It is not always wise to try and get a longer range of motion.
Keep you back flat for spinal safety.
There is really only one exercise that would allow you to round your lower back in a bent-over position, and that is during the stiff-legged deadlift (SDL), but that is primarily a lower-back, not a hamstring or a glute exercise. The trouble is that the SDL and the romanian Deadlifts (RDL) are closely related at first glance. Knowing how the two exercises differ and when to attempt them is critical. Our focus is to correct the romanian-style deadlift, for which there is never any rounding of the lower back, period. The RDL is a glutes and hams specialty move done with a flat back. If you round your back, you not only remove emphasis from the hamstrings and glutes, but you could also risk injury.
The best way to correct an improper RDL is to go through a series of steps as you get into position. First, begin the RDL from a standing position. You can adjust your knees (unlocked to slightly bent), low back (flat and tight) and chest (up/big) all at once. Then as you bend over, you need to keep thinking “chest up, back flat, knees bent”. Along with a flat back and bent knees, the path of the bar is different during the RDL and the SDL. During the RDL, you need to keep the bar very close to your legs throughout the move. Conversely during the SDL, you actually allow the bar to drift away from your body. So as you can see, even though both are bent-over barbell moves, ther are dramatically different in technique and target muscles.
Stand upright holding a barbell in front of your upper thighs with a pronated (overhand) grip. Keep your feet shoulder-width apart and a slight bend in your knees, with your chest up, abs tight and the natural arch locked in your lower back.
Keeping your back flat, lean forward from your hips, pushing them rearward until your torso is roughly parallel to the floor. As you lean forward, keep your arms straight and slide the bar down your thighs towards the floor until the bar reaches your shins. At the bottom, keep your back flat, head neutral with the bar very close to your legs. Flex your hamstrings and glutes to raise back up while pushing your hips forward until you bring the bar back to the start position.
Leg Remedy: Romanian Deadlift
There are very few exercises that have as much going on as the romanain deadlift, which is why going through a mental checklist is your best bet to secure good habits. Besides form, many athletes try and take the bar so far down to the ground in an effort to increase the range of motion (ROM) that they are either forced to round their backs, or if they do keep their backs straight, they actually touch the floor with the plates. (In fact, some people do a hybrid of an SDL and an RDL, in which they try and bounce the bar off the floor at the bottom, which can wreak havoc on your joints and is not recommended.) If you are doing an SDL, allow the weight to settle on the ground, hence the “dead” part (but that move is not specifically for legs, so we will not discuss it here). Second, during a correct RDL, the bar never touches the ground. So, go through your checklist, stay strict and smooth for the best RDL possible.