A trainer explains when you should be using intervals versus reps to clock your workouts

Despite the fact that the rise of digital fitness has given everyone access to some of the world’s top trainers at the push of a button (sometimes for $0!), every once and a while you’re bound to find yourself attempting to structure a strength-training workout of your own. Whenever I find myself in this situation, there’s one question that constantly haunts my brain, and one that I’ve never quite been able to figure out the answer to: When am I supposed to monitor my moves by reps, and when should I be using timed intervals?

Over the last few years, you’ve definitely seen interval-based training become more and more popular. Think: the four-move, seven-minute circuits in Kayla Itsine’s Sweat app, or the AMRAP (which, as I recently discovered, stands for “as many reps as possible”) that often serve as “closers” in HIIT classes. While these types of workouts can help get your heart pumping and muscles pounding all at once, there’s still a time and a place for good, old fashioned repetition work.

Here, Aaptiv trainer Kenta Seki answers the question once and for all of when you should be using reps versus timed intervals to calculate your workouts.

When to use reps: If you’re hitting the heavy weights, reps are the way to go. “Rep-based training is ideal for strength training and muscle building,” says Seki. “It generally doesn’t raise your heart rate as high as timed sets, and focuses on isolating the muscles you’re working.” He also notes that this type of training is best for documenting progress, because you can keep track of how much weight you lifted and how many times you were able to do it. That way, you can set goals for yourself and work your way up the next time you grab a set of dumbbells.

When to use intervals: Looking to ramp up your heart rate? Stick to intervals. “Timed intervals are generally ideal for faster pace workouts, and to keep your body in a calorie burning, metabolic state,”  says Seki. “They provide good structure so that you know how long to perform an exercise, as well as how long to recover. With timed training you’ll generally want to use lighter weights and focus on endurance.”

With all of that in mind, Seki offers one important disclosure worth considering: “It’s important to note that there is no general conclusion in the fitness industry regarding which method is more beneficial for muscle gain or fat loss,” he says. “Everyone’s body is different and responds differently to each style of training, so I always recommend trying different methods and finding one that works for you.”

If you need a little sweat-spiration, try this 20-minute HIIT running workout. And here are the 10 best Aaptiv workouts that our editors swear by.