[kashmir]

"sit ups" frequently do little for ab tone as many people do them using the hip flexors. Assuming it is safe (see above), a modified situp that targets the abdominis rectus would be to lie on your back, knees bent. Use your fingers to find where the hip flexor crosses from leg to pelvis (lift your leg and it will bulge), now start a sit up but stop as soon as the hip flexor kicks in. Hold 30s. Repeat.

In time your abs will strengthen and you can get up higher. You are also teaching your abs to work rather than your hip flexors.

Good luck.