Well, I can only explain by example. When I was a young man, I had a job with a construction company building a set of loading bays for a trucking company. For 10 hours a day I shoveled, spread, and compacted sand. I was so completely exhausted that I was physically shaking by the end of my shift. After going home, showering, eating a good meal, and trying to relax, I could not go to sleep. I was so tired, every muscle in my body ached. Once my muscles had relaxed, they began to cramp. I finally lost consciousness, I cant say I actually fell asleep, around 2am and had to be up and on the job site again by 7am the next morning. The next morning my muscles had "settled sore" and I could barely stand. Its what they used to refer to as being "bone tired".