1. Is it bad to do sit-ups
with your hands
behind your head?

2. YES !!!

3. Sit-ups can strengthen your belly muscles, but doing them
incorrectly can hurt your back. Sit-ups should be done while
you lie on your back with your knees bent enough for the soles
of your feet to touch the floor. Place both hands on your chest
and slowly raise your head off the ground. Raise your shoulders
about one foot and then lower them to the ground. Do this
slowly ten times, rest a few seconds and then do two more sets
of ten. After a week or two this exercise will feel easy, so add a
light weight held behind your neck or on your chest. As you
become stronger, you can use heavier weights.

4. According to one of the studies performed by the researchers at Harvard
University, there's no need to do more than 30 sit-ups in one workout. To
strengthen your belly muscles, you increase the resistance, not the number of
repetitions. Keep your knees bent to protect your back. If you do a sit-up with
your legs straight, you place a great force on the iliopsoas muscles that increase
the arch in your back, which can damage the ligaments and joints. If your belly
muscles are weak, you are likely to arch your back excessively when you sit up
and increase the chances of injury. If you are doing sit-ups to flatten your
stomach, you need to raise your head only about one foot because going higher
than that uses the quadriceps muscles in the front of your upper legs, not your
belly muscles.

5.  “Sit-ups the Right Way”, Gabe Mirkin, M.D.
http://www.drmirkin.com/fitness/sit_ups_correctly.html
 WINCH, M. (2004) Strength Training for Athletes. UK; Stanley L.
Hunt Ltd.
 http://www.netfit.co.uk/tech2.htm
 McARDLE, W. et al. (2000) Essentials of Exercise Physiology. 2nd ed.
Philadelphia: Lippincott Williams & Wilkins
 HERE ARE SOME VIDEOS WITH INSTRUCTIONS FOR PROPER
FORM OF SIT-UPS:
 http://www.youtube.com/watch?v=W7tsFCJ9C6c
 http://www.youtube.com/watch?v=1fbU_MkV7NE
 www.videojug.com/film/how-to-do-sit-ups-3