If it's any help, I did night fill work for almost a year before I had children (this is going back about 10 years)... and I am only a small woman (5ft tall and at the time 45kg) ... they never worried that I couldn't lift things that were too heavy, as most of the time, the heavy boxes are in the aisles they need to be in, and mainly night fill's job is making sure the shelves are stacked full.