Reflexologists posit that the blockage of an energy field, invisible life force, or Qi, can prevent healing. Another tenet of reflexology is the belief that practitioners can relieve stress and pain in other parts of the body through the manipulation of the feet. One claimed explanation is that the pressure received in the feet may send signals that 'balance' the nervous system or release chemicals such as endorphins that reduce stress and pain. These hypotheses are rejected by the medical community, who cite a lack of scientific evidence and the well-tested germ theory of disease.