DFA-Alpha1: HRV Logger App. 

The science behind using DFA-Alpha1 is very complex… and way over my head.  This technology is available to everyone with a heart strap (preferably Polar) and an iPhone.  Your HRV or heart rate variability is considered to be a barometer of your parasympathetic nervous system activity.  Your HRV or heart rate variability changes during exercise.  By detecting these changes, you can calculate the DFA-Alpha1. A DFA-Alpha1 value of 0.75 or above correlates with zone 1 or zone 2 heart rates.  A DFA-Alpha1 of 0.75 is your first lactic acid threshold or LT1.  



In the example above you can see a long Zone 2 ride with a heart rate between 105-137.  The picture on the right shows my DFA-Alpha1 as it was being displayed live on my phone.  Your HRV changes daily based on your sleep, as well as internal or external stressors.  So your DFA-Alpha1 level of 0.75 will vary.  One day it might be a heart rate of 135 and the next day it might be 141.  Therein lies the value of this app.  It enables you to stay in an aerobic zone and grab all the benefits and minimizes the chances of going into Zone 3 or no mans land. Many metabolic and other changes occur when you enter zone 3, so try out this app and stay above 0.75 for your long aerobic runs and rides.   The developer of the app, Marco Altini, posted this article about his app on Medium which goes into further detail.  


Heart rate: When you are in Zone 2 your heart rate is somewhere between 60-75% of your maximum heart rate. Using your heart rate (HR) is probably your best way to identify what zone you are in. for an accurate assessment, we need to know your maximum HR and your resting HR.  I’ve hit my max HR on some trail races- a most unpleasant experience!  Your resting HR is your HR in bed as soon as you wake up and before you get up.

The old method of calculating your maximal heart rate using 220- your age is not accurate for most people and should not be relied upon.  For those of you who know your maximum HR and your resting HR, Training Peaks and many other calculators are out there.
Mine calculates out to be 126-139. With respect to your perceived effort. Near the top end of zone 2, you may find it harder to hold a conversation without having to pause to breathe.  
Let’s spend a moment to talk about Cardiac drift.  Cardiac drift will occur for most of you near the end of a long run or ride. That means that your HR starts trending up despite holding the same wattage on the bike or minutes/ mile during a run. Dehydration can drive cardiac drift… but more likely in non-elite athletes, cardiac drift is likely due to metabolic stress. You are leaving Zone 2 and using more Type 2 muscle fibers. Those Type 2 fibers burn glucose and produce more lactate. The lactate causes your muscles to be less effective and consequently, you need to work harder to maintain the same effort.
What do you do if you see your heart rate drifting?  As you drift, adjust your effort. Don’t keep up the same effort.  As your training improves your metabolic flexibility, mitochondrial efficiency, as well as your muscles’ ability to clear lactate will improve.  As your training and fitness level improves, you will notice that the time when you start to drift will also improve.

Breathing: If you can speak easily and in full sentences without having to pause at all you are probably in Zone 1. If you can speak or sing, but need to pause occasionally to breathe you are probably in Zone 2. If you cannot speak more than a few words before you need to pause, you are in Zone 3 and above.

Elite athletes are tested extensively to determine precisely when they are in zone 2.  You and I do not have access to those labs. When elite athletes are in a lab they will measure their lactate to see when it starts to transition upwards after an initial plateau.  The scientists may also study their respiratory quotient (RQ).  That is a calculation that is based on expired CO2 to determine what fuel they are burning.  A respiratory quotient of 0.7 and you’re mostly burning fat, an RQ of 1 and you’re burning all glucose.  
Our latest Podcast on Mitochondrial health and longevity. 


How long should I spend in Zone 2? 

How much exercise is needed to improve Zone 2 fitness? Many professional coaches suggest that 90-minute sessions are needed.  If performance is your goal, then preferably twice a week.  That’s hard for many to do… but that appears to be the number. For those who are pressed for time… and want to get in a harder workout too, that’s fine. Try to ride 75-80 minutes in zone 2 then crank it up for some sprints at the end of the ride. That way you are also exercising at your lactic acid threshold, and perhaps your VO2 max.  In terms of cardiac health, short high-intensity sessions are also important.  Many find it easier to perform these higher intensity sessions at the end of a long Zone 2 effort.  
Wendy came out the following Sunday.  But she hadn’t been able to ride or run all week. She felt rundown.   We sat in the parking lot and discussed this before the rest of the group took off.  She wasn’t going to push it that day.  Turns out, she only made it one mile.  The next day she was in my favorite Cardiologist’s office.  


What is Overtraining?

Training hard seems like a logical way to proceed if your goal is to increase performance, and perhaps your lifespan.  Many athletes train really hard.  Uncoached, or poorly coached, some athletes and even causal runners and cyclists often train too hard.  Then you start to notice that your performance is suffering, or you just don’t feel good.  What’s going on?  

