The way I was told its about accessing the risk before the lift.
If YOU think you can lift ANY weight and you've done the risk assessment then go ahead,,
but the opposite is also true,,
If you've done the assessment and you DONT think you can lift the weight(whatever the weight is and for whatever reason)
no-one can make you lift it.
its the same reasoning behind working at heights-- ANY difference is a concern, from a street kerb, or chair, up to those silly idiots walking around on skyline cranes.
You do the accessment, examine the risks, and plan the work accordingly.

