Muscle Returns (マッスル・リターンズ) was a one-shot follow up story set 8 years after Kinnikuman's conclusion. It was publishied in Kadokawa Shoten's Kakutou Ace in 1996. It was an early attempt at restarting the Kinnikuman manga, which would eventually lead to Kinnikuman Nisei two years later.
It is notable for introducing Kevin Mask and Kuan. Featuring to date the only appearance of a teenage Meat in its original printing. Because of these oddities, the story doesn't fit into continuity very well and should probably be considered non-canon.
The story contained a few inconsistencies with Kinnikuman Nisei (which began publication two years later), one of which being the appearance of a young Kevin Mask. His mask was different, resembling his father's, and according to his starting age in Nisei (18) he shouldn't have even been born when this story takes place.
The story was edited/fixed and re-released in 2001 in the Jump Remix series. Kevin Mask's look was changed into that of a little boy in school clothes, rather than looking like he was 14 with Robin Mask's armor. Meat, who was supposed to be in suspended animation while the story took place, was replaced by Geronimo. This version is contained in Kinnikuman vol. 37.