Exercise is a healthy and, one might argue, necessary part of our lives. The guidelines from public health organizations tell you to aim for at least 150 minutes of “moderate” exercise each week, and that each minute of “vigorous” exercise counts double. That raises the question: What the hell is “moderate” exercise? How do you know when you’re doing it?
It’s not really about heart rate
The short answer, which is only slightly incorrect, is to say that “moderate” exercise is the same thing as “zone 2” cardio, which I’ve previously explained. (Zone 2 is is the second-lowest intensity in a five-zone system, and you need a heart rate monitor to know which zone you’re in.)
While “zone 2” and “moderate” are both wide ranges of intensity, they do overlap a good bit. If I had to pick which heart rate zone most closely matches “moderate” intensity exercise, I’d pick zone 2. But that doesn’t mean it’s the same thing.
Science measures “moderate” and “vigorous” in terms of METs
The research that led to these guidelines didn’t use heart rate as its metric. Instead, these scientists measured exercise in terms of metabolic equivalents, or METs.
One MET is the energy expended when you’re at rest—the amount of oxygen, calories, etc that it takes to keep you alive and breathing. (We use oxygen in the process of burning calories, so officially a MET is 3.2 milliliters of oxygen per kilogram of body weight per minute.)
Researchers can then put an oxygen mask on a person and measure how much oxygen they use while running, walking, playing guitar, etc. If an activity takes twice as much oxygen as sitting still, they say it takes two METs. Here are a few examples (taken from this scientific paper):
2 MET: washing dishes, playing croquet
3 MET: walking at 3 miles per hour (a pretty typical walking pace)
4 MET: table tennis, ice skating
5 MET: modern dance, fast-paced ballroom dance
6 MET: volleyball, singles tennis
7 MET: jogging, jumping rope
The numbers go up from there. Speed skating clocks in at 15 MET. To be clear, you will not be measuring METs directly when you exercise. The MET studies are done in labs so that we can use the information to get a sense of what MET values each common type of exercise tends to have.
Moderate exercise is 3 to 6 MET, and vigorous is 6 or more
The physical activity guidelines define “moderate” exercise as at least 3 MET, but less than 6. Vigorous is 6 MET or more.
Because METs are specific to the activity, not to how fit you are, it makes the most sense to look at METs in terms of the pace you run or the settings you use on your treadmill or other cardio machine. Here are paces and activities that have been clocked as between 3 and 6 METs:
Walking at 3-4 mph (a 15-minute to 20-minute mile)
Cycling, between 50 to 100 watts
Shooting baskets
Playing baseball
Taking a low-impact aerobics class
And these are vigorous (6 or more MET):
Race walking (5+ mph)
Walking uphill
Walking with a 12-pound pack
Jogging (a 12-minute mile is 8.0 MET; the faster you go, the higher the MET)
Bicycling at 12 miles per hour or faster
Swimming laps
Playing a game of basketball, soccer, or hockey
So how am I supposed to know when I’m doing moderate exercise if I can’t measure METs?
I’m going to say this again: Your fitness watch can’t measure METs. This is why people so often tell you to target “zone 2” instead—it’s not really correct, but at least it's an easy measurement that you can read off your watch.
In reality, this falls short in two ways: (1) different gadgets and systems use different cutoffs to define "zone 2", and (2) for most of us, zone 2 includes most moderate activities but also some vigorous activities. If you’re relatively fit, you can jog at a 12 minute-per-mile pace while keeping your heart rate in zone 2. That’s a “vigorous” activity in terms of METs, though.
The other reason heart rate isn’t accurate for this task is that your heart rate changes for all kinds of reasons. The hotter it is when you're working out, the higher your heart rate tends to be. Same goes for when you’re nervous or stressed. And as you get fitter, you’ll be able to do the same activities at a lower heart rate. Those activities might feel easier than they used to, but they’re not any less work.
That’s why you’ll want to refer to the bulleted lists above, or to a more fleshed-out chart like this one. To recap a couple of dividing lines:
Walking is moderate, jogging or running is vigorous.
Bicycling is moderate if it’s under about 12 miles per hour on the flat
Indoor cycling is moderate if it’s up to about 100 watts of power
All that said, you don’t have to overthink it. Those guidelines that mention “moderate” and “vigorous” activities aren’t asking you to monitor your heart rate or any other numeric metric. They want you thinking in terms of generalities: walking versus running, leisurely bike commuting versus sweating your heart out in a spin studio.
And honestly, if it’s easier to watch your heart rate than to worry about the above, that’s fine. For most of us, 150 minutes of Zone 2 is going to be at least 150 minutes of moderate exercise. So follow that guideline, and you’ll be an overachiever.