After that kind of massive success, it's only understandable that Yudetamago were seemingly never able to follow it up with another hit manga, though not for a lack of trying... until a decade later.
Following the end of Gourmand-kun in Kadokawa Shoten's Weekly Shonen Ace, which ran from 1994 to 1996 for just four volumes, Yudetamago made a 60-page one-shot called Muscle Returns that appeared in Fighting Ace magazine. This was actually a sequel to Kinnikuman, and astonishingly enough Shueisha simply allowed Kadokawa Shoten to publish it, apparently not caring about any potential royalties, as it had published the wideban edition of the original series back in 1994; times were different, people. Kadokawa would publish the one-shot with some new bonus Kinnikuman stories as its own book, & it might actually also be included in Volume 37 of Kinnikuman itself, which was the first new volume released by Shueisha when the manga returned to serialization in 2010/2011; it's currently now at 79 volumes, literally 2.19x the length it originally was in 1987! However, Muscle Returns still had a great response to it, so in 1997 Shueisha allowed Yudetamago to create another sequel one-shot in adult magazine Weekly Playboy (no relation to America's Playboy; that was the now-defunct Monthly Playboy), a solid decade after Kinnikuman had originally ended. This second one-shot, Kinnikuman II-Sei (as in "Nisei/Second Generation"), introduced a new generation of "Chojin/Superhumen" & was the first of a five-part "Legendary Prologue" that would then lead to a proper serialized run in Weekly Playboy from mid-1998 to mid-2004, totaling 29 volumes. The success of Kinnikuman II-Sei also resulted in a "Revival Manga Boom", where old classics were brought back with "next generation" sequels, like Akatsuki!! Otokojuku, Ginga Legend Weed, & Ring ni Kakero 2. Despite being made precisely for adults who more than likely read the original Kinnikuman as children back in the day, there was definitely an appeal to Kinnikuman II-Sei that could be marketed to younger audiences as well, & this included Toei, which obviously would be all for recapturing the success that the original manga's anime had.
So, to celebrate the 20th Anniversary of the 2002 TV anime adaptation of Kinnikuman II-Sei, better known abroad as Ultimate Muscle: The Kinnikuman Legacy, let's start a four-part series of anime reviews with a theatrical "Double Feature"!