It is all the muscles that act on and stabalized the trunk area of the body. The core has many functions. It can flex, extend, laterally flex, rotate and stabalize. For example, in sports, most of this happens simultaneously. So why, then, for most of us does core training for most of us consist of merely sit-ups and ab crunch machines? That is similar to taking that sports car and driving strictly straight ahead. Remember, the core is all of the muscles. Those include your butt, hamstrings, quads, low back, abs and obliques. So, get off the ab machine and grab a medicine ball.
Start with your feet about shoulder width apart. Bring the ball down to your right foot by pivoting on your toes to the right, bending your knees and leaning forward with your trunk to the left. Do that 10 times, then repeat by bringing the ball from your left foot over your right shoulder. Start slowly and gradually increase the speed of your movement and weight of the ball.
You have just flexed, extended, laterally flexed, rotated and stabalized as well as activated the musculature in your lower body. Don’t be surprised if this movement looks eerily similar to an overhead serve in tennis, a home-run swing in softball, or even turning the soil over in the garden—train the way you use your body. Emphasize the “go” muscles rather than the “show” muscles.