What Women Need to Know About Zone 2 Training

coaching intensity training Oct 23, 2023
Stacy Sims Zone 2

Zone 2 training is a hot topic, but like many things, it’s different for females.

If you even remotely follow fitness and training media, you will have seen the buzz around “Zone 2 training”--generally known as steady, conversationally-paced exercise–and how it is the golden child of endurance (and health) training programs. And while there’s no doubt that easy days have an important place in health, fitness, and training, for women specifically, the benefits are being oversold. 

When we talk about “zone training”, we mean breaking down our training intensities into heart rate or power ranges that are used to form a structured training plan or workout. The purpose of stratifying intensities in this way is to achieve specific physiological and metabolic adaptations through our training. In this structure, Zone 2 is relatively easy and long (60 to 70% of max for 45+ min) and you should feel like you can go for hours. The current recommendation is to have the bulk of your exercise sessions–three to four training sessions a week–be in Zone 2.

Why the emphasis on Zone 2? The thought is that Zone 2 is a low enough intensity to stimulate mitochondrial and other adaptations within the muscle cell that improve the skeletal muscle’s ability to use fat as a fuel, spare carbohydrate; improve metabolic flexibility (the ability to rapidly switch between fat and carbohydrate oxidation), as well as to better clear lactate during higher intensity exercise. 

Female Muscle Makeup & Zone 2 Training

To understand what this all means for female athletes, let’s dig into how this works within our specific muscle fibers. We have two primary types of muscle fibers: Type 1 fibers, called “slow twitch” fibers, and Type 2 fibers, called “fast twitch” fibers (which are broken down into subtypes Type IIa and Type IIb). Type 1 fibers have the greatest mitochondrial density (mitochondria are the “powerhouses”of the cell) and are highly oxidative, meaning they are very efficient at using fat as a fuel. As intensity heats up and muscle contractile speed increases, we need more energy than Type 1 fibers can generate using fat, so Type IIa and then Type IIb fibers are recruited. Type II fibers have lower mitochondrial density and high capacity to use glucose for energy.  

Because Type I fibers are so efficient at using fat as a fuel to keep going, the concept around Zone 2 training is that by spending more time tapping into Type I fibers, we can increase their mitochondria density and respiratory rates (the metabolic reactions that require oxygen to convert fatty acids into the usable ATP), as well as increasing the transport proteins (MCT-1) needed to clear lactate quickly and efficiently (during exercise, lactate is produced by the Type II fibers, but primarily cleared by Type 1).  

Sounds great, doesn't it? Go long and easy to boost your metabolic health, endurance capacity, and improve your overall performance! 

But, hold on a minute. If the main goal of Zone 2 training is to increase the number and functionality of mitochondria within the skeletal muscle, and increase fatty acid utilization, we need to question the validity of this concept for women. 

Yes, you’ve got it. This conversation around Zone 2 benefits does not take into account sex differences. Research shows that females (e.g. XX chromosomes), have more oxidative (Type 1) fibers, have greater fatigue-resistant muscles, have greater autophagy activity and a higher reliance on lipid (fat) metabolism as compared to males(e.g. XY chromosomes). We know that training status does have an impact on muscle mitochondria adaptations (basically increasing muscle oxidative capacity), but when we look at equivalently trained women and men, we see that there are differences, specific to skeletal muscle, in mitochondrial oxidative functional capacity. Women have approximately one-third greater mitochondrial intrinsic respiratory rates (the amount of mitochondrial respiration occurring for a given amount of mitochondrial protein) and greater mitochondrial oxygen affinity (p50mito) than men.

What about increasing fatty acid utilization then? Should women spend time in Zone 2 to increase their ability to use fat? Again, no. 

Research shows, as compared to similarly trained men, women have a greater amount of intramyocellular lipid droplets (aka, fat particles stored in skeletal muscle cells), a greater amount of the plasma membrane fatty acid transporter protein CD36 which increase fatty acid uptake into the cell, and also a greater sensitivity to malonyl-CoA (M-CoA) (a metabolite that can inhibit fatty acids getting into the mitochondria), 

What about metabolic flexibility? Women are already metabolically flexible! Not only do women oxidize more fat and less carbohydrate during prolonged exercise; women also have greater metabolic flexibility because there is a greater ability to switch between fatty acid and glucose use,depending on what nutrients are available. 

Finally, when it comes to improved lactate clearance, it may be more important for men to improve their clearance capacity. We see that men exhibit greater MCT-4 and MCT-1. For one, because they have a greater ratio of Type II to Type 1 fibers (remember type II is glycolytic, which produces lactate); men rely more on carbohydrate metabolism than fat metabolism during exercise, and they have higher circulating plasma lactate levels per unit of workload (fitness matched to women). When it comes to endurance training, men upregulate MCT-1 more so than women, which may look like a bad thing, but in the big picture, it makes sense with respect to sex differences in glycolytic fibers and circulating lactate during exercise. Should women be concerned about a reduced expression of MCT-1? The short answer is no. Because women’s bodies fuel exercise differently than men, although we do produce lactate, because we have less overall glycolytic activity, we will have less overall expression of MCT-1. When women do a block of specific high intensity work, and up regulate MCT-4 (the transporters that pull lactate out of cells) there is also a response to upregulate MCT-1 (to clear the lactate). 

Where does all this leave women in the Zone 2 conversation? For women, Zone 2 training is great for active recovery, a certain amount of base building for endurance athletes, and social exercise. Yes, there is merit in Zone 2 BUT if you are planning the bulk of your exercise time for Zone 2 training to enhance mitochondria function and fatty acid utilization, you may want to revisit that concept (more on that in a future post).

One final and very important point is that when you do train in Zone 2, really make it Zone 2. This is really one of the biggest problems I see: women spending too much time going hard or “kind of hard” and not enough time going truly easy. When Zone 2 training enters the conversation, they think they need to spend hours doing this, but it very often becomes hours doing moderate intensity that is harder than Zone 2 and is eventually counterproductive in that it just makes them worn out without the training gains. Stay tuned for part 2 where I’ll talk about what training scheme works best for women.

Stay Connected!

Subscribe to get email notifications which include the 'Extra Bit'. In this segment, I relax (a bit) and talk about my less formal life.

We hate SPAM. We will never sell your information, for any reason.