sit ups vs. sit downs

I remember when I did PE in grade school I could do close to 50 sit-ups in one minute. Some kids could do over 60! Sit-ups back then were so easy! Like taking a stride when running...
So how come now they're so hard??! Funny how they call them "sit"-ups, because even though now all I do all day is "sit", but it hurts even to do 10 of them, while back then sit-ups were so easy and we hated sitting. I guess that's where the "up" part comes in... Maybe the thick skin that's been building up in the area that hurts when I do sit-ups has something to do with it. I've asked it many times to go away. Oh how stubborn it is.