This upper/lower split allows you to hit everything hard twice per week. There's a minimal negative carryover from each workout.
"Negative carryover" is when one workout will negatively affect the following workout. For example, if you train your arms hard on Monday and then hit chest and back on Tuesday, your performance will be negatively affected in that second workout. The upper/lower split avoids that issue.
Here's what the weekly split could look like:
- Monday: Lower Body One
- Tuesday: Off
- Wednesday: Upper Body One
- Thursday: Off
- Friday: Lower Body Two
- Saturday: Upper Body Two
- Sunday: Off
This split also allows you to include more volume and target work for individual muscles than the whole-body split. There are fewer big compound lifts compared to a whole-body workout, which makes it more manageable for many.
The only drawback is that an upper/lower split can be psychologically demotivating for some. Lots of guys don't like to train legs. Having half the workouts solely devoted to legs can kill motivation for those who hate lower-body sessions. Something can be physiologically optimal, but if it kills your will to train hard, it won't work.
But if you like training the lower body, this is one of the best spits (if not the best) you can use. It's the most versatile. It can be used for every training goal: strength, size, athletic performance, fat loss, or health. And there are several ways to do it.
You can use the typical approach where you hit the whole upper body twice per week and the whole lower body twice. This is the best option for muscle growth.
You can use a lift-specific division. For example, using the Wendler 5/3/1 split you'd have a squat day, a bench day, a deadlift day, and a military press day. You train the big lift and then add 3-5 assistance exercises for the muscles involved in that day's lift. This is a good way to train for strength since you can easily fix the weak links in your big lifts.
There's also the option of using a movement plane or pattern split. You could do an upper-body workout with vertical pulling and pressing, and a second workout with horizontal pulling and pressing. For the lower body, you'd go anterior chain one day and posterior chain the other day. This is great for balanced development and to minimize injuries.