Across our various media, you have surely heard us reference Zone 2 exercise several times. It is such an integral component of our exercise framework and important longevity tool that we are constantly stressing its importance. Yet, we recently realized that although we have brought it up so often and had countless questions asked on it, we never had a centralized spot that described all things Zone 2 in detail. The hope is that this post can now be that spot. While we will continue to write about certain applications of Zone 2, important things to track with it, modifications of it, etc., this should provide a comprehensive starting point for anyone looking to integrate this training into their regimen.
Generally, what is Zone 2?
As we increase in physical exertion from resting, to light exercise, to harder exercise, and all-out-intensity exercise, we progress through “zones” of exertion, unsurprisingly called Zones 1-6 (see Nick’s recent post for an additional overview of the zones). During most of the day, we find ourselves in Zone 1, not really pushing ourselves in any serious way. By the time we reach Zone 5, we are in a full sprint or some equivalent activity, pushing virtually as hard as we can (or, as hard as we think we can; you’ll notice there’s actually a Zone 6, which we’ll perhaps discuss sometime). Zone 2 exercise, meanwhile, is a low-to-moderate intensity zone, where we actually should spend a lot of our time training. During this type of exercise, you are not destroying yourself, but going at what some might call an “all-day pace.” In this zone, we are maximally training our aerobic oxidative energy system, driven by our mitochondria. This is the primary mechanism by which we can enhance mitochondrial function and quantity, which are fundamental to our cellular metabolism.
Why should Zone 2 training be done?
Mitochondrial dysfunction, or poor mitochondrial health, has been linked to nearly every major chronic disease process. Mechanistically, it is quite clear that it plays a role in the onset of type 2 diabetes (T2D) and associated metabolic diseases, which themselves seemingly underlie the development of countless other diseases.
From the work of Dr. Gerald Shulman at Yale School of Medicine, it’s been shown that the origins of insulin resistance, the hallmark of T2D, appear to trace back to muscle mitochondria. In particular, sedentary behavior, or a lack of movement and stimulus for mitochondrial adaptation, results in few mitochondria, as well as a poor oxidative (“burning”) capacity. The subsequent accumulation of fat in the muscle drives a number of downstream processes, only eventually resulting in the high blood sugar that gives a diagnosis of T2D. The point? Insulin resistance is a spectrum. You do not want to find yourself shifted at all towards metabolic ill health, let alone seeing those elevations in blood sugar. By the time those come around, the disease process is already quite advanced. Only the insulin sensitive (completely non-insulin resistant) end of the spectrum can make us confident we might avoid downstream consequences. Therefore, keeping our mitochondria in tip-top shape prevents this pathology from progressing to any level of harm.
If that was not convincing enough, the mitochondria appear to contribute to other major killers independently of this foundational metabolic disease role. Without going into exhaustive detail now, a century-old discovery showed that dysfunctional mitochondria is a signature of cancer (one we still cannot entirely explain), suggesting that altered mitochondrial function may play a major role in tumor propagation. Additionally, it even seems that a defect in the aerobic mitochondrial metabolism of neurons may play a role in neurodegeneration and dementia. Needless to say, this organelle is integral to our existence as cellular beings, and any breakdown in its condition can cause major problems.
How can you determine if you’re in Zone 2?
There are many ways to formally define Zone 2. Most accurately, Zone 2 is defined as the maximal exertion at which lactate levels remain steady below 2 mM. When we exceed the capacity of our mitochondria to burn fuel (i.e. we train harder than zone 2), we start to seek alternative methods to quickly produce energy. The primary system we turn to is glycolytic metabolism, which occurs outside the mitochondria and produces lactate. That way, elevations in lactate can show us that we are no longer maximally utilizing our mitochondria.
Most people do not have lactate meters, however, due to the hassle of pricking your finger and the outrageous cost. Luckily, there are alternative ways to gauge if you are in Zone 2. One of the best tests, believe it or not, is trying to see if you can talk. If you are able to carry a conversation (maybe not super comfortably, but mildly strained), you are most likely in the right place. Your heart rate should be around 70-80% of your max heart rate, which for me and most people in their 20s will be in the ballpark of 135-150 bpm. Your Apple Watch now even estimates your zones by heart rate, which you can check out on our Instagram. Lastly, trust your relative perceived exertion (RPE), or how you assess the level of difficulty. If you are completely straining, unable to sustain the pace for a long period of time, unable to speak, having to constantly breathe out of your mouth rather than mostly nose, etc., you are probably going too hard.
How much Zone 2 do I need to do?
Dr. Iñigo San Millán at the University of Colorado School of Medicine has demonstrated significant improvement in Zone 2 exercise capacity, performance, and mitochondrial biogenesis at a level of 3 or more hours of training per week, divided into sessions of at least 45 minutes. That means, if you want to get the most bang for your buck, you are looking to do either 3 hour-long sessions a week, or 4 bouts of 45 minutes per week. It seems that over and beyond 3 hours a week one will still get a benefit, but the biggest inflection point is at 3 hours for one to get most of the benefit.
What are the best forms of exercise for Zone 2?
Any form of exercise where you can keep your level of exertion pretty much constant and low-to-moderate will work. I often use a stationary bike, because I know exactly what pace I should be going at and I can keep virtually all variables constant. Outdoors on a bike, I find there are too many factors that make it hard to stay right in my zone, either making me work less or more than desired. Starting out, you can easily do incline treadmill walking, and adjust the slope and speed as needed. Nick is certainly fond of using a stairclimber, both for Zone 2 and more intense forms of cardio. My favorite form of Zone 2 is a rowing machine when I have access to one due to the full-body effort of it, though technique is vital to preserve if rowing for Zone 2. Recently, I have even gotten into jumping rope, which is very accessible and seems to have me right in a Zone 2 sweet spot. The one form of cardio I might not recommend for Zone 2, though, is running. For most people, running is just a little too taxing to stay in Zone 2, and normally it kicks them into higher-intensity zones. Those are still useful to train, which is why running clearly remains a generally healthy activity, even if not ideal for Zone 2 purposes.
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