I split wood for my Father-In-Law this weekend (who is usually on the other end of good deeds). I stay active, but never do anything like this. After 2 afternoons, my back and legs were a little sore, but my hands were just shot. My wife had to button my shirt for me. Yes, I'm pathetic.
When we don't use muscles, they grow weak. This is why I've struggled with memory work. (I'd gotten out of the habit). What muscles do you need to work?
Reading? Prayer? Good deeds? Better go muscle up!