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This is Part 1 of a three-part NiaHealth series on exercise intensity. This post covers how intensity is defined. Part 2 looks at why it matters for your health. Part 3 gives you a practical way to measure intensity during exercise. Two companion pieces go deeper — one on wearables and heart-rate zones, and one on the role of Zone 2 training.
There's no argument that exercise is one of the most powerful things you can do for your health. It helps prevent and delay chronic disease, and it can add healthy years to your life.1 Exercise also raises your VO₂ max — a key measure of fitness that brings its own benefits for how long, and how well, you live.
But when you sit down to plan an exercise program, two questions turn out to be surprisingly hard to answer:
- How hard should you work out?
- How do you know how hard you're working out?
Both questions come back to one thing: exercise intensity.
In scientific terms, intensity is the rate of energy your body demands during exercise. In plain terms, it's how fast your body uses oxygen and burns calories.
Even more simply, intensity is just how hard you're working. It's the difference between an easy walk and an all-out sprint. And intensity matters for almost every benefit of exercise — living longer, raising your VO₂ max, and performing better. For example, people who get more "moderate-to-vigorous activity" (often shortened to MVPA) tend to live longer, have a higher VO₂ max, and develop fewer chronic health conditions.1
But what exactly is MVPA? How much should you do? And how do you know when you're actually doing it? This is where things get tricky, because different fields of science talk about intensity in completely different ways.
Public health guidelines use the words "light," "moderate," and "vigorous" (plus that catch-all, MVPA). Exercise scientists sort intensity by something called metabolic thresholds. And the athletic performance world leans on training zones. So your doctor might call a workout "moderate intensity," an exercise scientist might call it "threshold intensity," and an endurance coach might call it "Zone 3." They may all mean the same thing — or, here's the catch, they may not.
To make things more confusing, there are also many ways to measure intensity: how hard the effort feels to you, your heart rate, your pace or power, or how much energy an "average person" would burn doing that activity. And once again, each field tends to favour its own method.
If this feels like a lot to take in, you're not alone. It took me a month of digging through the research to get it straight — and I'm still learning!
In this post, I'll walk you through the different ways of defining exercise intensity. In Part 2, I'll cover why it matters for your health. In Part 3, I'll give you a practical way to set and measure your own intensity targets. By the end of the series, you'll have a clearer way to think about intensity — and a few simple ways to measure it.
First, the basics: what is "intensity," anyway?
Let's start with two definitions. Physical activity is any body movement that burns more energy than sitting still. Exercise is one type of physical activity: it's planned, structured, and done on purpose to improve your fitness or health.2 In this post, we'll mostly be talking about exercise — the planned, purposeful kind.
When health professionals design an exercise plan, they often use a checklist called the FITT(VP) framework.2 The letters stand for:
- F — Frequency (how often)
- I — Intensity (how hard)
- T — Time (how long each session lasts)
- T — Type (what kind of exercise)
- V — Volume (the total amount)
- P — Progression (how you build over time)
Most of these are easy to plan and measure. You can count your sessions each week, time each workout with a clock, and easily say whether you went for a run or lifted weights. But one letter is a real challenge: intensity.
At its core, intensity is how fast your body demands energy during exercise. The harder you work, the faster you burn fuel.
As intensity goes up, two things happen:
- Your external load goes up. External load is what you're doing on the outside: your speed, your power output, and the activity's difficulty on the MET scale (more on METs soon).
- Your internal load goes up. Internal load is what's happening on the inside: your heart rate, your oxygen use, how hard the effort feels, and whether you can still hold a conversation.
Measuring any of these can tell you something about how hard you're working. But on their own, the numbers — speed, heart rate, effort — don't tell you much about what a workout will actually do for your health or fitness. For that, you need a way to sort intensity into categories.
And that's the problem we turn to next: there are multiple ways to sort intensity into categories, and none of them is perfect.
Three different fields, three different intensity frameworks
There isn't one single way to sort intensity into categories. There are several, and they come from three different fields. Knowing how these fields differ matters, because the best framework for you depends on your goal — general health, or sports performance.
The public health field: "One-size-fits-all" physical activity guidelines
This is the world most of us know. Public health guidelines like Canada's 24-Hour Movement Guidelines, the World Health Organization's Guidelines for Physical Activity, and the Physical Activity Guidelines for Americans sort activity into three levels: light, moderate, and vigorous.2
How do they decide which is which? Mostly by two things: METs (a measure we'll explain in a moment) and how the activity feels. Moderate activities have MET values between 3 and 6. They make you "sweat and breathe a bit harder" — about a 5 or 6 out of 10 in effort. Vigorous activities have MET values of 6 or more. They make you "breathe the hardest" — about a 7 or 8 out of 10.
