If you want the most gains, you should train each muscle group twice a week, according to a new review in the journal Sports Medicine.

Scientists analysed 10 prior studies that compared muscle growth in people who trained each muscle group once, twice, or three times per week over a couple months. =

They found that when you work a muscle twice a week, it grows more than when you train it once a week.

You might be thinking, “duh.” But check this out: That’s keeping the total number of sets and reps you do each week equal.

Doing three sets of biceps curls on Monday and three sets on Thursday will make for bigger biceps than if you do six sets on Monday.

Scientists aren’t sure why this is the case, says study author Brad Schoenfeld, Ph.D., an assistant professor in exercise science at Lehman College. But here’s the theory:

Lifting triggers growth in a muscle for a window of time that lasts about 48 hours, Schoenfeld says. When that window is over, you’re no longer building muscle—unless you train the muscle again, which will spur growth for another 48 hours.

That growth will accumulate over time, so the more frequently you put your body into a muscle-building state, the bigger your muscles will be.

That’s only up to a point, though. In studies where people spread their sets over three days per week, they had similar gains to people who trained twice a week. Scientists aren’t sure why this is the case.

Schoenfeld’s recommendations: If you split your training into upper- and lower-body days, do two upper-body days and two lower-body days per week.

But if you like total-body training and want to maximize your gains, Schoenfeld advises lifting three times per week. That’s because it’s hard to fit in the work required to build muscle into just two total-body workouts per week, he says.