your legs are heavy enough that if you know how to use them, you do not need any equipment to target a lot of muscles, especially your core. a lot of back pains and bad posture issues are related to weakness in your glutes and lower back, and tightness of the two main hip flexor muscles(iliacus and psoas major).
this exercise is going to help you solve those problems. I give two variations for this exercise but no matter which one you choose, try to point the toes out. also, make sure you control the speed and do not hyperextend as well(do not exaggerate it!).