physiology

The entire purpose of running by heart rate zones is to create a specific pattern of physiological response in your body when running in each zone. “Zone 2 training,” for example, has specific (purported) fitness benefits that are connected with the physiological responses seen below LT1, the first lactate threshold.

There is a similar connection between “Zone 4 training” and LT2: the purpose is to be close to, but not above, LT2, the second lactate threshold.

In Part I of this series on heart rate zones, we covered the science of maximal heart rate (HRmax), %HRmax, heart rate reserve (HRR), and %HRR. We also covered the science of LT1 and LT2 (which is really just a method of estimating SSmax), and why LT1 and LT2 must be at the foundation of any scientifically based zone training model.

(Note that reading Part I is not a strict pre-requisite; if you are familiar with the basics of %HRmax, %HRreserve, LT1, and LT2 you should be able to follow along just fine).

We concluded Part I with two simple questions: (1) what heart rates correspond to LT1 and LT2? and (2) can specific heart rate zones correctly position you below or above these key thresholds?

Our purpose in this article is to answer these questions. More specifically, we are interested in the individual variation in the heart rates that correspond to LT1 and LT2. It would be wonderful—and suspiciously convenient—if LT1 and LT2 corresponded to exactly 70% and 90% of HRmax, as some zone models claim. Unfortunately, heart rate zones are neither this convenient nor this consistent.

In short, LT1 and LT2 can occur at a shockingly broad range of heart rate values, even when expressed as %HRmax or %HRreserve. Here's the key data:

Threshold	Maximal heart rate (HRmax): 90% range	Heart rate reserve (HRR): 90% range
LT1	69 – 94% HRmax	65 – 90% HRR
LT2	80 – 98% HRmax	73 – 95% HRR

...and those are 90% ranges, meaning that one in ten runners will be outside these ranges!

If Zone 2 training is important to you, you need to be running at less than 70% HRmax, or less than 65% HRR to be 95% sure you are in Zone 2.^[1]

These numbers come from a meta-analysis of 412 runners from 25 different scientific studies, the results of which I present below.

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Heart rate zones are all the rage for many runners. Various experts and authorities tout the benefits of different “zones,” though exactly how many zones there are—and what heart rates define each zone—are points of vociferous disagreement.

Instead of tackling the behemoth that is heart rate training in one post, this article is focused on one narrow but critical aspect of heart rate training: how and why heart rate zones are connected to LT1 and LT2, the first and second lactate thresholds.

My goal is to convince you that the entire point of heart rate zones—as they are currently used and advertised—is to be able to tell whether you are running below LT1, above LT1, or above LT2. The (purported?) benefits and (supposed?) scientific rationale of heart rate zones in popular training advice is almost always couched, either explicitly or implicitly, in the physiology of LT1 and LT2.

We will focus only on the connection between heart rate, heart rate zones, and LT1 & LT2 as key physiological signposts. Our goals are to answer the following questions:

• What is heart rate trying to measure?
• Which measurements of heart rate are most appropriate for quantifying training intensity?
• What is the purpose and underlying science behind heart rate zones as a concept?

Along the way, we will understand %HRmax, %HRreserve (%HRR), and some (but not all) of their flaws.

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