The Romanian Deadlift also known as an RDL helps strengthen your entire posterior chain, so your lower back and hamstrings! Not only that, but the trapezius and abdominal muscles are recruited for both power and stability. The Romanian Deadlift is known as a compound exercise, one that recruits many primary and secondary muscle groups and effectively works many of the same muscles that contribute to a strong squat.
Hamstrings: By keeping your knees slightly bent when performing a RDL, you recruit your hamstrings more than a traditional stiff-legged deadlift. Your hamstrings work as they cross your hip joint to help pull your torso upright 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 are too bent.
Spinal Erectors: Your spinal erectors are the long muscles that run up and down either side of your lower back. They help maintain your posture during normal circumstances. But during a RDL, they keep you from rounding your back and provide power, both resisting the weight on your way down and serving as a prime mover on the way up.
Abdominals: Your abdominals contract to keep you from folding in half during a RDL. While the main muscle of your abdominals works to pull your pelvis and torso together, when performing a RDL, 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 a RDL. Your obliques (the muscles at the sides of your waist) contract to help keep you from leaning to one side or the other.
Tapezius: Your trapezius (traps) is the large muscle that covers much of your upper back and it also helps maintain proper posture. This muscle maintains the position of your shoulder bales during a RDL, and when you lean forward, it helps maintain the alignment of your upper spinal column. When you pull the weight upward, your traps contract to help generate power in conduction with your spinal erectors.