If your back hurts, clasp your hands behind your head and lift your head up. You need to engage your core and maintain the feeling of pushing your back against the floor while moving your legs. If your core is weak, your back will lift when you lower your legs, and exercising in that position will hurt your back, so lower your legs just before they lift and pause slightly before coming back up. This way, you can strengthen your core without hurting your back. You can challenge yourself slowly until you reach a completely straight position! I wonder if I even have abs; when I first started, I couldn't lower my legs past 30 degrees, but now I think I can lower them to 45 degrees, haha. Fighting while protecting your back! I'm so envious of those who can already lower their legs straight!