This refers to the gradual increase in the various forms of stress imposed on the human body in order to improve its ability to withstand such stress. It is commonly employed in resistance training where people are encouraged to increase the weights during exercises in order to gain strength. The principle was first proposed by American physician Thomas DeLorme while working on the rehabilitation of American soldiers in the Second World War. It is based on the idea that the body, when exposed to increased stress in a gradual manner, adapts to it through an appropriate response mechanism.
