No one should ever do sit ups or crunches anywhere, for any reason, ever. Not ever.

Medical organizations and medical professionals have come out against the exercises for reasons dependent on their focus: Orthopedic and geriatric organizations are concerned about the inordinate pressure exerted on the thoracic spine and the propensity for back injury. OB/Gyn's and professionals interested in child development note the inordinate pressure exerted on the pelvic and abdominal organs and on the pelvic floor (from a direction that promotes prolapse).

Exercise specialists disown the exercises because they don't work and because they recruit the use of and end up working the psoas, a muscle you don't want to work in this way or you run the risk of chronic low back pain. There is a finite number of flexes in your spine before it fatigues, leaving it prone to disc herniation. This is called 'fatigue life'; sit ups and crunches are pronounced flexions.

All abdominal work should be done by engaging the transverse abdominus (by pushing out the TA just above the pubic bone) and moving the legs. This can be done with a person in plank position or on her back.

But never, ever do sit ups or crunches. Your back and your bottom will thank you.