JFYI, here (Italy, possibly all EU, but I am not sure) the max weight of a sack of cement (or similar, gypsum, plaster etc.) is since quite a few years 25 kg, but it once was 50 kg.

A "sane" max limit is 30 kg for a "normal" worker in good shape, so the 25 kg makes a lot of sense.

There was a precise technique to lift the 50 kg sack and put it on your shoulder, one single, tiny mistake in the speed or in the amplitude of the movement and it was very, very likely you would strain your back.

In the old times it was rather common as an accident on building sites, and anyway even people that never made a mistake have had their backs ruined over the years.

The "main" study/manual on the matter is actually US originated, by NIOSH, dating to 1994, that has been recently updated:

https://www.cdc.gov/niosh/docs/94-110/default.html

not limited to construction only, it applies on whatever activities imply moving loads, particularly if in a repetitive manner.