Exercise of the week: Core Strengthening Exercises
If you only do crunches and sit-ups then you’re training abdominal muscles without natural movement. Their main function is to provide stability during movement.
Crunches only target the "six-pack" muscle running down the front of your torso. You contract the abs as you crunch up and in real life you rarely need the muscle to work this way. The abdominal muscles role is to prevent excessive extension of the spine.  Research proves it’s better to do stabilization exercises like planks and side-planks.
Other core exercises that are important are full body exercises such as deadlifts and pushups with good form. 
 Here's the deadlift:
Engage your core and pull up on your pelvic floor before lifting.  Raise the shoulders and use your legs for the start of the movement.  Use your back and abdominal muscles for the end of the movement.  Return the bar slowly to the ground keeping your core strong.  Get instruction from a knowledgeable trainer to get help.