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If you follow trends in fitness, you’ll notice that certain ideas go in and out of popularity in cycles. Things that were considered great for your health a few decades ago fell out of favor 10 years ago, and now they’re all the rage again in 2024.  Are eggs good for you or bad for you?  Is running good for your knees or bad for them?  It can be hard to know what’s true and what you should do about it when every few years it seems that the consensus changes.  Aerobic exercise is another perfect example of this swinging pendulum in fitness.  In 1968, Dr. Kenneth Cooper published Aerobics, which popularized low intensity, long distance cardiovascular activities. 

In the 80’s and 90’s, strength training became increasingly popular with less focus on cardio, largely due to the popularity of icons such as Arnold Schwarzenegger and the rising influence of weight training in sports such as Major League Baseball.  If you remember players such as McGuire, Sosa, and Bonds chasing the single-season home run record, you’ll recognize how influential strength training (and perhaps some, ahem, “enhancement”) had become by the mid 90’s.  In the early 2000’s, low intensity cardio fell out of popularity, in favor of the trendier “high-intensity interval training.” In recent years, high-intensity interval training has fallen out of favor, with popular influencers such as Peter Attia bringing more attention to a lower intensity style of cardiovascular training, often referred to as “Zone 2 training.”  

Aerobic fitness and your health

One of the reasons that Zone 2 training has become so popular in recent years is that many successful influencers, coaches and practitioners have used this style of training to help improve their athlete or patient’s VO2max.  If you’re not familiar with the term VO2max, you can think of this as your maximum exercise capacity, defined as your body’s ability to do 3 important tasks:

• Uptake oxygen through the respiratory system
• Deliver oxygenated blood to working muscles via the cardiovascular system
• Utilize oxygen at the local muscular level

VO2max is an important and useful metric.  VO2max is strongly correlated with athletic performance, particularly in endurance-related sports such as cycling, running, and swimming.  Most importantly, an individual’s VO2max appears to be strongly predictive of cardiovascular disease risk:

“Low VO2max has been shown to be comparable with elevated systolic blood pressure, smoking, obesity, and diabetes in importance as a risk factor for mortality, as well as a predictor of coronary artery disease and the progression of atherosclerosis,” (Laukkanen et al 2002).

Clearly, VO2max is a big deal when it comes to performance, longevity, and healthspan.  Before you sign up to have your VO2max tested, it is important to note that your exact VO2max value is less important than identifying and improving the underlying systems which directly contribute to your score.  Your VO2max value is simply a proxy for the health of each of these important systems.  Let’s identify what these systems are and then get into how to improve each.

As mentioned above, VO2max can be defined as the integrated capacity for your respiratory system to uptake oxygen, your cardiovascular system to deliver oxygen to working muscles, and the muscular system’s ability to utilize oxygen.  In fact, these systems are represented in the actual formula used to determine VO2max, known as the Fick Equation:

VO2max = Q x (AO2-VO2)

To break this formula down into simple terms:

• Q=Cardiac Output= Stroke volume x Heart Rate, where stroke volume is the amount of blood your heart pumps out with each beat and heart rate is the speed at which your heart is beating, expressed in beats per minute (bpm).  This is the cardiovascular component of VO2max, most related to the strength and efficiency of your heart and blood vessels.  If your heart’s ability to keep up with the demands of exercise is diminished, either via insufficient stroke volume or heart rate, cardiac output will be the factor which is most limiting your VO2max.
• AO2= Arterial oxygen saturation.  When you go to the doctor and they put a pulse oximeter on the end of your finger, this is what they’re testing- the percentage of your arterial blood which is saturated with oxygen, typically above 95% at rest in healthy individuals.  This is the respiratory component of your VO2max.  If your respiratory system is limited in its’ ability to either bring oxygen in or get carbon dioxide out, your arterial oxygen saturation levels will drop as exercise intensity increases.  A reduction in arterial oxygen saturation will lead to a decreased VO2max.
• VO2= Venous oxygen saturation.  This is arguably the least understood component of VO2max, largely because we were unable to accurately test venous oxygen saturation until somewhat recently.  This is the muscular system’s contribution to VO2max:  You could have a highly developed respiratory system, which brings a bunch of oxygen into the body and an excellent cardiovascular system which delivers plenty of oxygen to your working muscles.  However, if your muscles are unable to do anything with this influx of oxygen, your VO2max and your performance will suffer accordingly.  When your muscles work hard, they utilize a high percentage of the oxygen that’s available to them and as a result you’ll see deoxygenated blood leaving the muscles on its way back to the heart and lungs.  Therefore, the lower this number is, the better your muscular system is at utilizing oxygen, the higher your VO2max will be.  

