Cybernetics is the science of replacing normal biological parts with artificial ones that fill the same roll. In the beginning of the 20th century it began with pace-makers and artificial hearts and from there developed at a break-neck pace until before long any organ in the human body, with the exception of the spine and brain, could be easily replaced with an artificial equivalent. Once joints, bones, and even muscle tissue began to be effectively replaced and the ties into the nervous system became more reliable and faster, it did not take a fantastic leap of creativity to begin using this science to actually augment the parts in question.
Over the course of a thousand years the science began to build people that were 5, 10, even 50 and 100 times stronger than a normal person. People would actually replace their entire bodies with artificial parts, effectively transforming themselves into machines of war with internal, sometimes concealed weapons systems powered by on board nuclear fusion power plants to keep them running indefinitely.
Modern cybernetics are still used for organ and tissue replacement for medical purposes. Any augmentation of the body is highly illegal unless it is for military purposes on enlisted soldiers who, when their term of service is up, will have their upgrades removed and replaced with standard medical-grade cybernetics.