Good training information has never been more available. For runners who don’t work with a coach, that’s phenomenal. But it also can be overwhelming. There are endless options for canned plans. Daniels, Pfitzinger, Hudson. Hansons, Fitzgerald, Coogan. Not to mention dozens (hundreds?) of influencers who sell plans based on what they’ve done in the past.
It's tough to analyze all that information and separate good information from misguided noise. In the last post on The Running Nerd, we examined speed in the context of distance running. It’s hugely important, but how we address it in the context of our training is more important.
Today, we’re kicking off a new series: Thinking About Training. This series will break down key training principles and help you build a structured framework to think about training.
This series will have several parts. We’ll start with the key components of distance running performance, examining what they are, how they interact, and how we can improve them. Then, we’ll step back and look at the big picture, thinking about the structure of a training cycle. Finally, we’ll put it all together and look at some examples from various coaches and plans to see how these principles are applied in different systems.
So, without further ado, let’s look at the key components of performance!
Categorizing What’s Happening While You Run
Different intensities of effort result in different physiological responses. This is intuitive: you can only run all-out briefly before you blow up, but you can run at a relaxed pace for a very long time.
Coaches regularly prescribe different paces/intensities for workouts, targeting specific adaptations. There is a mind-numbing list of zone, pace, and intensity structures out there. With so many methods and terminologies floating around, it’s easy to get lost in the weeds. But all good training structures share one thing: they align with how our bodies respond to different intensities. That’s where exercise intensity domains come in.
Exercise scientists categorize the physiological response to exercise using a structure of “domains” (think zones).
There are four domains:
Moderate
Heavy
Severe
Extreme
NOTE: While the four-domain model is widely accepted, some researchers use different models.
Key markers separate these domains:
Lactate threshold (LT1), which separates the moderate and heavy domains
Critical speed (CS), which separates the heavy and severe domains
VO2max, which separates the severe and extreme domains
NOTE: Not all researchers agree (surprise!) on where to separate the domains. However, evidence supports using LT1, CS, and VO2max as anchor points to separate domains, and I’d argue it’s the most consistent with real-world experience.
So, what are those markers, and why do they matter?
The Key Markers
LT1
LT1 represents the first lactate threshold, which is a physiological marker that separates the moderate domain from the heavy domain.
Let’s step back for a quick, massively oversimplified review of lactate. Lactate results from carbohydrate metabolism. The faster you run, the more your body relies on glycogen rather than fat. Breaking down glycogen produces pyruvate, which can be turned into lactate. The faster you run, the more glycogen you burn, and the more lactate you produce. Some of that lactate is used as fuel by the mitochondria in the muscle fiber it’s produced in. Some of that lactate is shipped off for another muscle fiber to use. As lactate production increases, lactate can get shuttled from your muscle fibers into your blood for the heart, kidneys and liver to use. Lactate is continuously produced and shuttled; when it exceeds what your body can process and use, it builds up in your bloodstream.
LT1 is determined by the amount of lactate in your blood. It’s commonly defined as the speed at which you experience a significant deviation from baseline blood lactate levels. To test blood lactate, you’d prick your finger (ears are also common) and apply clean blood to a strip to test blood lactate. Then, a lactate meter analyzes the lactate content. There are a lot of different methodologies to determine what “significant” means, ranging from “baseline + 1 millimole (mmol)” or “baseline + 0.5 mmol” to advanced statistical analysis based on the series of blood draws. That’s all pretty unimportant for most runners.
LT1 is a useful marker because it allows us to easily assess the boundary between mostly easy, very sustainable efforts and steady, moderate efforts. Lactate levels are not the cause of changes to your physiological state but are rather an indicator of those changes.
Three critical things to understand about LT1:
Below LT1, in the moderate domain, your oxygen consumption and blood lactate levels are mostly constant after the first few minutes of running. Your body can quickly provide the oxygen your muscles need. This is a very sustainable effort.
Above LT1 and below critical speed, in the heavy domain, oxygen consumption and blood lactate levels will increase. Eventually, they will stabilize for a period of time at elevated levels. How long they remain stable is dependent on other factors.
It’s easiest to think of LT1 as the speed at which easy running turns into a moderate effort.
In running circles, LT1 goes by many names. It’s often referred to as the Aerobic Threshold (AeT). Some researchers simply call it “the lactate threshold.” You’ll hear other people refer to it as the first threshold. It’s related to two other thresholds, the gas-exchange threshold (GET) and the first ventilatory threshold (VT1), which are essentially attempts at estimating the same boundary via cardiovascular markers. It’s also typically the top end of “Zone 2” in common usage.
CS
Critical speed is the fastest pace you can run with a stable physiological state. At paces below critical speed, you can keep running for a long time. At paces above critical speed, the wheels will come off pretty quickly.
In the heavy domain, oxygen consumption and blood lactate will more or less stabilize at an elevated level. Fatigue will increase significantly, but it’s manageable.
