Loaded question, all depends on individual factors, history and level of training.
If you are having back pain it could be several issues: tight lumbar erectors, tight hamstrings, rotation of the pelvis, herinated discs, weak or imbalanced hip flexors / lower abdominal etc.
The first thing you need is an assessment of your posture, specifically rotation of the pelvis and then ROM testing for your spinal erectors and hamstrings. Then you need a test of the strength of your lower abdominal area and then a coordination/strength test of the anterior flexor chain to look for muscle imbalances.
Traditionally full situps strain the back because they over recruit the hip flexors and lead to increased anterior pelvic tilt which increases shearing of the lumbar vertebrae during many exercises, especially ab work. They can also aggrevate any disc problems or can express latent disc problems that have not been diagnosed yet.
The full sit up /crunch is useful for many athletes for most sports that involve running, jumping & throwing need a coordinated effort between the hip flexors and trunk flexors (abs) the problems arise when they are placed into a routine too soon and cause the above mentioned changes is pelvic rotation leading to muscle imbalances and possible injuries.
The abs always need to be trained: Transverse abdominus activiation first, lower abs next , obliques and then upper abdominal region to enusre correct neurological coordination and actviation. ONLY when there is balanced development in all muscle groups do you add in a full ROM sit up. Unfortunately most idiot trainers and BS magazines know about 2 ab exercises: Crunches & machine crunches.
Crunch machines can wreck your lower back for if you don't set them right and you flex too far forward and have your 12th rib cross the axis of rotation of the machine the tension of the weight stack leaves the abdominal wall and is transmitted to the quadratus lumborum (deep muscles in your lower back) and these muscles are unprepared for the sudden load for the QL is an extensor of the spine but when you go too far forward it is forced to become a flexor of the spine in a weak mechanical postion and you get tremendous forces and eventually a QL strain.
So once again we see we need individual assessment, magazine training is bullshit, most trainers are f'in morons and people who rely on crunch machines keep buying BMW's & beach houses for Chiropractors, Orthopedists and Physical therapists.
Later!! (gotta drive my BMW to my beach house! - long live cybex, nautilus & hammer strength $$$$$$)
S
