Where anorexics think they are too fat when they are actually too thin, bigorexics think they look too puny when in actual fact they are massive.
The Body Dysmorphic Disorder Foundation in the UK say one in 10 men training at gyms could suffer from this muscle dysmorphia, which can lead to depression and steroid abuse, and it's on the rise.
Sports psychologist Sara Chatwin joined Paul Henry to discuss the phenomenon.
Watch the video for the full interview with Ms Chatwin.
