In Robert Louis Stevenson's "Dr. Jekyll and Mr. Hyde", the latter character was depicted as, while vicious and sinister, an ordinary human in terms of physical strength and endurance. However later works tend to paint Mr. Edmund Hyde as someone with strength and endurance of superhuman levels more comparable to the Incredible Hulk- when did this trend start?
Something of this idea is present in the original text of The Strange Case of Dr. Jekyll and Mr. Hyde (1886), with Jekyll's final account saying:
I became, in my own person, a creature eaten up and emptied by fever, languidly weak both in body and mind, and solely occupied by one thought: the horror of my other self. But when I slept, or when the virtue of the medicine wore off, I would leap almost without transition (for the pangs of transformation grew daily less marked) into the possession of a fancy brimming with images of terror, a soul boiling with causeless hatreds, and a body that seemed not strong enough to contain the raging energies of life. The powers of Hyde seemed to have grown with the sickliness of Jekyll.
This is in relation to their levels of energy rather than size or strength as such, and it's not suggested that Hyde has more physical capacity than a human. He is certainly described as smaller than Jekyll (the doctor's clothes are loose on him), but hairy and "ape-like" - therefore possessing a primitive and unrestrained strength that the civilized Jekyll lacks.
I think the first work to really seize on this was the League of Extraordinary Gentlemen comic by Alan Moore and Kevin O'Neill . In its first volume (issue 6; or p134 of the collated volume, 1999), Jekyll ruefully remarks that Hyde is currently too large to need a firearm, asking "Do you know, I was once taller than he was?" This Hyde is a towering gorilla-like figure. The reason is explained in the second volume (issue 8; or p121, 2000):
Hyde: I mean, when I started out, good God, I was practically a ****ing dwarf. Jekyll, on the other hand, a great big strapping fellow. Since then, though, my growth's been unrestricted, while he's wasted away to nothing. Obvious, really. Without me, you see, Jekyll has no drives... and without him, I have no restraints.
This is a development of what was happening at the end of Stevenson's original, if we imagine Jekyll/Hyde's survival and make his comparative strength more literal.
I think that most adaptations have been interested in Hyde's depravity in the context of his origin story, with League being unusual in putting him in something of a superpower ensemble that still draws from the themes of the original text. I also have the sense, from the way that Hyde is presented here, that Moore is expecting the reader to notice his size as unusual - it's not taken for granted that a super-strong Hyde is something that we're meant to recognize.
If you count the Marvel Comics version of Mr. Hyde, he made his first appearance in December 1963, and it was emphasised from the start that his strength was superhuman.
However, his alter-ego was Calvin Zabo rather than Henry Jekyll, and Robert Louis Stevenson's original novella was referenced as a work of fiction within the Marvel universe.
HYDE: He has no way of knowing that my strength has been magnified a dozen times, so that I am as powerful as any twelve normal human beings!
Journey Into Mystery Vol. 1 #99 (December, 1963)
I went through the list of films based on the Jekyll/Hide story on (German) Wikipedia.
Physical transformation seems to have been a part of the trope from the beginning, but more often towards a smaller/more "depraved" looking persons. Physical violence did not seem superhuman at first, but more a "berserker" type of thing (normal human strength enhanced by animal rage rather than any truly supernatural thing).
The first in that list that mentions a physical transformation into a creature of superhuman strength is the 1976 "Blaxploitation" movie "Dr Black, Mr Hide", in which the protagonist (named Mr. Pride, clearly alluding to Hide) changes into
a white-skinned Frankensteinian monster with superhuman strength and invincibility
The English language wikipedia mentions an earlier Indian adaption of the story, Karutha Rathrikal from 1967 in which a medical doctor
develops a medicine, which transforms a person into a monstrous creature
So at least as far as Wikipedia-worthy movie adapations are concerned, this seems to be the first one that unambiguously refers to superhuman powers. That is, unless we are allow to stray away enough from the source material to include cartoons - in the Looney Toons cartoon Dr. Jerkyl's Hide from 1954 Sylvester the cat is apparently transformed into a monster cat after he
accidentally ingests Dr. Jerkyl's formula (thinking it is soda pop)
That Warner Bros. parodied this suggests this was already a trope, so there might be earlier examples.
For what it's worth, the original text specifically refers to Mr. Hyde as a Juggernaut, in Chapter 1 (Story of the door):
Well, sir, the two ran into one another naturally enough at the corner; and then came the horrible part of the thing; for the man trampled calmly over the child’s body and left her screaming on the ground. It sounds nothing to hear, but it was hellish to see. It wasn’t like a man; it was like some damned Juggernaut.