Peter hurts his knee is a running gag on Family Guy. It often occurs when either Peter Griffin or another character hurts their knee or another part of their body, sits where they are, grabs that part of their body, winces, inhales sharply through their teeth, and repeatedly whines "Ah!" an excessive number of times.
Peter's Silver Scroll
In "Wasted Talent", after Peter receives the last silver scroll in a bottle of Pawtucket Patriot Ale, someone tells him to run home as fast as he can. Peter does so a la Charlie from Willy Wonka and the Chocolate Factory to the music of "I Got a Golden Ticket". When he finally gets to his house, he trips over himself and hurts his knee.
Peter and the Chumbawumbas
In the same episode, after being banned from the tour of the Pawtucket Brewery, Peter asks Pawtucket Pat if he even gets a Chumbawumba song. Pat obliges, but the Chumbawumbas only sing part of the song before kicking Peter in the knee.
Peter as John "Hannibal" Smith
In "Brian Goes Back to College", in opening credits for the new incarnation of The A-Team, Peter plays John "Hannibal" Smith. In the theme sequence, his Jeep rolls over and he grabs his knee, and begins wincing in pain.
Lois and the Fox News Contract
In "FOX-y Lady", Lois is now the victim. After signing a contract with Fox News Channel, her boss tells her to run home as fast as she can, but she trips. Unlike Peter's initial trip, she actually grabs hold of her breast.
The Imperial AT-AT Walker
In Something, Something, Something, Dark Side, a snowspeeder ropes an Imperial AT-AT Walker by the legs, and the walker falls. After getting up grabs its "knee" and begins wincing in "pain".
Chap of the Manor
In "Family Guy Viewer Mail No. 2", British Peter "Neville" falls down and grabs his knee, hissing "bugger" through the pain after being chased Benny Hill-style by the London Bobbies for accidentally killing Queen Elizabeth II.