The Edge of the Envelope: Functional Threshold, Lactate Threshold, and Critical Power.

It’s a one-parameter model that most of us use, so it’s worth understanding the parameter.

As soon as you start reading about training, you can’t get away from this notion of “threshold”. There are a number of physiological and performance-oriented definitions of it. Some people would have you believe that the nuances matter. Unless you are a researcher, they kind of don’t. In this post, I dig into this idea of a threshold — what it is, why it matters, and how you can use it.

Threshold: the point at which you stop caring that you are drooling on yourself.

“the point at which a physiological or psychological effect begins to be produced” — Merriam-Webster

I like this definition because it helps make the point that a threshold might be physiological or psychological. In this case, I’d argue that it’s both. Well, not literally, but I’ll get to that later.

While this post is rooted in cycling, the same concepts [1], [2] are used in running.

What it is…

Threshold is a number we use in planning training and tracking progress. In that sense, it is a parameter of a model. We correlate it with performance and physiological indicators to the point that some people might say it is defined by those indicators. I find that view to be backwards — what matters is the parameter not the various indicators. Correlating it to things like blood lactate or 95% of a 20 minute all out effort is useful to measure it or try to understand how to improve it, but threshold is still just a number we use in a performance model or to track our progress.

OK, so why THIS number? What makes it so special? Because it is the threshold between aerobic and anaerobic response. It is the threshold between sustainable and unsustainable pace. It is the difference between feeling… not terrible and feeling totally awful. Taking this a bit too far, it is the difference between winning and losing. If you increase your threshold compared to your competition, they have to go at an unsustainable pace (for them) to keep up with you.

…it is the difference between winning and losing.

There’s more to it of course — but basically, that’s it. Threshold is a number we use in a (numerical) model of performance that helps predict the threshold between what’s possible for some extended period of time and what’s not.

Why it matters…

I’m going to take a little detour and illustrate some details of the three kinds of threshold described above in the context of describing why the general phenomena of threshold matters. Note: I could dump on each of these in the paragraphs below but I won’t. They each have value and drawbacks — I’m focusing on the value here.

Let’s start with Functional Threshold Power. It’s pretty simple in that it just integrates all the physiological and psychological factors into one big fat “how hard can you suffer for one hour?”. I mean, you can’t really argue with that being a reasonable estimate of what pace is sustainable and what pace isn’t. It’s a complete integrator that is fantastic for setting training zones and tracking progress in a very wholistic way.


This is a feature of threshold that is really important: It’s a wholistic estimate of your capacity that’s pretty much physiological (so it’s good for tracking physiological progress) but also a measure of psychological stamina and focus.

Lactate threshold is a measure of a physiological response that correlates to the other forms of threshold pretty well. Depending on the athlete, there are going to be differences, but the idea is that above some level of exertion you overwhelm your bodies ability to flush byproducts of burning energy.

Illustration of lactate threshold vs workout intensity
Illustration of lactate threshold vs workout intensity

Looking at threshold in this purely physiological way is a good illustration of its value to training. The capacity of the machine that is your cardiovascular and energy systems is plastic. Building a visceral sense for the functional characteristics of that machine that limit or enable you to perform is useful.

The third form of threshold I’ll discuss here is critical power. CP is an interesting one — and my personal favorite. It is explicitly a parameter of a model; the critical power model. The CP model is an intensity duration curve that describes the maximal power a person can do for given time increments. Threshold, CP, is the asymptote of the curve where power flattens out as duration increases. (see here for a discussion of CP and FTP) The CP model has a related parameter of work capacity above CP called the W’. More on that in a later blog post, but the point here is that the data-driven formulation of CP lends its self to this kind of thing which I see as a real strength.


If we look at threshold in this way— as something of a lower limit of an intensity duration curve derived from lots of data, we start to really understand why it matters. You can look at it as the floor of your fitness. Understanding what that base level is helps you understand what your capacity should be and how good you are going in general.

Whether you look at threshold as a wholistic functional parameter, a physiological response parameter, or a parameter of a data-driven curve fitting exercise, I hope these paragraphs have given you some insight into how it’s a super useful number to be aware of when setting training targets and tracking your progress.

How you can use it…

There are three uses I’ll discuss here — 1) setting training zones, 2) as a parameter in a performance model, and 3) as a metric to track training progress.

Once you know how to do a given workout and you know basically how it should feel, you can figure out by feel if you are going too hard or not hard enough. However, that assumes you know the workout and have enough experience to figure it out by feel. If you don’t, or you don’t want to think about it in the moment, using your threshold as a guide to set the intensity of a workout is super useful. I won’t go into any details on it here, there are plenty of zone calculators or coaches who will help you through that.

In my last post on “what works” I talked about performance models as a thing that works. The driver of a performance model is a daily estimate of training load. Estimates of training load are some non-linear function that uses an estimate of threshold to control the non-linear response of training load accumulation. So at the end of the day, threshold is the controlling parameter of the performance model. As your fitness goes up, if your threshold is set too low, the performance model will say you are training too hard when you are just in shape. On the flip side, if your threshold is set too high, the performance model will say you should be super fresh when you are actually just out of shape. Adjusting your threshold as your fitness changes will ensure the driver of your performance model is accurate and the performance model remains an accurate estimate of your fitness and freshness.

Over the long run, understanding where your threshold is gives you a really good sense for how much work you have to do or how well what you’ve been doing is going. While not really that important, the psychological importance of being able to size up and prepare for a challenge ahead or step back and survey your achievement can not go under emphasized. Using a consistent and relevant metric for this is invaluable.

Something to think about…

Are you psyched? I’m psyched.

I ask because, while we are physiological creatures, there’s a ghost in this machine that can really make a difference in our performance.

Imagine two scenarios — or better yet, do both and report back, would love to hear about the experience.

Scenario 1) You are in a well lit studio with music that you enjoy riding to. You have three or four friends who are also doing a power test that day to cheer you on. You do a power test with a clear mind and a focused and supportive atmosphere with your friends.

Scenario 2) You are in your basement with headphones on. Your dog is interested in your trainer, but no one knows you are doing a power test and no one cares. You do a power test with the day’s work BS in the back of your mind.

How much better would your performance be in scenario 1? What does this say about what “threshold” is? Obviously it’s not completely physiological, but mostly it is. If you get things right enough, the mental part can be thought of as part of the error that’s inherent in one-off testing. But it’s also something to pay attention to and be thoughtful about. If you consistently train in a good space with people who care and will push you, you’ll push the physiology further as a result — and vice versa.

It’s wholistic like that… which complicates things. But don’t get lost in the complication, just don’t take it too seriously and stay psyched.

In closing, remember the infinite wisdom of LeVar Burton,

“But you don’t have to take my word for it.”

Recovering coached athlete focused on road and cyclocross racing for a decade. Father, cyclist, hydrologic information specialist. My opinions are my own.

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