Sit Up Myth

You should not do straight legged sit ups - true or false?

THIS IS MOSTLY TRUE

Many people who performing a straight leg sit up will arch their lower back, which places a strain on the cervical spine and lower back. If you do this exercise, you should concentrate on keeping the lower back on the ground, even place your hand under the arch of your back to press down against it.
Not only does it potentially damage your back, the straight leg sit up does not work the abdominal muscles effectively, which I assume is the aim of doing the exercise. The straight leg sit up works the quadriceps and hip flexors more so that the abdominal muscles.

sit-up with weights

Instead of the straight leg sit up, you can do a similar exercise with the legs bent, either being held or loose.