Chi is not abstraction of physical strength at all. While physical strength is generated by muscles and speed, chi is generated and stored at dan tien (about 2 inch under the navel)and directed by mind. Of course, every one naturally has chi but to develop chi and use chi is not easy. It requires that you train under qualified teachers if not it will be even dengerous for your health. Chi can be applied not only for health (through chikung exercises) but also in combat. Howerver, It seem that there are fewer and fewer mastial art masters can truly apply chi im combat. It is a pity.
If you are really interested in chi, then I would recommend you log on to following website. In my oppinion it is an interesting one by a master who is knowledgeable about both chikung and martial art:
http://wongkk.com/. I myself like the section: answers to questions on this website.