For the past 10 years or so, high-intensity exercise like HIIT (high-intensity interval training) has been all the rage, where a 20-minute full-out session a few times a week is all you may need to stay fit. Now the trend appears to be for lower-intensity (relatively easy) but longer workouts, often referred to as Zone 2, that supposedly can yield similar, if not better, results.
There’s no standard definition, but in general, you’re working out in Zone 2 when you’re exercising at a relatively moderate (sometimes called low-moderate) intensity. Zone 2 activities would be those you could do for a long time—even an hour or more (if you cared to)—like going for a steady but unhurried walk, a leisurely bike ride on flat terrain, or a gentle swim.
Zones, from 1 to 5
Exercise intensity can basically be divided into five zones that are associated with heart rate, among other parameters. Though the exact numbers are still being debated for all zones, a Zone 2 workout is estimated to involve a heart rate that’s about 60 to 70 percent of maximum. By comparison, high-intensity workouts in Zone 4 or 5 categories, for instance, would involve heart rates of at least 80 percent of maximum heart rate.
The best way to determine your maximum heart rate is with an exercise stress test (a test that puts your heart under stress while you work out but is typically reserved for people having symptoms that could indicate cardiovascular disease). If you’re healthy, you can estimate your maximum heart rate by multiplying your age by 0.7 and subtracting the total from 208. For example, if you’re 65 years old, your maximum heart rate would be 162.5; 60 to 70 percent of that max is approximately 98 to 114, meaning that your heart rate should fall in that range for a Zone 2 workout.
Short of having a heart rate monitor to know when you are in Zone 2, you can easily approximate the intensity of your workout by doing what’s called the “talk test.” That means being able to easily hold a conversation while exercising without becoming short of breath.
The different zones correlate with what fuel (carbs versus fat) the body is primarily using to create energy during the workout. In Zone 2 workouts, your body is mostly using fat versus carbs for energy. At higher zones (3 to 5), the body uses more carbs and much less fat.
Zone 2 rewards
Working out regularly in Zone 2 may boost what’s referred to as metabolic flexibility, a term that means your cells can efficiently switch between using carbohydrates and fats for energy. It’s thought that type 2 diabetes is one of several chronic conditions associated with metabolic inflexibility, whereby the body over-relies on carbohydrates and doesn’t efficiently switch from carbohydrates to fats for energy.
Mitochondria are involved in generating energy within cells, and having more mitochondria that are healthy and work efficiently is a key part of metabolic flexibility. (In other words, with a lot of healthy and efficient mitochondria, the body is better able to use fats and carbohydrates for energy and switch between them when necessary.) Exercising a lot, whether it’s a long session or many sessions a week, is linked with an increase in the production of mitochondria. Because Zone 2 workouts can be done for longer than high-intensity workouts, you’ll be able to do more exercise overall and, therefore, produce more mitochondria.
In particular, having more metabolic flexibility can be advantageous in endurance sports. If you’re better able to use fat for energy (versus carbs) during an endurance event, you may perform better. And since the body has a very large supply of fat, compared to carbs, if you’re working out in Zone 2, you’re not needing as large a supply of carbs and so won’t “hit the wall” (which happens when you burn through your carb stores). A study in Sports Medicine in 2023 found that, among professional cyclists, those who were better at burning fat for energy had a competitive edge over those who were less able to.
In addition, after engaging in a Zone 2 workout, you likely won’t experience as much muscle soreness and fatigue, and, as a result, you may be able to do an additional workout the next day, compared to if you did a high-intensity workout that takes your body longer to recover from. Importantly, because of all these advantages of Zone 2 exercise, you may also enjoy working out more.
Putting it into action
There’s no hard and fast rule as to how much time you should spend working out at a slower pace versus a high-intensity pace, but exercise physiologists and trainers generally suggest that 70 to 80 percent of your overall training time be spent in Zone 2 while the rest (20 to 30 percent) be spent at higher intensities—Zone 3 or Zone 4, for example.
As mentioned, workouts that would generally meet Zone 2 intensity include that unhurried walk, bike ride, or swim. And remember, you can estimate that you’re in Zone 2 when you can do the “talk test” and not be short of breath.
Engaging in Zone 2 workouts doesn’t mean eliminating high-intensity workouts, however. You can, for example, add in some high-intensity exercise on days you do Zone 2 workouts, or do your high-intensity workouts on a different day of the week.
While working out at lower-intensity levels is likely to be a safer bet for many individuals, if you have been sedentary, have any serious health conditions, previous or current injuries, or biomechanical problems that limit your ability to exercise, it’s always a good idea to talk with your healthcare provider before starting any exercise program.
