OK, the issue here is your self-definition of my statement "Lean Muscle Mass"
Lean muscle mass can be anything that is muscle and does not include fat. Not necessarily BULK. A lot of Triathletes have lean muscle mass, but are very thin. Of course, they have built more slow-twitch VS fast-twitch muscle.
When I started loading trucks, I was a gym rat, a Triathlete and pretty damn buff. Loading has helped me stay that way. Now, if you want 30" biceps, a 46 inch chest and a 32 waist, sure, you have to go to the Gym, lift like an animal, do some quality cardio and watch your diet all after loading trailers. But if you simply want lean muscle VS fat, loading trucks and a sensible diet will do that for you. Especially the trailers I have loaded. When most of the General Public think of UPS parcels, they think of 2 or 3 lb packages they get from Amazon. They don't think of the 45 lb. boxes of facing bricks I have loaded by the thousands.
I just can't see how someone could build any type of muscle mass by loading, probably unloading. All you're mostly doing when loading trailers is taking stuff out of your power zone and putting them on the ground, the heavy ones you guys would usually drop right on the floor instead of challenging yourself on putting them above your shoulders. You have to be really proactive and have zero lazy tendencies to build any type of muscle mass. Like picking up over 70ibs all the time and going fast all the time and never slowing down on purpose.