Here I demonstrate a fantastic (and not too easy I might add) abdominal exercise. It also works arms, chest, shoulders and triceps:

~ line yourself with your chest directly over the ball.
~ set your shoulder blades firmly into the sides of your back.
~ body is parallel to the ground.
~ lift one leg stiffly with the foot flexed and touch the opposite heal.
~ alternate with bringing one knee at a time in toward the ball.

I recommend that med ball planks be done with total stability. No moving the back out of neutral.
*Common mistakes: body is NOT parallel to the floor (less abdominal activation), the lower back moves, and shoulders hunch & round.
*To increase the difficulty, try a hard, less stable, light med ball. It feels great!

Thank you for reading,
MK