The bends, or decompression sickness, is the name given to a range of symptoms by a person exposed to a rapid elevation of atmospheric pressure around them. As a result, nitrogen bubbles in an individuals blood causing joint pain. This generally happens in the outer environment of space, where there is no air pressure, or in deep sea diving. To prevent the bends, personnel will spend time in a decompression chambers.
As a result of the testing conducted on James T. Kirk by the Vians in their underground lab, Doctor Leonard McCoy diagnosed his symptoms as those found in someone suffering from the bends. McCoy was unable to treat Kirk for his symptoms, nor was he even sure how someone could get the bends underground. Kirk was, nevertheless, cured by the touch of Gem, an empath, and without the assistance of a decompression chamber. (TOS: "The Empath")