A lot of it is due to repetition. I know most people associate working out with repetition, but in manual labor tasks you are usually performing a much narrower and less wholly engaging set of movements for longer more sustained periods. This changes the activity from exercise into a session of stress on the body that can exceed its ability to manage the wear and tear done by those activities. Also, workouts moves are done with the intent of being healthy and getting fit, doing work is just getting a job done, wether it requires walking or bending or whatever.
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