This is a very interesting subject shrouded in the great fog of prehistory. Almost every culture throughout history has used some form of energy medicine. It would be impossible to answer the question of when energy medicine was founded as it most likely was used in some form or another before we had written history. However we can trace a few lineages of varying types of energy medicine back pretty far at least 4 thousand years ago. The term energy as we use it today is also known by the word Chi in china, Ki in Japan, and Prana in India. I am a practitioner of two types of energy medicine, Reiki and Chi Kung, Reiki has a lineage that goes back to a Japanese man named Mikao Usui. while the tradition of Chi Gung was studied by Buddhist monks in the Shaolin temple for over a thousand years some say much longer, Chi Kung was brought to china along with Buddhism by an Indian by the name of Bodhidharma. following the history back to india, the yogis of ancient india practiced yoga which is based on tapping into and using the chakras or energy centers in the body. To answer your question regarding strength and flexibility, Kung fu practitioners use chi or energy excercises to gather chi and store it for future use. A skilled practitioner can release the chi energy to assist with situations that require great strength, chi energy can also be used after exerting great force to help rebuild and balance the body and speed recovery from injury. In the chinese tradition of energy medicine it is thought that blockages in the flow of energy can cause disease. The idea is to keep the energy flowing freely throughout your body in a similar way that your blood flows freely. Tibet also has unique tradition of energy healing, as well as native american traditions that involve energetic medicine. Also it is believed that plants have different energetic properties and can be used to balance the energy of people.