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This article is about Muscle Man's older brother. For Margaret's cousin, see here.
John Sorrenstein, a.k.a. "Muscle Bro", is a character who first appeared in the episode "My Mom".
Even though Muscle Man resembles a zombie, monster, or a disfigured humanoid with green skin, his brother has pale red skin and resembles a tall, plump, grown man. It is revealed that he's older than Muscle Man because he calls him 'little brother' on the radio. He arrives to teach Mordecai and Rigby a lesson on making fun of their mom, though this was simply a prank they had planned. He also appears in "Trucker Hall of Fame", where he delivers the grim news of Muscle Dad's death to Muscle Man, and then he's seen in a flashback where he strikes a piñata and scorpions fall on him. He also makes a cameo appearance in "Pie Contest". He reappears in "Trailer Trashed" where he helps Muscle Man save his trailer with his truck.
- Unlike Muscle Man and their parents, he looks more like a human being.
- His hair looks like the hair on the picture of Muscle Man's mom, which was shown to Mordecai and Rigby.
- He bears a resemblance to Lolly Poopdeck from The Marvelous Misadventures of Flapjack, which JG Quintel also worked on, as well as Carl Brutananadilewski from Aqua Teen Hunger Force.
- His legs are oddly long for his body.
- In "Trucker Hall of Fame" and "Trailer Trashed", Muscle Man calls him Muscle Bro.
- It is revealed in "Trucker Hall of Fame" that he was pranked on his birthday when Muscle Dad and Muscle Man put scorpions in his piñata instead of candy.