lactate

New web app: Predicting LT1 pace and Zone 2 pace from 5k time

I’m excited to launch a new app for predicting training paces: my LT1 and Zone 2 pace calculator is now live, and provides a simple and accurate way to estimate your first lactate threshold, or LT1 pace, as well as an upper limit for your easy run pace, a.k.a. Zone 2 pace. 

📲 Check out my LT1 / Zone 2 app here! 

Normally, LT1 requires a standardized blood lactate test for reliable results. But with this app, all you need is your 5k time (or a reasonable estimate of your current 5k fitness).

Under the hood, this calculator relies on real scientific data from thousands of runners and state-of-the-art statistical techniques to come up with its estimates. 

Try out the calculator at the link above, or read on if you want more details on the science behind how it works.

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Kristoffer Ingebrigtsen’s Norwegian Single Threshold training approach

Perhaps the most unlikely sporting hero to come from Norway’s famous Ingebrigtsen family is Kristoffer Ingebrigtsen.

Though he has no Olympic medals, European titles, or world records like his brothers Jakob, Filip, and Henrik, the eldest Ingebrigtsen brother has accomplished a much more relatable goal for many recreational and amateur runners: losing 25 kg, getting back in shape in his mid-30s, and running very respectable times (32:58 in the 10k and 1:12:11 for the half marathon) while training once per day as a family man with a full-time job.

Kristoffer Ingebrigtsen’s training, set by his brother Henrik, has served as a source of inspiration for a burgeoning Norwegian Singles Approach to training, which attempts to apply the principles of the Norwegian model of training (pioneered by Marius Bakken and later advanced by the speedier Ingebrigtsen brothers) to self-described “hobby joggers”: ordinary people with ordinary jobs, who still have ambitions of improving their times.

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A runner’s guide to LT1, the first lactate threshold

LT1 is the first lactate threshold: one of two key thresholds that define your body’s pattern of physiological response to different running speeds (the other being steady-state max or “SSmax”).

Your LT1 can be intuitively understood as the boundary between paces that feel easy versus paces that feel moderate, steady, or strong.

On a physiological level, LT1 represents the point at which your aerobic system faces its first real disturbance to homeostasis.

Now, that does not mean that paces above LT1 are anaerobic—instead, your aerobic system rises to meet this challenge, and maintains an aerobic steady-state. However, the signs of this greater aerobic challenge become evident in key physiological metrics like blood lactate and oxygen consumption.

A number of my recent posts have brushed up against LT1, like my article on individual variation in heart rate zones, my transcript and commentary on Marius Bakken’s double threshold training, my article on physiological resilience for marathon runners, and of course my article linked above on SSmax, the other key physiological transition.

However, I haven’t done a full article yet dedicated to LT1, and since there’s a ton of wrongheaded information out there about exactly what LT1 represents, it’s high time we take a deep dive into the science behind LT1, the mess of various acronyms and terms related to LT1, and the practical relevance of LT1 for training.

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Individual variation in heart rates at LT1 and LT2 in runners, and the implications for zone training

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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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LT1, LT2, and the scientific basis of heart rate zones for runners

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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New app for calculating threshold, CV, and VO2max pace

I’m excited to share my latest web app: a calculator for threshold pace, CV pace, and VO2max pace! 

📱 Check out my new threshold, CV, and VO2max calculator here! 📱

This calculator allows you to predict physiological training paces from any race distance from 800m to 10 km. 

This app was developed by analyzing over 10,000 race performances using the critical speed model, the gold standard in physiology for determining metabolically stable and metabolically unstable paces. 

The app also includes uncertainty ranges so you can get an idea of the range of different physiological profiles that you might find among runners at the same level of performance.

Below the app, you’ll find a detailed write-up of how it works and what differences you’ll notice between this app and other CV pace, threshold pace, and VO2max pace calculators and tables that you might’ve used in the past. 

As with all of my apps, it is 100% free and open-source. If you find any bugs, have any suggestions for improvements, or spot something that doesn’t look right,  drop me an email. 

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Predicting threshold pace, CV pace, and VO2max pace from 5k times

Most serious runners do workouts at threshold pace, CV pace, and VO2max pace. But how can you know if you're running the right pace for your threshold, CV ("critical velocity," a.k.a. critical speed), or VO2max sessions? There are plenty of conversion charts online, but it's not always clear whether they're making predictions that are accurate for most runners.

