We've all done it right, gone to do some sit ups and put our feet under a bar, bench, radiator or anything to fix our feet inn place so we can perform a true 'sit up'.
Restricting your feet for a sit up exercise
It's a common sight in most gym's, I know I've seen it many times, however. If you're doing sit ups like this, you're doing them wrong. Fixing the feet enables you to pull harder with the hip flexor, in turn this puts increased stress on your lower spine, essentially this enables you to sit up without bending your spine. What happens from this action is that the effort required from the hip flexors is increased while the effort required from the abdominals is decreased, exactly opposite to why most people would perform the sit up exercise with restricted feet.
Overall the sit up is one of the least effective abdominal exercises as during the sit up exercise your abdominal muscles spend very little time under tension, even when performed unrestricted, it's your hip flexors which start the movement, followed by some abdominal activity before finishing with momentum making the exercise ineffective at stimulating the rectus abdominals, our primary core muscle and the the one responsible for flexion of the spine get very little stimulus.
The same is true with straight leg sit ups which put enormous pressure on the lower spine, requiring a huge effort from your bio mechanically disadvantaged hip flexors. This will cause lower back discomfort and likewise stimulate the abdominals for a very small amount of time during the movement.
So, what can you do?
The Crunch or Abdominal Curl Exercise
How to perform
Remember not to thrust your upper body up off the floor; if you feel discomfort in your neck you can place your tongue on the top of your mouth to reduce this.