I would have to say this is a very big MYTH! And the amount of people that ask me 'If I do 100's of sit ups will I get a flat belly'? the answer I give is usually something along the lines of 'more than likely you'll end up with a hernia'!
Although sit ups do work the rectus abdominus (the long, flat muscle that extends vertically at the front of the abdomen between the pubis and the 5th, 6th and 7th ribs). Sit ups also work the hip flexors (front of thighs). Sit ups alone will not flatten out the stomach. The only way to get a ripped stomach where you can see definition is to reduce the amount of fat overall in the body, do HIIT (High Intensity Interval Training, see 26/11/11 blog) and have a clean sensible diet.
Also why do people think it is necessary to do 100's of sit ups (I've heard of some trainers advocate a minimum of 1000 sit ups!!). The rectus abdominus is a muscle just like say our biceps or our chest muscles. But I rarely see someone doing 100 bicep curls or 100 barbell chest presses!
As I mentioned earlier sit ups use the hip flexors more than the stomach muscles especially if you sit up past 90 degrees. This creates a pull on the lumbar spine which may exacerbate an already weak back or may well create a new injury.
What else can we do?
Well for one stop doing sit ups! Find an experienced trainer (like myself) to give you a 'core' programme to work on your deep stabilising muscles. Your Transverse Abdominus (TVA) acts like an invisible corset, when this is strong we have a better supported and protected back. You'll also notice differences in your posture and how you carry yourself, this too can give the illusion of a flatter stomach.
Nutrition - over processed and refined foods, wheat, potatoes, sugars, too much alcohol and takeaways can add inches to our bellies. Cutting a lot of these foods or completing ditching them will make a difference to ones stomach. Don't take my word for it, try it and see.
De stress - too much stress will cause too much cortisol, this stress hormone will cause your body to store fat rather than use it!
HIIT as mentioned earlier, and PHA (see 22/01/12 blog) training moves that incorporate whole body movements rather than isolated ones such as your bicep curls and tricep kickbacks, will have a profound effect on your body composition all over.
Do these things and in time you may start to see a 6 pack rather than a 6 barrel showing through!