Looking for a new exercise to try? Give these weighted good morning's a shot... When done properly, this movement works the entire length of the erector spinae (not just the lower back) and engages all of your posterior chain... That's a lot of muscles! I see a lot of people doing this exercise, but many of them do it with poor form. short garments good for formal occasions
This exercise is a controlled hip hinge- it is not meant to be performed with stiff legs or locked out knees. Thoracic spine should be in arched position, lumbar spine should also be in an arched, natural position so that the middle-back is working to support the bar. Glutes should be recruited and engaged as well.
If you try and do this with straight legs in an upright position, you're going to hurt yourself and/or fall over. Your feet should be firmly planted throughout the entire movement and you should be in control of your balance.