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Heart Rate Zones

Zone 2 and restoring your mitochondrial health



This will be the first in a series of articles looking at the most important muscle in your body, your heart. Your heart provides a host of valuable data and insight, from HRV (heart rate variability) to RHR (resting heart rate), about your fitness and health status. These data can be helpful in understanding the benefits that you can gain from training at various heart rate ranges, commonly called heart rate zones.  

But before we get into discussing HRV, RHR, and other cardio related metrics, I want to make sure you have an understanding of the various exercise zones and the nuances to training in each of these zones. 

Heart rate zones are estimated ranges of beats per minute (bpm) that are used to measure the intensity of your workout. There are typically five heart rate zones, each with its own benefits and potential drawbacks.  Let's explore each of these zones in detail.


Zone 1: Warm-Up (50-60% of maximum heart rate)

This is the lowest intensity zone, typically used for brisk walking and for warm-up exercises. The heart rate in this zone is relatively low, and it helps to gradually increase your heart rate, prepare your body for exercise, and reduce the risk of injury. This zone is also an excellent time to focus on your breathing, flexibility, and form, exercising in this zone burns fat as the primary fuel source.


Zone 2: Endurance (60-70% of maximum heart rate)

This zone is typically used for longer, lower intensity workouts like steady-state cardio or long-distance runs. The heart rate in this zone is moderate, and it helps to improve cardiovascular endurance, boost energy levels, and increase lung capacity. Exercising in this zone burns fat as the primary fuel source. 

Zone 3: Tempo (Roughly 70-80% of maximum heart rate)

This moderate zone is a step up in intensity from the endurance zone, and it's ideal for workouts that require more effort, such as interval training or hill repeats. The heart rate in this zone is high, and it helps to build cardiovascular strength. This is the zone where the body begins to fuel your physical activity by burning predominantly glucose as opposed to fat. 

Zone 4: Power/endurance (Roughly 80%-90% max heart rate)

This high intensity zone, typically used for short, high-intensity workouts like short sprint intervals of 10 seconds, or hard effort shorter distance steady state runs . The heart rate in this zone is high, and it helps to build muscular endurance, increase power, and improve overall fitness. Training in this zone can also increase cardiovascular strength and endurance. This zone is fueled almost entirely by burning glucose.

Zone 5: Power (Roughly  90-100% of maximum heart rate)

This is the highest intensity zone, typically used for maximum effort exercises, like all-out sprints for 20 seconds or more. The heart rate in this zone is at its highest, and it helps to build maximum power and strength. Training in this zone can also improve cardiovascular endurance and overall fitness, and help to burn fat more efficiently.


Demystifying Heart Rate Zones


The best way to help you understand these zones is to understand what is going on energetically within the body at varying intensities of exercise. 

Your body has three energy systems that work together to produce energy for physical activity. These three energy systems are:

  1. Phosphagen or phoshocreatine System: This energy system is responsible for providing energy for high-intensity, very short-duration activities, such as weightlifting or sprinting. It involves the breakdown of stored ATP (adenosine triphosphate) to release energy. This system operates quickly, but has limited fuel stores, meaning it can only sustain high-intensity activities for about 10 seconds before the glycolytic system must begin providing a higher % ATP and intensity must come down. at about 60 seconds this energy system will be fully depleted and a recovery period will be needed. 

  2. Glycolytic System: This energy system provides energy for moderate-intensity, moderate-duration activities, such as non all out sprint running or cycling. It involves the breakdown of glucose (from carbohydrates stored in the muscles and liver) to produce energy. This system operates somewhat more slowly than the phosphagen system, but provides a larger energy reserve.

  3. Aerobic System: This energy system provides energy for low-intensity, long-duration activities, such as distance running or cycling. It involves the breakdown of carbohydrates, fats and proteins to produce energy in the presence of oxygen. This system provides the largest energy reserve and can sustain activities for several hours, but takes longer to start producing energy than the other two systems.