You’re overreaching… the first stage of overtraining.  If you’re tired, feeling run down, your times are increasing and you feel sorer than usual then you are in an overreaching phase.  

If you monitor your heart rate, you see that your resting heart rate is increasing.  Your heart rate variability will likely be dropping.  Your body temperature is elevated.  If you have a Peloton you notice that your wattage is decreasing.  Maybe not dramatically, but you’re achieving less work with a higher heart rate.  

“Recovery is a weapon.. respect your body’s need to rest”


Recovery from running is critical


What are the warning signs of overtraining?

Some of the more obvious signs of overtraining include: 

  • fatigue
  • increased soreness
  • you’re less than enthusiastic about training today
  • moodiness: you may be more depressed or angry
  • your motivation lags

Physical manifestations and warning signs of overtraining include: 

  • You don’t feel well.
  • Increased resting pulse or blood pressure
  • weight loss
  • Gastrointestinal issues such as constipation.


Please Avoid Overtraining: 

  1. Going hard every workout will increase your risk of injury.  Stress fractures and various tendinopathies will occur more frequently when overtraining. 
  2. Overtraining: Overtraining can be a problem for weekend warriors as well as professionals. Some of you, maybe even most of you won’t be able to recognize that you are overtraining until it’s too late.  I will discuss how causal runners and cyclists go off the rails later on. 
  3. Overtraining doesn’t necessarily result in sore muscles… it results in higher resting heart rates, higher basal body temperature, lower heart rate variability (HRV), and a gradual decline in your performance. At its worse, it starts to affect your mood, your immune system, and even your desire to exercise. Overtraining has ruined the career of many athletes.  
  4. In the past, 20-40 years ago, a watt was considered a watt… so coaches just pushed people… hard. And then we started to follow certain physiological and performance parameters like those mentioned above and we started to see overtraining and its manifestations: illness, burnout, and injuries.
  5. Over the long term, chronic overtraining and overexertion can lead to permanent heart damage.  

Overtraining and Maladaptation

Many elite training programs use a polarized training mix of 80/20. 80% of training is in zone 2. 20% are harder efforts- therefore, threshold or VO2 max levels. Surely these trainers and sports physiologists know a little more about training than us? 

Training too hard and too often might actually take years off your life 

Wendy’s tests revealed that she had a dilated heart.  Another test also revealed an arrhythmia… an abnormal heart rhythm.  This was one of those arrhythmias that result in a higher risk of death.  So, Wendy, had a defibrillator placed.  That would shock her heart if she needed it.   Wendy hasn’t needed her defibrillator yet… but she also isn’t able to exercise aggressively.  Her heart is too weak.  
Your heart is a muscle.  It can wear out.  Let’s assume you have 3-4 Billion beats in your heart before it tires out.  That’s one reason why folks with lower heart rates live longer.  Drive your car less it and you’ll keep it longer too.  

The Reverse J Curve of  All-Cause Mortality Benefit From Exercise: 

Current studies suggest that 2.5 to 5 hours/week of moderate or vigorous physical activity will confer maximal all-cause mortality benefits. But studies dating back decades have shown that more than 10 hours/week may reduce those health benefits.  This is known as the reverse J curve of exercise.  The most physically active among us in middle age have a predicted life expectancy eight years longer than those who are more sedentary.  But the proper “dose” of exercise is critically important.    
There are large improvements in the risk of all-cause mortality (ACM) for people who manage to walk 6,000 steps per day.  Your ACM risk continues to decrease to a certain point. Say around 15-20 miles per week for runners.  This assumes that you’re not running at your lactic threshold the entire time.  After 10-15 miles/week, you begin to lose some of the ACM benefits of exercise.  For those who train 50 miles/ week or more, you may give back as much as 38% of the risk of dying from ACM. 
Zone 2 heart rate training offers profound benefits for all of us.  Regardless of whether or not you plan to PR your next race, or live longer to play with your grandchildren.  Overtraining should be avoided.  It can have awful short-term and long-term consequences.  


It’s very easy to get lost in the weeds if you spend any amount of time reading about how to exercise online.  Whether you are optimizing for longevity or performance, zone 2 heart rate training — or base training — offers very significant benefits. Many of the world’s top trainers and athletes follow a polarized training schedule which emphasizes that 80% of your training is aerobic and in Zone 2.  Perhaps we should take our queues from the best :-). 

Disclaimer:  this information is for your education and should not be considered medical advice regarding diagnosis or treatment recommendations. Some links on this page may be affiliate links. Read the full disclaimer.

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About the author:

Howard J. Luks, MD

Howard J. Luks, MD

A Board Certified Orthopedic Surgeon in Hawthorne, NY. Dr. Howard Luks specializes in the treatment of the shoulder, knee, elbow, and ankle. He has a very "social" patient centric approach and believes that the more you understand about your issue, the more informed your decisions will be. Ultimately your treatments and his recommendations will be based on proper communications, proper understanding, and shared decision-making principles – all geared to improve your quality of life.