THE ABDOMINAL MUSCLES
These share the task of keeping the spine upright by exerting a pull down the front of the trunk that counterbalances that exerted by the back muscles. They also help the spine to bend, by pulling the front of the rib cage closer to the pelvis.
Abdominal muscles also control twisting actions between the shoulders and the pelvis – no golfer could do without them – and they are used when pushing, and for holding the posture when leaning backwards. When the body is bent sideways, they share the work with the back muscles on that side.
A muscle called psoas (from the Greek for loin) passes from the lumbar vertebral bodies, round the pelvis, and over each hip joint to the upper end of each thigh-bone. It contracts when you sit up from lying down. When it is active, it pulls on the lumbar vertebrae, compressing the discs.
There is also an indirect mechanism by which the abdominal muscles support the spine. When a weight is being lifted, these muscles, in conjunction with the back muscles and the other muscles forming the abdominal cavity, tighten automatically. This increases the pressure inside the cavity, making it load bearing (in the same way that inflating a balloon makes it able to support a weight), and as the compressed abdomen presses against the spine, it absorbs some of the load on the spine, and helps the back to straighten up.
Weight-lifters deliberately increase their intra-abdominal pressure by wearing a special belt. For ordinary people, it is enough to keep the abdominal muscles in good condition: this is very important for preventing backache.
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