Core strengthening refers to the strengthening of the muscles that function in trunk movement and stability.  This includes traditional spinal flexors, extensors and rotators, as well as deep abdominal and back musculature.  It also can include muscles that stabilize the hip and shoulder girdle. 

There is growing evidence in the literature of both biomechanical and EMG studies, to support the notion that there are two muscle systems that work to maintain the stability of the spine:  the global muscle system and the local muscle system.  Each functions differently, depending on the loading conditions placed on the spine. 

The global muscle system includes the large, torque producing muscles that move the spine and pelvis.  They do not have a direct attachment to the spine.  This includes the thoracic part of the paraspinal muscles (longissimus and iliocostalis lumborum), the quadratus lumborum (lateral portion), rectus abdominus and, to some extent, the external oblique muscle. 

The local muscle system involves the deep muscles with direct attachments to the spine, including the lumbar multifidus, transverses abdominus, diaphragm, internal oblique and lumbar part of the paraspinal muscles. 

Core strengthening in physical therapy programs is usually directed at improving the ability of the local muscle system to provide segmental stabilization.  The muscles that are focused on include the transverse abdominus (a deep stomach muscle), multifidus (a deep muscle that rotates low back joints), the internal obliques and the diaphragm.  The hip stabilizing muscles, the gluteus maximus, gluteus medius and hip adductors, are also often addressed with core strengthening.  Patients may be taught exercises for strengthening these muscles in different positions, such as on the back or stomach, side lying, sitting on a large physioball and/or standing, as some muscles can be trained better in certain positions and, also, core muscles need to be strengthened in the functional positions of sitting and standing so that they will be used more during daily activities.