Q: When doing sit-ups, does hooking your feet on something (so they don't move, and so your legs don't extend) decrease the value of the excercise? I can't seem to keep in the same position when doing sit-ups, I find myself moving my feet around.

A: It doesn't necessarily diminish the value of the exercise, but it does change what muscles do how much work. By keeping feet unanchored, or especially keeping them up in the air (which is how I prefer to do it) you force your front abdominal muscles do almost all of the work. By anchoring your feet, you bring in hip flexor muscles into play by offloading some of the work to them. Neither way is really wrong, it's just a matter of what muscles you want to target. I would recommend doing it both ways.

When your feet are unachored, don't try to do a full sit-up, but instead concentrate on curling up your body, so your shoulder blades no longer touch the floor while your lower back is pretty much immobile -- this emphasizes abdominal action. When your feet are anchored, go for a full situp with straight back.