ceiling crack

[boyzi]

A crack appeared in by ceiling about 6 months ago at first it looks like a simple hair line crack but recently it now looks like some one has drawn a thick pencil line running across the ceiling near the chimney breast. my lodger has taken to doing exercise in the room above. would the exercise cause this to widen or could the cause lye elsewhere . however the hair line crack appeared around the time my loger started exercing. There is a dispute between my lodger and myself that the exercises could be the cause. my lodger disagrees that it is the cause. My lodger states that these exersices are minor using a twist and step machine. working out to about 1hr a day 2 times aweek over six months or more. I will also add that all the ceilings in other rooms were replastered at the same time over 2 years ago. there are no cracks anywhere else. can you suggest what could be the cause.