A one-shot chapter set eight yeas after canon events in Kinnikuman. It is non-canon.
Muscle Returns (マッスル・リターンズ) was a one-shot follow up story set eight years after Kinnikuman's conclusion. It was published 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. It features - to date - the only appearance of a teenage Meat in its original printing. Because of these contradictions and inconsistencies, the story does not fit into any current timeline or continuity, and as such it is considered non-canon. It is also worth nothing that this one-piece was rewritten at a later date, in an attempt to make it closer to the canon of Kinnikuman Nisei.
(More to Come)
The story begins with a brief summary of past events. 
It describes how the Seigi Chojin began an era of peace, which led to them being unprepared for forces of evil. The story takes place on July 10th, at the Nippon Budokan. It is the first Chojin Ultimate Championship. A new chojin - known only as Buki Boy - enters the contest. 
He defeats Warsman, and proceeds to defeat Buffaloman in the ring during the semi-finals, with a record for defeating all his opponents in less than five minutes. He breaks Buffaloman's neck, and it requires all of Harabote Muscle and Terryman's strength to stop Buki Boy from murdering Buffaloman even after the match has ended. 
Buki Boy challenges Terryman to the finals, with Geronimo as Terryman's second for the fight. He dislocates Terryman's limbs, in order to tie him up, and defeats him without any real competition, before claiming the championship belt. The crowd cheers for Buki Boy. 
Robin Mask swears he will defeat Buki Boy, despite being forced to sit out of training - and the contest - due to an injured rib-cage. He will not call back Kinnikuman, whom he thinks deserves a rest, and is determined to fight Buki Boy in the Tokyo Dome three days from Buki Boy's victory. Meanwhile, in the Southern Alps in Nagano, Kinnikuman can be seen fishing with his feet. It is revealed he has made friends with a teenage boy named Kuan. A young Kevin Mask comes out of the woods to find Kinnikuman. 
Kevin reveals that if Robin fights, his rib will pierce his lung and kill him, and so he begs Kinnikuman to fight on his father's behalf, but Kinnikuman pretends to be an anonymous nobody and hides. Kuan promises a devastated Kevin that he will convince Kinnikuman to fight for him. Even though Kinnikuman loves to wrestle and still trains, he is reluctant to fight, but the Yujo Power convinces him to fight. 
He goes to the Tokyo Dome at the appointed time, only to fight Buki Boy soundly defeating Robin Mask without any competition, and Kinnikuman intervenes to take over the match. Buki Boy initially has the upper-hand, but the cheers from the crowd inspire Kinnikuman onward. Terryman cries with joy, as he watches on television from his hospital room, as Kinnikuman defeats Buki Boy with a Butt Buster. The story ends with the crowd and Seigi Chojin celebrating the win, as Kinnikuman clumsily falls out of the ring. 
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 fourteen with Robin Mask's armour. Meat, who was supposed to be in suspended animation while the story took place, was replaced by Geronimo. This version is contained in Kinnikuman volume thirty-seven.