Calculations are made of abdominal muscle loads and stresses associated with the development of intra-abdominal pressure during slow, symmetrical lifts. The muscles considered are the rectus, transversus and the external and internal obliques. Muscle loads and stresses have been calculated in an abdominal cross-section at about the level of the third lumbar vertebra. For four cases examined, maximum stress levels for men in the 25–35 years age range appear to be 3p-4p for the rectus muscles, 15p-25p for the transversus and 4p-6p for the obliques, where p is the intra-abdominal pressure. Corresponding figures for a man aged 66 years are about 8p, 21p and 8p respectively.
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2.
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