What is the difference between Zone 1 and Zone 2? Between Zone 4 and 5? Between 8 and 9?
What? You didn’t know there were 9 zones? What kind of crap is this!?
As a coach and exercise physiologist, I’m often asked the differences and exact heart rate thresholds between Zone 3 and 4, or Zone 1 and 2. “Well, my old coach said Z1 stopped at 140, and now you’re telling me it stops at 147? Which is right?
The truth is, with two exceptions (and even exceptions to those exceptions), zones are a fairly arbitrary set of numbers, which is why there are dozens of ways to calculate “Your Zones.”
Most athletes assume that there is a physiological change (“Threshold”) that happens between each zone, and that training 1-2 beats higher or lower effs everything up. This is partially true, but not entirely true, as you’ll see below.
The best way to understand how your body works is to think of a Graded Exercise Test. Imagine you are on a treadmill, and every 60 seconds the speed and grade go up. Through this process, your heart rate, lactate readings and perceived effort level go up, and up, and up. As the effort increases, your body changes how it makes energy for the muscle, progressing from mostly fat using oxygen, to mostly sugar without oxygen.
At some point, you cross the first threshold and go from a “resting” state with stable breathing and lactate levels, to a “working a bit” state, with lactate levels that aren’t really climbing, but they’re certainly above resting. This threshold has a lot of names (listed at the end of the article) but we will simply call it Threshold 1 (T1). Under T1, your body is preferentially burning fat, targeting mitochondria, and giving you the best bang for your buck for training. You can do a LOT of this work, and it requires very little recovery time. Above T1, your body is working harder, using both fat and sugar energy sources, but overall you are at a very sustainable effort level.
As intensity keeps increasing above T1 in the Graded Exercise Test, you’re eventually going to pass a threshold beyond which your body is no longer in a sustainable state. Let’s call this Threshold 2 (T2). Once you cross T2, even if the treadmill doesn’t speed up any more, your lactate, heart rate, and oxygen intake are all going to continue to climb FAST. Once you cross T2, even if only a little, you have at most 40 minutes before your muscles become so acidic that you fall off the treadmill. The simplest set of zones is simply to have three zones demarcated by these two thresholds.
In a perfect world, I’d send an athlete to a lab where they would have their lactate and oxygen measured in a Graded Exercise Test, and we’d know at what speed/power/heart rate their T1 and T2 are.
If the athlete can’t do that, then we go to an estimation. The most common way to do this is to perform a maximum effort test and some math to estimate your T1, T2, and zones. You can test and build zones off of your maximum heart rate, your lactate threshold heart rate, your functional threshold power, lactate threshold running speed, etc.
Then, we commonly use some scientist’s population-level research to set zones. The one I use most commonly in this situation is the work of Dr. Andrew Coggan. He found that over a huge amount of people, their T1 HR on average was 83% of their T2 HR. While it’s really not as good as testing to find your T1 (because some people’s T1 might actually be 75% of T2, and indeed some elite athletes might have a T1 that’s up to 90% of T2), it’s not a bad place to start.
Finally, there are several ways to estimate your T1 without referencing T2 or maximum HR, all with varying levels of accuracy. Some will use a nose-breathing test, but some athletes can nose breathe well above T1 so that isn’t always accurate. Some will use a steady state treadmill test, and if your HR drifts up over time, you’re probably above T1. This is challenging because if you’re on a treadmill you’re almost certainly inside where the air is hot and still, and your cardiovascular system will turn up the HR just to keep your core temperature down. So even at speeds actually under your T1, you could be getting a rising HR over time.
Next, now that we have established what your T1 and T2 are, how do we end up with four, five, or even nine zone systems?
Well, we start with the three zones from above:
Z1: Under T1
Z2: Between T1 and T2
Z3: Above T2
And we know that trained athletes usually recover better with easy efforts than sitting on their bony asses, so we want a nice easy training zone in there, so we have:
Z1: <85% of T1
Z2: 85% of T1 up to T1
Z3 Between T1 and T2
Z4: Above T2
Then let’s say this athlete’s race is something short, like 20 minutes. Since we know that they can hold T2 for something like 40-60 minutes, we can guess that a 20 minute race will result in HR’s significantly higher than T2. So we make a zone for that
Z1: <85% of T1
Z2: 85% of T1 up to T1
Z3 Between T1 and T2
Z4: T2 up to 5% above T2
Z5: 5% above T2 and greater
Now we have a pretty sweet zone-5 system.
NOTE: Remember when I said there are exceptions to the exceptions? Some coaches (including Dr. Coggans) don’t even USE T2 as a marker between zones, they put it inside Z4.
You’ll often see differences in the zones for different sports, and even different distances of sport (some triathlon coaches will use slightly different zones for short vs long-course racers). I sometimes even only use a three zone system. However, almost ALL endurance sports use T1 as a barrier for their low intensity work (80-90% of your training should be below T1).
So what does this mean for YOU the athlete?
- A huge part of endurance training is targeting your energy systems
- The closer your zones are to matching your physiology the better you are able to target the energy systems you want to target.
- The best way to find T1 and T2 is through testing at a physiology lab.
- The second best way to find T1 and T2 is through working with an experienced coach.
- Once you have those zones, set up a set of zones that makes sense for you and your coach, or you and your sport.
NOTE: Other names for T1: Gas Exchange Threshold (GET). Lactate Threshold (NOT the lactate threshold you’re thinking about, this is a recent change in the literature which is confusing to no end), Ventilatory Threshold 1 (VT1), Lactate Threshold 1 (LT1), Aerobic Threshold (AeT).
Other Names for T2: Lactate Threshold (It’s like they’re TRYING to confuse us) (LT), Maximal Lactate Steady State (MLSS), Onset of Blood Lactate Accumulation (OBLA), Anaerobic Threshold (AT), Ventilatory Threshold 2 (VT2), Lactate Threshold 2 (LT2).
