Jess Nevins, author of The Encyclopedia of Pulp Heroes, gave the paper “Those Who Cannot Remember Doc Savage Are Condemned To Repeat Him: The 20th Century Backlash Against Posthuman Bodybuilders” at the December 4, 2009 IEET seminar on the Biopolitics of Popular Culture in Irvine California. MP3
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I've read some Doc Savage, and I noticed that he seemed to be getting less powerful over time. Interesting that the same thing happened with the Avenger and the Shadow and several others.
Perhaps they just decided to leave the superpowers to the comics, which were doing just fine with super-duper characters.
What about Tarzan? He never got weak, never lost his ability to swing through the trees, never lost the ability to track by scent.