Counter Effect: Action and reaction.
When lifting something heavy, in this case a barrel, will effect more than just the shoulder. The rib cage will dip in closer to the pelvis on that particular side.
Gravity will make the lifter lift quickly and drop it on his shoulder almost instantly to fight the pull of gravity.
This character is use to lifting heavy stuff so it will not phase them as much. But for a standard character, there would be a bit of strain and wobble.
This has been a personal test in animation.
Counter Effect: Action and Reaction.
I think it still could use some work. Add a bit of bounce to it instead of just a dip then rise but I think I did decent work.