Unstable exercises like this one below make your abs work harder. The most important part of your core is actually surrounding your spine. It is responsible for posture as well as daily movements which support your back and organs. The transverse abdominis (inner core) acts as a corset to hold your body in, which gives you a sleek slender look. Using things like the stability ball, dina disc, bosu activate this must as you move. Work smarter to get the most out of your workout!
Start by laying on your back on the BOSU, feet planted firmly on the ground. Place your hands lightly behind your head, but be sure not to use your hands to pull your head forward.
Raise one leg off the ground and try to touch your knee with your opposite elbow. Be sure to do this by using your abs and core and twisting your torso, rather than by pulling or twisting your neck (source).