this is common. I do this sometimes too. What your doing is that you're rushing, and your hands find the floor, and your arms bend, and you hit your head because the rush suddenly puts your body on your arms very quickly, and so they bend. This will work if you aren't stretching your body out, which could also be a factor in why you're hitting your head: If don't do so already, start with your hands over your head. DO NOT RUSH YOUR HEAD BEFORE YOUR HANDS! Just let your hands lead the way, slowly, until you reach the ground. Just focus on them and you'll be fine.