Each of the energy systems is used depending on the intensity and duration of the physical activity being performed. For example, if you were to try to run your best 3 miles, when you first start to run, your body primarily relies on the phosphagen system, but as you run, it switches to the glycolytic system, and eventually to the aerobic system as fatigue sets in in each system. 

A mistake I often see on line is people talking as if these three systems work independently of one another via on off switches, but this is not the case. 

All three energy systems are constantly working together to provide energy to the body, but the dominance of one system over the others will vary depending on the intensity and duration of the physical activity.

Now with some basic physiology out of the way, we can begin to understand how we can use these HR zones to target the varying energy systems to optimize our health and fitness.

Mitochondrial Dysfunction and Chronic Disease


Researchers over the last decade have increasingly taken the view that mitochondrial health is paramount to optimal health and aging. So much so that myriad pharmaceuticals that are being researched to combat aging target the mitochondria. 

As humans, in the absence of genetic pathology, we are born with robust mitochondrial health. In our natural environment, high in daily physical activity and maintaining a healthy weight, we preserve robust mitochondrial health across the lifespan. 

Unfortunately, thanks to the modern environment of sedentary lifestyles and high calorie, processed food laden diets, our mitochondria are not only unhealthy, they are largely dysfunctional.

What exactly does that mean? Mitochondrial dysfunction is a common underlying factor in most chronic diseases. Remember earlier when I said that the mitochondria are responsible for generating the majority of a cell's energy supply? Well, that's important because any disruption in their function can have far-reaching consequences. 

Mitochondrial dysfunction can lead to decreased energy production, altered metabolism, increased oxidative stress, and inflammation, and impaired glucose disposal, all of which can contribute to the development of chronic diseases such as cardiovascular disease, type 2 diabetes, neurodegenerative disorders, and cancer. 

In addition to energy metabolism, mitochondria play a critical role in maintaining cellular health and initiating cell death, or apoptosis, when necessary. When mitochondria aren't functioning properly, this balance is disrupted, leading to cellular stress and damage that can contribute to chronic disease. This is exactly why, as I mentioned earlier, targeting mitochondrial dysfunction has become an area of focus for many researchers in the quest for new treatments for chronic diseases as well as life extension.

In order to regain our mitochondrial function and health, look to the GH, Hunter Gatherer tribes. 

Modern HG are the closest that we can get to observing our ancestors. They live in a very similar environment to that which we evolved in for more than 500k years, and, most importantly, before the Neolithic, or agricultural revolution. 

Prior to the agricultural revolution, all humans, just like the modern GH, spent most of their day working to acquire food. This would have been in the form of hunting prey, or digging up tubers or gathering berries. The point is, our species was always, and still is, highly physically active outside of the modern environment. 

In fact, most modern HG tribes cover an average of 10 to 15 miles by foot every day.  Most of this distance is done by brisk walking. Next would be moving more quickly when tracking/chasing an animal to exhaustion, and the smallest portion of this distance would be covered by brief periods of hard running or sprinting.  Sprinkled in to their daily activity would be climbing, carrying loads, such as animal carcasses, or other activities that we would consider resistance exercise. This evolutionarily consistent pattern of physical activity gives us a rough template which we can use to model our own training after, if our goal is optimal health. 

Remember earlier when we talked about energy systems and heart rate zones? The vast majority of HG physical activity is low intensity zone 1 and 2 work, Remember that this type of physical activity is fueled by the mitochondria metabolizing fatty acids.

This is the reason that you may have seen an emphasis being placed on zone 2 exercise more frequently on social media the past few years. 

This is because zone 2 is the zone in which you generate the highest energy output before going Into zone 3, where glycolysis, or burning glucose as the primary source of fuel takes over. 

Zone 2 is the zone where the mitochondria are being taxed the most. Think of it as exercising your mitochondria.

