I would expect the max stress to be slightly higher when sitting in the chair because of the fact that some people tend to plop down into them. Doing pull ups will create a lot more cycles of near max load.
Note that wood, if that is what it is connected to, has the unusual ability to better resist shorter term loads. An impact load, with a very short duration, doubles it! Even snow loads, that last much longer but are not there all the time, increase the ability to carry the load by 25%.
I doubt that you have much to worry about, assuming that the hook was appropriately sized and attached for the chair.