Strong evidence

Once weekly volume is matched, splitting it across more sessions doesn't add growth.

In practice

Train each muscle as often as your week allows. Three sessions or six makes little difference to growth if the total number of hard sets is the same — so pick the split you'll actually turn up for.

What it doesn't say

Higher frequency can still help indirectly: more sessions make a large weekly volume easier to fit and easier to recover between. This is about frequency itself, not about what frequency lets you do.

Source

• Schoenfeld BJ, Grgic J, Krieger J (2019). How many times per week should a muscle be trained to maximize muscle hypertrophy? A systematic review and meta-analysis of studies examining the effects of resistance training frequency. Journal of Sports Sciences, 37(11):1286–1295. doi.org/10.1080/02640414.2018.1555906