If you're strong and athletic, with powerfully developed muscles, you can describe yourself as muscular.
Weight lifters are muscular, and so is the Incredible Hulk. If your grandmother has worked as a cattle farmer her entire life, she is probably pretty muscular too. Anyone with strong muscles can be described as muscular. In the 1600's, the word simply meant "pertaining to muscles," but by the mid-1700's, muscular specifically referred to well-developed muscles. The root word is the Latin musculus, which, oddly enough, means both "muscle" and "little mouse."