In contrast, when you exceed critical speed, you’re running at a physiologically unstable pace. That means your oxygen consumption will continue to increase (until it hits VO2max, the next marker we’ll talk about), even while running at the same pace. Your lactate levels increase. Your muscles burn. Fatigue increases rapidly and dramatically. Basically, you’re on the clock — if you continue at that pace, you will blow up. The farther above CS you’re running, the sooner that will happen.
It’s very important to note that CS can drastically differ between runners with identical personal bests. That’s due to various factors, including anaerobic contributions to a race and individual muscle fiber makeup. Similarly, some runners can do more work above critical speed than others. We can calculate how long you have at a given pace with a little testing. We’ll revisit all this in a later post.
If running just below critical speed sounds awfully similar to the effort runners and coaches often describe as “threshold,” that’s because it is.
Three critical things to understand about CS:
CS is a validated estimation of the boundary, or “threshold,” between physiologically stable and unstable paces.
Running in the heavy domain below CS is sustainable. You can run that fast for a relatively long time. Running above CS is unsustainable. Your physiology is unstable, and you will have to stop or slow down.
It’s easiest to think of CS as the fastest pace you can run that feels controlled and sustainable.
VO2max
VO2max is the maximum volume of oxygen that your body can consume. It’s measured in milliliters per minute.
Typically, this is tested via a ramp test on a treadmill. You’ll start out running easily while wearing a gas exchange mask. Over time, the intensity is increased. As the intensity goes up, so does your oxygen intake. However, at some point, even though the intensity is still increasing, your oxygen intake will plateau. You’ve hit VO2max.
It’s useful to think about VO2max as a measure of cardiovascular capability—or how much oxygen your heart can deliver to your muscles via blood.
We already discussed how working at paces faster than critical speed eventually causes oxygen consumption to reach VO2max. That leads us to a significant point.
No single, specific pace (like 3K or 5K pace) leads to VO2max. If you’re running faster than critical speed, you will eventually reach VO2max (or your functional max/peak, which happens when another system gives out before your oxygen consumption plateaus), assuming you don’t stop or slow down.
Three critical things to understand about VO2max:
Working above critical speed but below VO2max is unsustainable, and the fatigue you experience is predictable.
Working above VO2max is limited by muscle output or nervous system breakdown. You cannot do work in the extreme domain for a long period of time.
It’s easiest to think of VO2max as a measure of your maximal cardiovascular output.
How Long Can You Go?
Running performance has been studied in laboratories worldwide for the better part of the past century. Most of that research has been centered on three key “factors” of performance (two of which also happen to be markers we’ve already talked about): VO2max, lactate threshold, and running economy.
Running Economy
Running economy is essentially a measurement of energy demand of running at a specific speed.
Let’s say we had two runners with identical VO2max measurements and the same weight, running at 6:00 per mile pace. The first uses 45 ml/kg/min of oxygen, while the second uses 50 ml/kg/min. Since the first runner uses less oxygen for the same speed, we can say they have better running economy.
Oxygen consumption doesn’t tell the whole story; one study defines running economy as “a complex, multifactorial concept that represents the sum of metabolic, cardiorespiratory, biomechanical and neuromuscular efficiency during running.” Many factors, like shoes, muscle fiber type, biomechanics, etc, can influence individual running economy, but it’s easiest to think of running economy as a measure of energy efficiency while running.
Fatigue Resistance
Researchers have used VO2max, lactate threshold, and running economy to model running performance. Perhaps most famously, Michael Joyner used them to predict that a hypothetical runner could run under 2 hours in the marathon. That equation became a key part of the lab testing and performance modeling in the Breaking 2 project, which played an integral role in our understanding of the fourth factor: fatigue resistance, or how well a runner could maintain their peak metrics in the three other factors over a long period of time.
Essentially, due to a variety of reasons, your metrics decline over a long effort. For example, say you have a VO2max of 60 ml/kg/min at the start line of a marathon. Two hours later, that might have dropped to 53 ml/kg/min. Or maybe your critical speed slowed from 6:00 per mile to 6:15 per mile. Or, you may experience a 3% drop in running economy due to inadequate fueling and neuromuscular fatigue.
Fatigue resistance is still an emerging concept in the literature. It is also known as “physiological resilience” and “durability.” Our understanding of what causes and alters fatigue resistance is limited, although much of the research suggests that typical endurance training (more mileage, consistent strength training) increases it.
A Quick Recap
First, we can separate running intensities into domains, or zones, via three key markers: LT1, critical speed, and VO2max.
Your body responds differently to training in each zone. In the moderate domain, efforts are sustainable, and your physiology is stable. Things start to get harder in the heavy domain, but your physiology still reaches a stable point. You are on the clock in the severe domain, quickly headed towards exhaustion. And in the extreme domain, your efforts are highly unsustainable.
Raw numbers don’t tell the whole story, though. Running economy is a measure of your energy cost at certain speeds. And over long efforts, your key markers deteriorate, reducing your performance. How well you can prevent that deterioration is determined by your fatigue resistance.
Where We Go From Here
Now that we’ve laid the foundation, we’ll zoom in on each marker and discuss how to train effectively to improve them. What adaptations do different training intensities produce? How should we structure workouts for specific adaptations?
Stay tuned for the next installment of Thinking About Training!