This article introduces a simple framework for determining threshold, CV, and VO2max pace based off your current 5k fitness, using percentages of 5k pace to set workout paces. Below, I'll show how these three training paces can be derived from the critical speed model, and present an analysis on thousands of race performances that result in the following training guidelines:

Pace Estimate Meaning Also known as
Threshold 90% 5k Fastest pace that produces a metabolic steady-state SSmax, LT2, CS-, T pace
CV pace 96% 5k Boundary separating metabolically sustainable and unsustainable paces CS, critical speed
VO2max pace 100% 5k Slowest speed that produces metabolic instability and drives oxygen consumption to VO2max CS+

Don’t forget that I have a handy pace percentage calculator for quickly calculating pace percentages!

These predictions are accurate for >90% of runners ("accurate" here meaning "producing the intended physiological effects") and come from data on hundreds of runners with 5k PRs from 14:00 to 25:00. They'll probably work pretty well outside that range too.

Naturally, these predictions work great for full-spectrum percentage-based training, but they work just as well in other systems too.

Want to know where these calculations come from? Read on!

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Marius Bakken in his own words: double threshold, lactate testing, altitude, periodization, and more

Double threshold training is one of the hottest topics in distance training right now, with many runners aiming to replicate Jakob Ingebrigtsen's incredible success in the 1500m, 3000m, and 5000m. The double threshold method does not originate with the Ingebrigtsens, though—it was first pioneered by Norwegian 5k runner Marius Bakken.

Marius Bakken’s 2022 blog post on the Norwegian Model of training has become a key resource for English-speaking runners interested in the practical implementation of double threshold training. Aside from that blog post and an excellent 2025 follow-up, though, English-language resources on double threshold training are lacking—most of us have to make do with second-hand recaps of Norwegian-language articles or interviews.

One of the main resources for Norwegian-speaking runners has been Bakken’s appearances on the Norwegian-language podcast I det lange løp (“In the Long Run”). That podcast, which ran from 2019–2023, hosted Marius Bakken several times to talk about double threshold training.

In this blog post, I am presenting what I hope will be another significant English-language resource for understanding the practical use of double threshold training: a translated version of Marius Bakken’s two-part interview on I det lange løp, plus commentary and footnotes.

The interview covers Bakken’s personal training history, the development of the double threshold method, the use of altitude training, intervals versus continuous runs for improving threshold, and more. I also wrote up a short commentary that follows the text of the interview.

Make yourself your favorite beverage and find a comfy spot to sit down, because there is a ton of in-depth training wisdom in this interview—the original interview runs nearly two hours, and even in writing, it’s quite a long read!

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Steady-state max (SSmax) for runners: The maximal metabolic steady state

Steady-state max is the single most important concept in exercise physiology. That’s a bold statement, but I’ve come to believe it.

The term “steady-state max” and its abbreviation “SSmax” (just pronounced “steady-state max”) refers to the highest metabolic power output that your body can achieve while maintaining a metabolic equilibrium—i.e. an internal state in your muscles and bloodstream that’s stable over time.

SSmax is connected to topics you’re probably familiar with, like lactate threshold (LT2, technically speaking) and critical speed, but they are not quite the same thing—metrics like LT2 and critical speed are ways of estimating SSmax, which is the actual underlying biological capability.

Understanding steady-state max is absolutely essential for building a modern, physiologically-based approach to training, so let’s dive in.

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The science of critical speed, critical velocity (CV), and critical power training for runners

Female athlete running in front of scientific graphic paper

Critical speed is the boundary that separates running speeds that can be sustained at a metabolic steady-state from speeds that cannot. Sometimes called critical velocity or “CV,” critical speed is known in the running world in partly due to its popularization by Tom “Tinman” Schwartz and his proteges, including Drew Hunter.[1]

Critical speed is increasingly becoming the gold standard among physiologists for identifying the limit of what runners would call “high-end aerobic” or “steady-state” running speeds, and is gaining traction as a training tool as well. The critical speed model explains the body’s response to different speeds better than older models based on the lactate threshold.

Among exercise physiologists, critical speed (or a semi-related concept, the maximum lactate steady state, which we’ll also discuss) is rapidly becoming the gold standard for capturing the aerobic fitness of athletes.

Critical speed has its roots in early work in the 1960s, 70s, and 80s, but didn’t really start to emerge as the strongest physiological model for intense exercise until the last 15 years or so.

In this article, we’ll take a detailed look at the critical speed phenomenon, understand how it works on a mathematical and physiological level, see some of the problems and controversies surrounding it, and learn how to apply the concept of critical speed in your own training.

⏱️ Reminder: I have a free critical speed calculator here ⏱️

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Check out my new book on marathon training!