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  • Writer's pictureRyan Allen

Systemic vs. localized effects of aerobic training

By now, anyone familiar with our blog has probably heard us emphasize the importance of incorporating Zone 2 training into one’s exercise routine for mitochondrial efficiency. A common question with this category of exercise is, what form of cardiovascular training (i.e. running, biking, rowing, stairs, etc.) is optimal for it? I think this is a loaded question and the answer will very much depend on the individual and their preferences.

For me, the definite first requirement would be to make sure the exercise can be performed while staying in Zone 2. For example, many will find it difficult to remain in Zone 2 with an exercise as demanding as running, while a brisk walk is not quite intense enough to get them up into Zone 2. I personally like stationary biking because it’s very controlled and easy to titrate your speed, effort, heart rate, etc. such that you remain in this optimal zone of exertion.

Another important aspect to consider when making your exercise choice would regard the part of the body that the training affects. If I’m doing a form of exercise that mostly works just one or a few muscles, are the benefits of Zone 2 only being realized in that muscle tissue, or is mitochondrial efficiency systemically improved? The answer is not currently well established in the literature, but my intuition tells me that mitochondrial benefits would largely be seen in the main muscles being used. In fact, my minor suspicion of localized mitochondrial benefits (as opposed to systemic) is the only thing that might concern me about stationary biking, an exercise that predominantly works the legs rather than the full body.

The reason I feel this is the case is because the demand for increased exertion and energy utilization is taking place at the muscle doing the work; the respiration that will cause the muscle to fire and relax is taking place there in those cells. So, if it’s that local tissue doing the work, it seems intuitive that that tissue would see the benefit of its own training. To put things another way, if you’re doing a single leg squat on your right leg, you probably wouldn’t think you’d see nearly the same benefit on your biceps or your left quad as you will on your right quad.

As it turns out, there may be some evidence to suggest that may not entirely be the case. Some studies have shown that certain effects of endurance exercise, such as changes in gene expression, have been observed in non-exercising tissues. In addition, many of the studies surrounding Zone 2 and evaluation of aerobic fitness by mitochondrial function used cyclists as the subjects, and of course significant improvements in metrics like lactate levels (a product of glycolysis or anaerobic metabolism) were still shown as a result of this exercise.

All of this being said, I am still intrigued by full-body endurance training with exercises like rowing. To share a quick anecdote, a few months back, I had come to find more or less automatic results with my Zone 2 on the stationary bike. I had been doing my training very regularly, and reached a pretty steady state in that I knew what speed I could go at, heart rate I could be at, etc. to remain in Zone 2. At the time, I could also practically predict what I’d see on my CGM, which was always a blood sugar drop to around 110 mg/dL, but definitely no lower than that.

During this time period, there was one occasion where I had access to a rowing machine, and I decided to try it out for my Zone 2 training, titrating my heart rate and perceived exertion to match my normal experience on the bike and stay in Zone 2. After doing a normal 45-minute bout on the rowing machine instead of the bike, I saw a staggering drop in my blood glucose to 84 mg/dL. While I’m not sure entirely what to make of this as it relates to Zone 2, I can only think that this was due to the full-body nature of the exercise. Working more muscles meant that I had more tissues with cells readily taking in glucose for energy usage (insulin-independent glucose uptake). To put it a different way, I had more places actively utilizing glucose from my blood. If this sort of blood sugar-lowering effect of glucose uptake was taking place in a more systemic way across the board in all the muscles I was working, perhaps other beneficial processes of endurance exercise could be as well with full-body exercise.

To be clear, there is absolutely no reason to believe that the stationary bike or non-full-body forms of cardiovascular training lack significant positive effects on metabolic health; if I didn’t think it conferred a great benefit to me, I wouldn’t be using a bike 4 hours a week for my Zone 2. The suspicions I’ve expressed in this post are purely speculative, and ones that I would like to follow up on. It could even be that these exercises do systemically upregulate production and efficiency of mitochondria just as well as rowing, but as far as I know, any studies with muscle biopsies that might tell us just aren’t out there yet. If I can find easier access to a rowing machine, I would be really curious to experiment with this and other forms of endurance exercise further on myself, at least from the point of view of glucose levels and any other metrics I can measure.


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