Legs. We have two of them to carry us around, and their musculature is very complex. This is due to the fact that despite appearances, only one joint – the knee – is (almost) simple flexing joint with one degree of freedom. Hip joints and ankles both have three degrees of freedom and therefore require accordingly complicated musculature to use them.
Legs also have the biggest and strongest muscle in the human body – the musculus gluteus maximus. It has to be big and strong, because it is the main source of propulsion, the other muscles being more responsible for keeping balance and maneuvering.
Professor Kos remarked, in a slightly yet not entirely creepy way, how working of this muscle can be minutely observed with ease in modern times due to the fashion of young women wearing tight jeans. I admit that I like to observe the changes in curvature of such working musculus gluteus maximus, and from what I gather not only am I not alone in this, but it is not exclusive to my gender either.
However I am also aware that it is rude to stare at someone’s ass – and from what I gather ignorance of this unfortunately is almost exclusive to my gender, as demonstrated by a respectable elderly professor talking about oggling asses with lecherous grin to a bunch of students half of whom were young women quarter his age.