As a multisport coach, one question I often get is, “Why so much Z2, Coach?”

First, let me define how I use Z2. Since we don’t often have access to a lab, I often use heart rate threshold data. I do NOT use power or pace for Z2, because if you have excess fatigue, you might be riding or running in Z2 pace or power but your heart rate is working too hard and in Zone 3.

First, let me define how I use Z2. Since we don’t often have access to a lab, I often use heart rate threshold data. I do NOT use power or pace for Z2, because if you have excess fatigue, you might be riding or running in Z2 pace or power but your heart rate is working too hard and in Z3.

I compared many of the different methods you could use for calculating heart rate. From Dr’s Seiler to Cougan, Milan, Coach Joe Friel, and coaching methods from USAT, Cyclesmart, and others, it is a pretty crazy spread. Using my bike heart rate data of 155 threshold heart rate, the low end of these zones would put my heart rate in the mid-’90s to low 100s heart rate, while some of these Z2 calculations would put me all the way up to 143 for Z2. Wow, I would be crushed if I went for a 3hr Z2 ride at a 143 heart rate, that’s closer to a Half-Ironman effort- that’s 92% of my threshold!\

Physiologists use 65-75% of max heart rate to get Z2, assuming that threshold is roughly 90% of max. Therefore we can assume 75-85% threshold is our Z2. Since the threshold is the most applicable and probably the most frequent data we can get from races, this makes sense. If you have very valid max heart rate data it’s worth running both numbers at a percent of threshold and percent max heart rates to see where your Z2 lines up.

I’m a firm believer in Dr’s Inigo San-Millan and Seiler. World-class athletes are not born in a lab. Yet when we reverse engineer their success across decades and across multiple disciplines from running, rowing, cycling, cross-country skiing, and triathlon, we find some common themes. They create a huge aerobic base with lots of aerobic work at 65-75% of heart rate max. Ideally 70% of max, or 80% threshold. There is so much work you can do at this level, without excessive fatigue or risk of injury.

We know too, that lots of aerobic Z2 work increase the number and size of mitochondria in the muscle cells. In fact, according to Dr. Millan, Z2 is the greatest stress for the mitochondria because it uses the most fat oxidation and the body is able to transport lactic acid into the cell and use it as energy. As you may remember, mitochondria produce ATP. Now the more mitochondria you have the more lactic acid you can utilize for fuel and clear from the bloodstream and the more apt you can produce. However, the excess lactic acid does not allow for mitochondrial growth; in fact, it’s an inhibitor of growth. Therefore, from a physiological view, you don’t really want to be doing a ton of high-intensity work. Too much lactic acid is unable to be brought into the mitochondria, which creates hydrogen ions that are acidic and toxic. Since focused Z2 work improved our metabolic efficiency through fat oxidation, lactate clearing, and lactate production, this allows us to be able to do more work. All this Z2 benefit is really rewarded in Z4, thus further improving Z4 power and speed.

If you run or ride, simply limit yourself to a Z2 heart rate- ignore the pace, ignore the power. I’m willing to bet with patience and consistency, you will grow your endurance, increase the pace and power at that Z2 without any excess effort and improve your PA:HR or PW:HR ratio (aka your cardiac drift will become smaller ratio.) A ratio of less than 5% of PA:HR is pretty good for an endurance run or ride without intervals. Once you have mastered heart rate and your aerobic engine is strong, your threshold and VO2 intensity as well as duration or repeatability of intervals- will likely be better than before! Good Z2 work is not necessarily easy. Using a talk test, you can hold a conversation, but the other person definitely knows you’re exercising. Z2 is very sustainable for 60-90 minutes; however, after 2-3-4 hrs you might be feeling a pretty good amount of fatigue.

If you have the data and the metrics to show your recovery, such as an Oura ring that tracks HRV and resting HR, you might want to play around with how much and how often you do intensity. That’s the micro level of heart rate variability. However, on a macro level, you might notice fatigue exists if you’re struggling to get your heart rate into normal zones. Should you do a hard session anyway even if it’s on the schedule? I recommend getting some sleep, increasing your glycogen through a higher-carb meal and getting hydrated, and doing that higher-intensity session the next day. But talk with your coach about this.

In reviewing all this fascinating research and information from the best physiologists, we can determine the following:

– Z2 is at 80% threshold or 70% max heart rate.

– Z2 frequency and duration would dictate 80% of your workouts should be at this intensity

-Z2 grows mitochondria

-Z2-improves fat oxidation

-Z2-improves lactate clearing.

-Z2 is repeatable with low risk, however, the reward is like any investment it takes time.

I hope you have your best year ever in 2023, and I hope you spend plenty of time in Z2 and that brings you nothing but personaly bests! Good luck!

Coach Jim Hallberg
4x National Champion
USAT Certified Coach

Coaching is

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