Split or full body

The question is almost always framed wrong. It isn't which split is best, it's how you spread the week's volume across the days you actually have.

Volume decides, frequency distributes

The factor that most clearly drives muscle growth is how many hard sets you do per muscle group per week. Frequency — how many sessions you spread those sets over — matters mainly because it makes the volume doable.

Comparisons between training a muscle once a week and twice a week tend to favour twice when total volume is matched. The difference isn't dramatic, and above twice a week the evidence thins out.

Choose by number of days

Note that the classic bodybuilding split with one muscle group per day isn't on the list. It gives a frequency of one, and it requires you to fit a whole week's volume for a muscle into a single session. That works for someone already big with plenty of time. Rarely for anyone else.

Full body

Every session has something for the lower body, something pushing and something pulling. Sessions get longer but there are fewer of them.

Suits anyone training two or three times a week, anyone who often misses a session — miss one of three and every muscle still gets two — and beginners, who gain from practising the movements often.

Downside: the last exercises in a session get worse quality because you're tired. Rotate which exercise goes first.

Upper / lower

Two upper sessions and two lower sessions a week. Frequency of two for everything, and shorter sessions than full body.

Suits anyone with four days who wants a split that's easy to count volume in. For most people it's the best trade-off between frequency and session length.

Push, pull, legs

Push covers chest, shoulders and triceps. Pull covers back and biceps. Then legs. Run twice through and you get six days at a frequency of two.

Suits anyone actually training six days a week. Run once through and frequency drops to one, which loses the point of the split.

What to actually count

Whatever the split: count hard sets per muscle group per week and stay between ten and twenty. A hard set is one taken near failure, not a warm-up set.

Only muscles that are primary in a movement count fully. Bench press is chest. Triceps working doesn't make it a triceps exercise.

Don't change split because you stalled

When progress stops it's almost never the split's fault. More common causes are too little sleep, too little food, more volume than you can recover from, or simply not having added weight for several weeks.

Work through those in that order before changing anything structural.