By looking at your aerobic fitness through the lens of VO2max, we can break an incredibly complex topic down into the three primary systems mentioned above.  Your VO2max will be most limited by one of these three systems, and each system will require a very different training strategy in order to maximize health outcomes.  At Paragon, we utilize a comprehensive assessment protocol to help identify which of these systems is most limiting for you, and devise a training program accordingly.  This provides you with a fully customized training plan designed to address your specific weaknesses.  In the absence of an assessment, it is a good idea to make sure that your training program addresses each of these systems on a weekly basis.

Next, let’s discuss how “Zone 2 training” fits into this big picture view of your health and identify if it’s the right choice for you at this time.

What is “Zone 2 Training?”

If you listen to health and fitness influencers such as Peter Attia or Andrew Huberman on a regular basis, you’ll often hear them refer to training within specific zones, and there is a trend within these spheres of influence at the time of this writing to push the concept of “Zone 2 Training.”  To understand whether Zone 2 training is right for you, we must first back up and ask ourselves:  What are training zones in the first place, and why should we use them?

The purpose behind using designated training zones is that the intended adaptations one might expect differ depending on which training zone your exercise is centered around.  We’d expect that spending 30 minutes training in Zone 1 would elicit different results from 30 minutes in Zone 5, for example.  If you use an activity tracker such as an Apple watch, Garmin, or Fitbit, you’ll notice that every time you record an activity, your watch will provide you with a breakdown of how much time you spent in each zone.  Below is a screenshot from my Garmin watch, which was recording my heart rate data during a recent 5-mile run:

While charts like the above make it seem as though each intensity zone is very distinct and separate from the others, it’s important to note that our body does not work in such neatly defined segments like you might expect based on the above.  These intensity levels and the benefits we gain from training in them exist on a continuum:  It’s not as though all of the benefits I’d get from working at a heart rate of 150 bpm (the top of my Zone 2) completely fall off a cliff as soon as my heart rate reaches 151 bpm.  For example, one of the benefits that often gets talked about with Zone 2 training is that it maximizes our body’s fat-burning capabilities.  This means that by working at lower intensity levels (zone 1 and 2), I am burning a greater percentage of fat in order to fuel my exercise as compared to glucose.  At higher intensities, the percentage of energy derived from glucose increases while the percentage of energy derived from fat decreases.  However, this does not mean that as soon as my heart rate goes above 150, I’m no longer using any fat for fuel, I am simply utilizing more and more glucose the higher than that my intensity level becomes. 

Since our body’s response to varying exercise intensities are less black and white than we’re led to believe, I find it simpler and more practical to use a 3-zone model rather than the standard 5 or 7-zone model found on most smartwatches and activity trackers.  The 3-zone model we utilize at Paragon is as follows:

As you can see, there is quite a bit more that goes into optimizing your cardiorespiratory fitness than simply picking an arbitrary zone from Dr. Attia’s podcast.  The best way to ensure that your training matches your goals and needs is to schedule an assessment and develop a customized training plan.  That being said, the popularized strategy of Zone 2 training could be exactly what you need, depending on your goals.  We’ll encourage our clients to do most of their aerobic activity outside of the gym in the Easy domain as described above.  The reason for this is simple: this Easy domain represents the biggest gap in what our clients need vs. what they’re getting on a regular basis.  When our clients come to Paragon, they’re usually lifting moderately heavy weights for a moderate duration of time.  They also typically finish their sessions with some form of high intensity interval training on the bike, rower, or VersaClimber.  Thus, our clients are typically checking several boxes in the Moderate and Hard domains, so the thing that they need to work on when they’re not with us tends to be that Easy domain of training.  In these cases, Zone 2 training as described by Attia, Huberman, et al, is perfectly appropriate.  Of course, this isn’t always the case.  If your typical routine consists of things like yoga, pilates, and long walks, and very little strength training and high intensity work, you’re likely already checking the boxes for that Easy domain, and so Zone 2 training is less likely to move the needle for you in terms of health and performance.