Here's why:
* Proper nouns are names of specific people, places, or things (e.g., John, London, the Eiffel Tower). They are always capitalized.
* Common nouns are general names for people, places, or things (e.g., man, city, tower). They are not capitalized unless they start a sentence.
"Leg" is a common noun, referring to a body part. It is not a specific person, place, or thing.