Crunches or sit-ups?  What’s the difference?  Crunches are about isolating the abdominal muscles.  Sit-ups go a step further and include the hip flexors.  As soon as the mid and lower back start to rise off the floor  the hip flexors begin to engage and the exercise starts to become a sit-up.

Crunches are a safer exercise for those with back or neck issues.  With a floor crunch the lower back stays on the floor and only the abdominal muscles are engaged.  The shoulders may or may not come up.  If the person is having neck issues or has forward head syndrome then it may be best to simply have him/her focus on tightening the abdominal muscles while also performing a chin tuck.  This way the neck muscles are not further strained in their already compromised posture and the spine is kept in a neutral position .

Both crunches and sit-ups have merit, but sit-ups should only be done if the person is not having any serious back or neck issues.

This  video explains the crunch with shoulder raise very well: