My mother was an OTR trucker during most of my teenage years, including driving for Walmart under contract (so not paid amazingly well, but regularly interacted with the Walmart employees at the DC in Tomah, WI). It's not always back breaking work, but access to (healthy) food is a major issue. For Walmart specifically, there's not much physical effort involved besides lowering the trailer landing gear, which is almost always done with a manual crank. The Walmart backroom staff handle unloading and the DC staff handle loading, which would be very very taxing to do constantly.