The Romanian deadlift -- originally developed by Romanian weightlifter, Nicu Vlad -- strengthens your lower back and hamstrings. Your trapezius and abdominals get recruited for power and stability. The Romanian deadlift was originally used to build power for pulling movements in Olympic weightlifting, but it effectively works many muscles that contribute to a strong squat. Consult a healthcare practitioner before beginning any strength-training program.
By keeping your knees slightly flexed when performing the Romanian deadlift, you recruit your hamstrings more than the traditional stiff-legged deadlift, according to a 2010 article in the "Strength and Conditioning Journal." Your hamstrings work as they cross your hip joint to help pull your torso erect as you stand up with the weight. Your hamstrings stretch on the way down, but you should never stretch to the point where your back rounds or your knees bend more.
Your spinal erectors -- the long muscles that run up and down either side of your lower back -- maintain your posture during normal circumstances. During the Romanian deadlift, they keep you from rounding your back and provide power, both resisting the weight on the way down and serving as a prime mover on the way up. These muscles work every time you lean forward. The more weight you use, or the farther you lean, the more they work. Do not lean forward to the point where your spine flexes; maintain an arch in your lower back at all times.
Your abdominals contract to keep you from folding in half during the Romanian deadlift. While the main muscle of your abdominals normally works to pull your pelvis and torso together, when performing the Romanian deadlift, it contracts in an isometric manner. An isometric contraction is one where no movement takes place, and this type of contraction by your abdominals keeps your chest from meeting your pelvis in a painful manner during the Romanian deadlift. Your obliques -- the muscles at the sides of your waist -- contract to help keep you from leaning to one side or the other.
Your trapezius -- the large muscle that covers much of your upper back -- also helps maintain proper posture. This muscle maintains the position of your shoulder blades during the lift, and when you lean forward, it helps maintain the alignment of your upper spinal column. When you pull the bar upward, your trapezius contracts to help generate power in conjunction with your spinal erectors.