Bonjour Chris !

I know it's quite confusing, don't worry !
Here it's a case of needing to specify which legs as the context is not that explicit.
For example, the verb here is not reflexive, as in "Je me brosse les dents." (I brush myself the teeth.), in which case we're given the owner of the teeth within the sentence.
Here we need to say "mes jambes" as opposed to someone else's.
Have a look at the related lesson:
Special cases when you use mon/ma/mes/etc with parts of the body (French Possessive Adjectives)

I hope that's helpful!
À bientôt !

In the case of "J'ai des crampes dans les épaules", you use "les" because of the clear context set up in the previous sentence, but also of the use of "avoir" (J'ai des crampes) which emphasises the belonging of the shoulders to the person speaking, hence the use of "les".