Canada's guidelines, published in 2021, recommend that adults aged 18–64 get at least 150 minutes of moderate-to-vigorous activity each week.3 (They also recommend muscle-strengthening exercises at least twice a week, plus several hours a day of light activity like standing and walking. For adults 65 and older, Canadian guidelines add activities that challenge balance.)
Guidelines like these are built for entire populations. That's their strength — and also their weakness. They don't account for the fact that the same activity can affect two different bodies in completely different ways.
Meet the METs: a useful but flawed shortcut
One of the most common ways to talk about intensity — especially in physical activity guidelines — is the MET, short for "metabolic equivalent of task." A MET simply compares an activity to sitting quietly at rest. Resting equals 1 MET. An activity that burns energy three times faster than rest is 3 METs, and so on.
You can also describe a MET in terms of the oxygen you use or the calories you burn, per kilogram of body weight:
1 MET = 3.5 mL O₂/kg/min = 1 kcal/kg/hour
Scientists have built a giant reference list — the Adult Compendium of Physical Activities — that gives a MET value to hundreds of activities.4 From there, intensity gets sorted into simple buckets:
This is the system most public health guidelines lean on.
There's a catch, though. That "1 MET" number was originally based on a single person — a 70-kilogram, 40-year-old man.5,6 Later research showed that the real resting value is closer to 3.0 mL/kg/min for men and 2.8 mL/kg/min for women.7 In other words, the MET is a rough average, best treated as an approximation.8
METs have another drawback: they ignore your fitness level. A brisk walk might be moderate for the average adult. But that same walk could be light activity for a trained athlete — or vigorous for someone new to exercise or recovering from surgery.
The exercise science field, which we'll look at next, treats intensity as something that can — and should — be personalized.
The exercise science field: Personalized metabolic thresholds
Instead of asking "what is the MET value for this exercise?" exercise scientists ask "what is this exercise doing to this person's body?".2 They sort intensity by how much an activity disturbs your body's internal balance — a state called homeostasis.
The logic goes like this: your body only adapts and gets fitter when you push it hard enough to disturb that balance. So if you want to know whether a workout is actually making you fitter, you need to know how much it's stressing your system.
To do this, they rely on two key markers called metabolic thresholds:
- The first metabolic threshold (MT1) is the point where exercise starts to noticeably challenge your body. Below it, you could keep going comfortably for a long time. On a lab report, MT1 lines up closely with two other markers: the first ventilatory threshold (VT1) and the first lactate threshold (LT1). It's sometimes called the aerobic threshold.2
- The second metabolic threshold (MT2) is the point where the stress on your body climbs fast. Above it, fatigue builds quickly and you can't last long. It lines up with the second ventilatory threshold (VT2) and the second lactate threshold (LT2), once called the anaerobic threshold.2
Using these two thresholds, exercise scientists think about effort in three personalized categories:
- Below MT1 — comfortable, sustainable effort. Your body clears waste products like lactate about as fast as it makes them, so you could keep going for a long time.
- Between MT1 and MT2 — harder effort, where the strain is building but still manageable. Lactate rises above its resting level but holds steady.
- Above MT2 — hard effort you can only hold for a short time. Lactate builds up faster than your body can clear it, leaving you breathless and tired.
The big takeaway here isn't the labels — it's the anchors. To an exercise scientist, what makes a workout "easy" or "hard" is where it falls relative to your own thresholds. Picture two people walking side by side at the exact same speed. Depending on their fitness levels, that walk could sit below one person's MT1 and above the other person's MT2.
That example points to a key feature of the exercise science categories: they're personal. Where your thresholds fall depends on your age, sex, genetics, health conditions, and fitness.
So how do you find your thresholds? You'd need a graded exercise test — a workout that gets steadily harder while a lab measures your breathing and the gases you breathe out. The gold standard is a cardiopulmonary exercise test (CPET). Done and read correctly, it can measure your VO₂ max and pinpoint your personal MT1 and MT2. Once you know how your effort, heart rate, and pace or power line up with those anchors, it's much easier to hit the intensity you're aiming for.
This is a completely different way of dividing up intensity than the public health system, which leans on fixed MET values that don’t account for the differences between people.
The athletic performance field has come up with yet another way to sort intensity — training zones. These are related to the exercise science thresholds, but they aren't quite the same thing.
