Mulos de carga / Muscle Force

English Speakers: For this one, I was looking for an idiom in english that means something like "loading mules", you know, referring to people who works really hard using muscles force on their jobs... Do you think the joke is funny on this way, or maybe there's any idiom that I could have used better for that? 

Any help will be greeted!! :)

Woolin' cheers!!