Upper / Lower: four days, balanced

Upper / Lower alternates between upper-body and lower-body days, typically four days a week.

Monday: Upper (heavy)
Tuesday: Lower (heavy)
Thursday: Upper (volume)
Friday: Lower (volume)

The heavy days centre on compound lifts in the 4–6 rep range. Volume days move to 8–15 reps with more isolation work.

This is arguably the best strength-to-time ratio available. You still hit everything twice a week, but with three rest days instead of one.

Progression: add weight on heavy days, add reps on volume days. When you hit the top of a rep range on all sets, increase the load and drop back down.