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Weider Magazines such as Muscle Builder and Power, now called Muscle and Fitness, had nicknames for most, if not all, the bodybuilders featured in its pages. Arnold Schwarzenegger was “The Austrian Oak”, Dave Draper was “The Blond Bomber”, 2015 Mr. Olympia Phil Heath is called, or maybe calls himself, “The Gift”, to name a few. Sergio Oliva was called “The Myth”. Let’s defer to Wikipedia on the origin of that moniker:
Sergio Oliva (July 4, 1941 – November 12, 2012) was a Cuban bodybuilder known as “The Myth”. This sobriquet was arguably given to him by bodybuilder/writer Rick Wayne but Oliva himself has doubted this claim. Supposedly Wayne had begun calling Oliva “The Myth” (because everyone who saw him at the 1967 Montreal World’s Fair said he was “Just unbelievable”).
It is generally agreed upon that the name “The Myth” means unbelievable, mythical, legendary, or something along those lines because there was no one around at the time with that much muscle mass and long muscle attachments combined with small joints and such a small waist. Now, let’s break down the name origin because Wikipedia says that bodybuilder and writer Rick Wayne coined the term “The Myth”. In the same sentence it says that Sergio Oliva didn’t believe that Rick Wayne coined the name. Footnote  gives us this information:
Oliva, Sergio; Marchante, Frank (2007). Sergio Oliva the Myth-Building the Ultimate Physique. Gras Publishing. ISBN 0-9779040-1-6.
Frank Marchante, a Florida-based professor and gym teacher, co-authored the highly researched book with Sergio Oliva. This book is apparently the source which states Sergio doubted that Rick Wayne coined the term. Note to authors: Please make your books available on Kindle. Kindle has a “voice to text” feature that reads the book to you. This allows the reader to audibly absorb the contents of the book while driving, lifting weights, lying in bed waiting to fall asleep…instead of taking valuable time away from that reader(or listener) who just wants to know the information in your book and doesn’t care if they hear or read that information. Knowledge is knowledge, it still counts if you hear it instead of reading it. Unless a book has life changing info, I’ll only purchase the Kindle version and ONLY if the Kindle version has “text to voice” enabled. Maybe someone has read the book and can tell us why Sergio doubts Rick Wayne’s claim.
Footnote is apparently where Rick Wayne claims to have coined the Myth term and the book is on Kindle with text-to speech enabled. Good for you, Rick Wayne!:
Wayne, Rick (1985). Muscle Wars. St. Martin’s Press. p. 257. ISBN 0-312-55353-6.
Right now, the only thing I can say with certainty is that the term “The Myth” was already firmly established in the April 1970 Muscle Builder and Power article by Rick Wayne entitled FROM WHERE I SAT. It is likely that Rick Wayne, at the very least, popularized Sergio’s nickname and there is evidence to back it up:
But the 1969 show promised the greatest treat of all. Sergio Oliva, the myth that breathes, that muscle phenomenon of the Weider generation who had already accounted for the demise of Poole, Sipes, and yes, some say even Larry Scott, would be facing the greatest threat to his bodybuilding supremacy. The fans had not forgotten the fact that no one dared show up against the Cuban emigre in 1968; and just when Muscledom was beginning to accept Sergio Oliva as its undisputed king, a comparatively unknown upstart had decided to challenge “The Myth’s’ right to his coveted position.
If you want to delve into the possible progenitors the “Austrian Oak” sobriquet/moniker/nickname, here is a start: https://www.builtreport.com/arnold-schwarzenegger/arnold-schwarzenegger-and-the-german-oak/