Err.. manual labor is definitely harder than office work. Think about it... 40+ hours in a factory where you are constantly lifting things, walking around, using your hands/body whenever needed.

OR

40+ hours in a office where you sit at a desk and type, fill out reports, etc.

I've yet to experience the office work, but I've been working in various factories for a while now and it takes a toll on your body. I'm constantly sore.