Pressure on the Body - Intensity and Duration

Pressure is a physical force applied to a designated area. It is applied perpendicular to the skin (as opposed to shearing/friction forces that are applied parallel to the skin).

Uniform pressure is generally not damaging to tissue. For example, deep-sea divers can cope with changes to pressure around their bodies underwater. 

Non-uniform or unrelieved point-pressure can be damaging to tissue.

To illustrate the different types of pressure, try the exercise in the picture. 

You would be able to maintain the pressure on your whole hand for a considerable amount of time.  However, after a few minutes, if you just apply pressure to one joint on your finger, you will begin to feel a great deal of discomfort.

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