I think your question is a bit vague.
What do you mean for "human"? Man, female, age, etc? But especially, how long the human has to apply the force?
Just one time in a day or for 8 hours continuously?
I suppose you have to see the NIOSH standards. You will find a table with different categories ordered by: age, sex, type of activity, duration, etc.
Here there is an example (attention, this is NOT exactly what you want)