The core is a group of muscles which provide static and dynamic stability to the spine, pelvis and ribcage. This group includes the transversus abdominus (TA), rectus abdominus (RA), external obliques, internal obliques, iliopsoas, multifidus, erector spinal, diaphragm, and pelvic floor muscles. Other muscles often included are the latissimus dorsi, gluteus maximus and trapezius. When working correctly they act as a weight belt or girdle by producing "hoop stress". (Imagine the iron bands around an old fashioned barrel). They are instrumental in the valsalva maneuver which is involved with lifting, excreting as well as during labor and delivery. This abdominal contraction produces hydraulic amplification by increasing intra-abdominal pressure and fluid compression of the paraspinals creating a semi-rigid column.
While most muscles propel your body, the core muscles resist movement statically and dynamically. When the resistance on the core is constant, static stability is required. Holding a low or high plank position requires static stability. When you transition from this position to a side plank you are exhibiting dynamic stability. In this example the forces exerted on the body are changing its plane of motion and the core muscle are responsible for postural reactions to changes in speed, motion and power. Throwing, lifting, running, bowling, tennis, gymnastics, basketball, football, baseball, shooting pool...all require proper function of the core muscles!
Failure of the core muscles to perform correctly may make a person more susceptible to injury. The importance of the TA is often discussed. It has been estimated that contraction of the TA and other muscles can reduce the vertical disc pressure by 40%, while failure to engage at the right time may lead to injury. Another study looked at the sequence of activation of the transversus abdominus and muscles involved with various movements. Subjects were asked to perform a lift with their upper extremities. Those with a history of back injury exhibited a delay in onset of activation of the TA. They contracted their deltoids first. Those without injury contracted their TA prior to the lift.
The most common exercise for the TA is the "vacuum" - drawing the bellybutton in towards the spine. Little can be done to increase the range you work through or the resistance, but the duration can be increased. A way to promote endurance is with a string. The string can be tired around the waist at the level of the naval while performing the vacuum. It is left on during the day to provide feedback. When the TA releases, pressure is felt on the string and you are reminded to contract again. Remember it's not possible to just work your rectus abdominus and get a flat stomach, the TA must be involved.
There are a great many variations of planks and swiss ball exercises that can be effective for activating the core, but based on an EMG study done by the Biomechanics Lab at San Diego State University, the following exercises have been shown to produce the greatest muscle activity: bicycle crunch, abdominal crunch on Swiss Ball, and captain's chair exercise. Remember! They must be done correctly without using non core muscles to perform the movement.
It's often difficult to be motivated to exercise muscles that aren't all that visible, but if you want to improve your strength, power and performance as well as potentially reduce your risk of injury give these muscles a good workout. Add some of these to your workout each day.
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