'Can't do' means what exactly? Does it hurt too much, is your muscles just not up to the work, is your back too stiff??? There is a limited range of movement on "average" for healthy people. On average means we take a sample of people, some can, some can't, some can nearly be a pretzel... from that sampling we have an "average" amount of movement that people are able to do. You might fall into the lower end of the range. Sit-ups and crunches are always done wrong. Folk always tend to lift with their legs and their lower back. The idea is to lift with the abdominals. Sounds like a great idea... but can you explain exactly HOW that is done? You can't, I can't. Having done those sorts of exercises I know that it is a cross between a tightening and a twist to get the body to "crunch" or to achieve a sit-up calling on the abdominals completely to achieve the movement. It took a bit of time to figure out HOW to do this, and still I can not explain how to do it. Crunches are better than sit ups since fewer people get injuries doing them. The biggest injury to sit-ups is lower back issues because people naturally rely upon the lower back, followed by the neck: http://www.stormingmedia.us/41/4123/A412393.html Sit-ups are meant to be a complete movement to here you touch your knees with your elbows (as example) crunches are not meant for the body to be doubled over. In fact just lifting the shoulders off the floor a couple of inches with the abdominal muscles and holding the position for a slow count of 5 to 10 then slowly lowering over the count of 3 is better for the muscles and works them out more effectively than trying to double up completely. Abdominals are usually the hardest ones to strengthen. Being that they are usually at near maximum load if you hold your proper posture all day long.

I have no idea what it is, but I have the exact same problem. I'm probably not quite as fit as you, but I'm pretty mobile. I try to do a sit up and it just won't happen. I can do it if i move my legs, but that's just propulsion. As for splits, I can get pretty far on forward ones and almost nowhere at all on straddle splits. Something that might be tendons always snags on something painfully every time I try the straddle splits, but that might just be normal inflexibility. I couldn't even begin to do a sit up or a crunch or anything like that to save my life. if someone were to hold my feet while I was lying down in the sit up position, and if i could not use my arms for propulsion or to grab my legs and pull me up, then I would not move. At all. Ever. It just wouldn't happen. I would stay very very still with no notion of how to move.