The core is a complex series of muscles, extending far beyond our abdominal muscles. They include pretty much every muscle group besides the muscles of the head, neck, arms, and legs. It is the usage of these muscles that allow mobility in the upper and lower portions of our bodies. Indeed, they are the foundation for all our movements, from getting out of bed to walking, running, climbing, and lifting.
The core muscles also act to counteract forces working against us. They stabilize the spine, ribcage, and pelvis against the stresses of movements and other external forces such as gravity. This stability prevents us from losing our balance and keeling over. It is, therefore important that the muscles incorporated in the core work together in symmetry. This is essential in order to transfer force evenly through the body and prevent us from having undesired back, hip, knee, and neck pain.