Fig 3.5 'Free body diagram' can be constructed to explain excessive inter-abdominal pressure.
Note: This diagram is extracted from my book 'Human Posture'; the writeup below is also from the same book. The topic of 'inter-abdominal pressure' came up in a discussion group - this blog was posted to throw more light on the subject.
....High intra-abdominal pressure has been cited as a reason for Hemorrhoids in this medical literature. From (Fig 3.5) we can try to understand how this could happen. Engineers use 'free body diagrams' to visualize the action of forces within a system. This is done by isolating the part of the body that needs to be studied by cutting it, and then introducing forces to keep the isolated body in equilibrium. The forces required to keep the body in equilibrium indicate the nature of forces acting within the body.
Since we wish to study the forces in the abdominal region we take one section across the abdomen and the other section at the upper end of the thigh so that the torso can be treated as a free body. In the abdominal plane tensile forces 'F1' and 'F2' are introduced to indicate the action of the stomach muscles and the back muscles respectively; the pressure exerted by the abdominal contents is denoted by 'p' and a compressive force 'C1' is introduced to indicate the load transferred to the spine. Lower down, the action of the muscles in the front and back of the thighs is indicated by tensile forces 'F3' and 'F4', while compressive force 'C2' is applied to the thigh bone. With correct values of these forces the torso is in equilibrium - which means, with these forces acting the torso will not move or rotate. Equilibrium can be achieved for different combination of 'p', 'F' and 'C'. Excessive inter-abdominal pressure refers to a condition where at equilibrium, pressure 'p' is abnormally large in relation to 'C1'. This will happen when instead of load of the upper body being passed through the spine, which is designed to take such loads, much of it is passed through the abdominal contents, due to faulty posture.
Once again we see here a problem which can be caused by purely mechanical causes. In this region of the body there are very complex arrangement of muscles which do not get the opportunity to achieve and maintain proper equilibrium.
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