The athletic performance field: All about training zones
This field is made up mostly of athletes and coaches, and their goal is to maximize performance. They also anchor their thinking to the metabolic thresholds above, but they use their own vocabulary.2 The athletic performance field has several different terminologies for how they classify exercise intensity, but the one most of us have heard of is "training zones."
The original training zone model had three zones, built around the metabolic thresholds: Zone 1 (below MT1), Zone 2 (between MT1 and MT2), and Zone 3 (above MT2). Over time, dozens of variations have appeared. The most popular today is a five-zone model, sometimes called the Norwegian 5-zone scale.9 In a 2025 survey of 778 endurance coaches and practitioners worldwide, most agreed that the five zones line up with the body's homeostatic thresholds like this:
- Zone 1: well below MT1
- Zone 2: approaching MT1
- Zone 3: between MT1 and MT2
- Zone 4: above MT2 but below VO₂ max
- Zone 5: above VO₂ max
One key feature of this model: it often uses stand-in measures of internal load — like heart rate — to decide which zone you're in. That's why training zones are so often called "heart rate zones."
So all three fields — public health, exercise science, and athletic performance — have their own way of describing intensity. Exercise science uses the metabolic thresholds as the dividing lines. The training zones map onto those thresholds fairly cleanly. But the public health categories were never built around the thresholds, so they don't line up as neatly.
The harmonization attempt: a shared language
A group of experts decided to do something about these different intensity frameworks. In 2025, the American College of Sports Medicine (ACSM) and Exercise and Sport Science Australia (ESSA) released a joint statement with one goal: to create a single, shared set of terms for exercise intensity.2
Their big idea was to anchor everything to the body's real biology — the metabolic thresholds — while keeping the language simple enough for everyday use. They proposed five categories: very low, low, moderate, high, and very high. Here's how those categories line up with the public health, exercise science, and athletic performance terms:
Exercise intensity categories proposed by the ACSM and ESSA
MT1 is the first metabolic threshold; MT2 is the second.
This table shows what a shared language could look like: one set of anchors everyone agrees on, with each field's words lined up against it. Keep in mind, though, that these are proposed categories. They haven't been widely adopted — at least not yet.
There's another important catch: the way these three fields are lined up hasn't actually been confirmed by research. Remember, the public health categories were never built around metabolic thresholds in the first place. They came from activity questionnaires, movement-sensor cut-offs, and MET tables.
Public health categories versus your metabolic thresholds
When researchers compare the public health categories (light, moderate, vigorous) with the metabolic thresholds, one thing becomes clear: they don't line up.
In one study, researchers tested 565 adults aged 18 to 83. They measured each person's thresholds (MT1 and MT2) in a graded exercise test, then converted the oxygen use at each threshold into METs.10 They wanted to see how much those threshold METs varied from person to person — and the results were striking. MT1 showed up anywhere from 2 to 13 METs, depending on the person. MT2 ranged from 3 to 18 METs. The authors concluded that giving everyone one fixed MET value badly misjudges how much stress a workout actually puts on a given person's body.
This means that even if everyone adopts the new shared terms, many people doing "moderate" exercise by the MET definition (say, a brisk walk with an assigned MET value of 4.8) won't actually be between MT1 and MT2.
So the real question becomes: does it matter? If you're doing an activity worth 3 to 6 METs, at a 5-6 out of 10 effort, do you get the benefits of "moderate intensity" — no matter where your thresholds sit? To answer that, we need to look at the research showing how exercise intensity affects your health. And that's exactly where Part 2 picks up.
The takeaway
When it comes to exercise, intensity refers to how hard you’re working. It’s a critical determinant of how much benefit you’ll get, but it’s also challenging to define. Public health guidelines sort activity into light, moderate, and vigorous, based mostly on METs and how hard something feels. Exercise scientists personalize intensity around your two metabolic thresholds, MT1 and MT2. And the athletic performance world uses training zones.
A 2025 effort by the ACSM and ESSA tries to translate all three into one shared set of terms. It's a promising step — but the way they line up is proposed, not proven. And as the research shows, the public health categories don't match your personal biology very well. The same MET value can land in very different places for different people.
That leaves us with the real question: if the categories don't line up with your biology, does it actually matter for your health? That's what we'll tackle in Part 2.
This article is for general education and isn't a substitute for personalized medical advice. Talk to a qualified health professional before starting a new exercise program, especially if you have an existing health condition.
Our editorial standards & process
At NiaHealth, our mission is to make proactive health possible for all Canadians—by combining science with humanity. We believe that rigorous, evidence-informed health information should never feel out of reach. Every word we publish is intentional. We choose language that empowers rather than overwhelms, clarifies rather than complicates, and respects the lived experiences behind every health question. Learn more here.
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