By locking your knees while performing straight-leg deadlifts, your lower back is forced to round (instead of hinging at your hips) and do all of the work to move the weight, which increases your risk for injury, says Nick Tumminello, certified strength coach and owner of Performance University. "It also makes the exercise less effective because it decreases the training stimulus on your glutes and hamstrings."
Maintaining a slight bend in your knees and hinging forward at your hips (without rounding your lower back) like you do to perform Romanian deadlifts, or RDLs, keeps the lower back in a much stronger and safer position and works the glutes and hamstrings—the muscles you want to be training, Tumminello says.
To do a Romanian deadlift (pictured), stand holding a weighted bar or dumbbells with knees slightly bent. Keeping abs engaged and back naturally arched, hinge forward at the hips, reaching weight toward the floor. Without rounding spine, return to start.