As with many research questions, I first noticed this difference in the body pose of jockeys more-or-less by chance. I actually first noticed the "standing upright" pose in a old lithograph in a hotel lobby and thought to myself, "how odd is that?"
Then, when I started looking at images of horse riders over time, it became clear that something changed around 1900--riders before that time rode their mounts in a near vertical pose, while after 1900 the jockeys would crouch closely to the horse. When you see videos of great horse runs (e.g., YouTube video of horse races ), the jockey's body keeps a steady position, almost floating above the horse.
On my first attempt to figure this out, I went looking for a historical series of jockey images--from 1800 up to 2000. Alas, my attempts to find such an album in both Books and Images just didn't pan out. I gave the attempts roughly 10 minutes each, looking down to pages 3 and 4 of the results.
But it just wasn't working!
I next tried to search [ horse jockey pose ] and found an article in Time on this very topic! ("Secrets of Jockeying: Why horses go fast" by Jeffrey Kluger, July 21, 2009) As surprised as I was, I followed the suggestion in that article to find the Science Magazine article with the query:
[ Science magazine racehorse speed jockey ]
Why "Science magazine"? Because I knew that just the word "Science" would be much to general, but the two-word phrase "Science magazine" would probably find articles in the journal "Science."
Why did I use "racehorse"? Because the article in Time said "...the greatest single increase in racehorse speed..." and I figured that was probably a direct quote. What's more, the word "racehorse" is a little unusual, and I was hoping that the Science article would have used it.
And sure enough, the Science article is in the top 5 of the search results page! Fantastic!
Unfortunately, if you don't subscribe to Science, it'll cost you a bundle to get the article.
So I did a search on the title of their article ("Modern Riding Style Improves Horse Racing Times") and found that lots of other reputable sources were quoting it extensively. Ah ha! This was good news because it suggested that the article was both widely read AND quoted by other reputable sites.
My strategy at this point was just to find another article that quoted the Science paper extensively.
This wasn't hard to do. The article "Jockey 'monkey crouch' helped improve horse speed" published in the HorseTalk web site confirmed my impression. Around 1897 stirrup length changed from long (with a near-vertical riding posture) to short, with the newer "monkey crouch" style of riding. With a little more reading, you'll find that Tod Sloan introduced the monkey crouch to US horse racing in 1897.
As the article goes on to say, the newer style is much easier on the horse.. .but harder work for the rider. Not only do you have to be small and strong, but also very cardio-fit to be a great jockey!
And... as Ach444 commented yesterday--another good query is [ jockey posture ] which has a high quality science blog as the first result. (Wish I'd done that one first!)
There are many ways to solve this particular search challenge--but in all cases, be sure to look for multiple links to the same resource so you can see who's writing about this topic. You want to know if you can believe them!