Where does the typical 23kg/32kg limit for checked in luggage for international flights come from?

It is not typical. Those are the allowances used by certain airlines on flights typically to or from the US they are the metric equivalents of 50/70lbs. They are not used by all (or even most) US airlines.

They are health and safety restrictions that apply to individual pieces of luggage and most airlines have different restrictions based on the class of ticket and the density of the flight. To quote from the Emirates website:

Enjoy one of the most generous baggage allowances in the world. Your free baggage allowance is based on either the total amount of weight or on how many pieces of luggage you have. This is called the weight or piece concept.

The weight concept applies to most routes. The piece concept applies only on flights to and from the Americas and from Africa. Your allowance depends on where you’re travelling, your fare, and your travel class.

I don't know when and where the limits for aviation have been set and would be interested to learn more about that but I do know a thing or two about ergonomics so here is some relevant background.

• Legal limits, when they exist are often considerably higher, certainly for men. The International Labour Organisation compiles them and at the end of the 1980s, the median value was 50 kg for men and 25 kg for women. In France, the absolute limit is 105 kg.

• A reference weight somewhere around 20-25 kg (or 50 lb. as in the revised NIOSH lifting equation) now frequently appears as a recommended maximum weight for handling and lifting in international norms and the scientific literature (for men). Reference weight for men means a few things things:

  • Preventing injury (either over-exertion or overuse over time) is much much more complicated than that, it's impossible to ensure work is safe based merely on some weight threshold.

  • It applies to working men (i.e. studies suggest it's acceptable for 90% of the male working population in the relevant country). It's also acceptable for 70% of women whereas a reference weight around 15-16 kg is appropriate for both men and women. It's possible to go higher, with additional training or individual permission (“special working population”).

  • It only applies in ideal conditions (lifting below the shoulder, an object that can be easily grasped with two hands, uniformly distributed load, without rotation or transport over large distance). The more contraints you add, the lower the weight you should aim for. The NIOSH lifting equation or the calculations in norms like EN 1005-2 or NF X35-109 are intended to take these risk factors into account.

  • In particular, these reference weights do not apply to repetitive lifting. All norms and recommendations (MIL-STD-1472G, etc.) stress that when you lift many objects (e.g. load/unload a vehicle), the weight that is safe to lift for one person is lower. The NIOSH lifting equation includes a “frequency multiplier” to take this into account.

I would not be surprised if 23 kg was the result of some compromise between ergonomic recommendations and economic imperative but considering what I just wrote about international norms, it seems hard to justify it that way. If you read and apply them properly to the aviation context, the safe limit is almost certainly lower (just look at the posture and cadence of people moving bags from the cart to the conveyer belt bringing bags to the hold of a narrow-body airplane).

It's even more difficult to find a source for the 32 kg threshold, except perhaps if you assume that everything between 23 and 32 kg should be handled by two people. Current recommendations are lower and cannot possibly justify it. Hard legal limits or older recommendations are higher (55 kg in this 1967 ILO recommendation, 90 lb in the first edition of the NIOSH lifting equation, 87 lb in the MIL-STD-1472G standard - the latter only up to ”bench height” or 1 m).

Also relevant and perhaps not a coincidence, air transport has an especially high rate of handling and lifting injuries and, together with human health activities (where workers, often women, frequently have to lift another person) account for over half of these injuries in the UK (I got that number fron the 3rd edition of Bodyspace by Stephen Pheasant and Christine M. Haslegrave).