Scientists aren’t sure why this is the case, says study author Dr Brad Schoenfeld, 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. 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 maximise 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.