I remembered the movie scene but I had to look it up:

In the film (Star Trek VI: The Undiscovered Country (1991)) - While imprisoned, Captain Kirk fights an alien and hits them in the knees, knocking them out … after the fight he is thankful that the brute had knees … but is told that ‘not all species have their genitals in the same place’

For alien anatomy … it would be pretty weird to have your genitals on your knees … you’d have to cover the entire planet in ‘baby bumpers’ and have a military industrial kneepad manufacturing complex