As the name suggests "core strength" refers to the strength of the muscles deep in the core of the body, around the spine. These muscles are vitally important to the strength of the spine and the upper and lower limbs which arise from the spinal column.
When most practitioners refer to "core strength" they usually refer to a range of large and small muscles which include: the abdominal muscles group (Rectus Abdominus, Internal and External Obliques, Transverses Abdominus) and the inter spinal muscles (muscles that run in between one or more vertebra) such as Multifidus and Rotatores.
Understanding Core Strength
The easiest way to understand how these muscles work is to imagine the spinal column is the mast on a yacht and the core muscles around the mast are the guy-wires which stabilise the spine. Therefore without the guy wires the mast will be weak and unstable.
It is also important to understand that although the core muscles support and strengthen the spine, they are just as important in strengthening and stabilizing the upper and lower limbs. As force production or power is generated in the trunk (spine) and is then transferred to the extremities.
Importance of Core Strength
In many cases where patients have experienced lower back pain, there is a weakness or deficiency in these core muscles. This creates an inherent weakness in the spine leading to injury. Another common fault are patients who only train one side of the spine (usually the abdominals). This only strengths one side of the spine, creating an imbalance between the muscles around the front and back of the spine which again can lead to or predispose to injury.
Current evidence suggests that those with reduced core strength maybe predisposed to injuries, this includes injuries to the lower extremities (especially in females). Whereas those who have strong core muscles have a reduced rate of lower back and lower extremity injuries, greater injury prevention as well as increased force production in the upper and lower extremities leading to improved sporting performance.

Types of core strengthening programs
Journal of Strength and Conditioning Research, Current Sports Medicine Reports, The Journal of the American Academy of Orthopaedic Surgeons, Sports Medicine, Medicine and Science in Sport and Exercise, Archive of Physical Medicine and Rehabilitation
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