This is exactly why Z2 training is thought to have a significant impact on mitochondrial health. Mitochondrial health is improved through a process called mitochondrial biogenesis, the increase in the number and size of mitochondria, as well as by enhancing their ability to produce energy, and reduce oxidative stress. This can lead to a range of health benefits, including improved cardiovascular health, increased endurance, and reduced risk of chronic diseases such as heart disease, stroke, and diabetes. (and though speculative, a younger biological age)

In contrast, when you engage in higher-intensity exercise (such as zones 3, 4 or 5), your body relies increasingly more on glycolytic metabolism to produce energy, which generates ATP more quickly but also produces a buildup of lactate in the muscles. Over time, this can lead to fatigue and damage to the muscles and other tissues. As I often say, as intensity goes up, frequency should come down. 

What does this mean to you?


Like our ancestors before us, and the HG of today, we should attempt to engage in as much low intensity, zone 1 and 2 physical activity as we can. 

We should then add in short bouts of zone 3-4 work, which would include high effort sets of strength training. 

Testing zone 5, or Max HR zone is something that I feel people over 40, who do not have at least a year of consistent physical activity under their belts, should not do without first getting clearance from a physician. 

This is because the stress placed upon the cardiovascular system is highest in z5, and I favor stressing, and therefore strengthening, the cardiovascular system slowly over time to allow plenty of time for adaptations to take place. And please remember, zone 5 is for short duration or interval work.

What about walking?


Walking at a brisk pace, on flat ground, is zone 1 for a metabolically healthy person, however, for someone with insulin resistance who is trying to reclaim their metabolic health, walking is very likely zone 2. 

How do you know what zone you are in?


In order to get a truly accurate idea of what zone you are in you would need to test your max heart rate, which may not be perfectly safe for someone who is new to exercise to do outside of a clinic.Smart devices such as an Apple Watch, Oura Ring, or Garmin can give an estimate of your heart rate zones, which you can use as one point of data, but since they aren't perfectly accurate, you should also pay attention to your perceived effort. Which should get you close enough. Remember, the zones are estimates anyway, and your energy systems are al working at the same time. As intensity/HR rises and falls, so does the amount of energy being provided by each of the 3 systems. So, it's not as if as soon as you cross a certain heart rate you go from z2 to z3 and all of a sudden you aren't training the mitochondria anymore. But, the further you get away from the lower, fat burning zones, the less and less you are training the mitochondria.

And, the worse your metabolic/mitochondrial health is, the less you can oxidize fat efficiently In the first place, So, the less fit you are, the more I suggest erring on the side of going slower if your goal is to train the mitochondria, 

  • Zone 1 should feel easy. You should feel like you can maintain z1 activity indefinitely. Think a brisk, but not maximal speed, walk, you should be able to have a full conversation with another person with no additional difficulty 

  • Zone 2 should feel less easy than zone 1, but it should not feel difficult at all. You should still be taking slow, smooth breaths with Z2. \You should be able to breathe through the nose only and you should be able to maintain a conversation during zone 2, as with zone 1, the difference here being that the exercise would make the person you are having the conversation with aware that you are exercising. 

                With zone 2, if you think "I wonder If I am going too fast?" You are probably going  too fast, Its not hard exercise, it is a nice, easily sustained pace.

  • Zone 3 will generally require some mouth breathing for most people. You would have to take breaths in the middle of sentences. Zone 3 should not feel like you can sustain it for hours on end. 

  • Zone 4 would be something like a 5k "race pace" or longer, 2-4 minute interval intensities

  • Zone 5 would be an effort you can sustain for less than 1 miunute max. 

Exercise across all of these zones will greatly improve your overall health and fitness. The fitter you become, the safer, and more beneficial, training your top end/zone 5 will be.But it doesn't take much! One HIIT session a week is plenty and two is the max I would suggest. 

And perhaps most importantly, the lions share of the reductions to mortality that can be achieved through exercise are seen in just going from sedentary to meeting the standard guidelines of 150 minutes per week of moderate exercise. This is as simple as a 50-60% reduction in mortality for an amount of physical activity that would be fully satisfied by 5 20 minute brisk walks and 2 20 minute strength sessions per week.

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