Legs

No muscle group in the body soaks up more total training volume than the legs, and that changes how you should program them: they can take — and need — more sets and more frequency than anything else to keep progressing, so treating leg day like any other day on the split usually leaves them undertrained relative to their size. That extra volume only pays off if it's split across the two patterns that move the leg, and they aren't interchangeable: the squat, which bends the knee and hip together (squat, Bulgarian split squat, sumo squat, leg press, lunges), and the hip hinge, which barely bends the knee at all (Romanian deadlift, dumbbell Romanian deadlift). The leg curl breaks that pattern entirely — it isolates the knee with no hip involvement, which is exactly why it's useful on a day when the lower back is already fried from something else. Among the squat variants, what shifts the balance between quads and glutes isn't which squat you pick but how you set up: a narrow stance with an upright torso loads the quads harder, while a wide stance or a forward lean shifts the work toward the glutes and posterior chain. The conventional deadlift sits between both patterns — a real hip hinge plus genuine knee bend near the floor — which is precisely what separates it from the Romanian deadlift, where the knee stays almost straight.