Ken and his pals have been made with a huge variety of bodies. Some articulated, some not. First was not, the straight leg dolls. Then they, as Barbie and friends, got bend legs and the next was a turn waist. Not so much twist on the boys. The 1968 body, or rather torso, is the one that has been in longest use and even Skipper's boyfriends got it, although in a shrunk version - weirdly enough still marked 1968. Then, in the 2000's or 2010's new bodies were developed. Ken and the boys also got a Play Line Model body with a "hip stance". In 2017 a whole new era began with two new body types, Broad and Slim.

I will add more body pictures as I get the time to shoot any new photos.
 

Ken, he's a doll! That is how he was introduced to the Barbie world. The early Ken dolls were made with at least two different body types. They differ in height and have some other tiny details that keeps them apart.

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In 1968 Mattel got the copyrights for a new body type. The old, lanky and kind of odd version of the first Ken dolls was changed to a beefier and more muscular body type. Ken's waist was a non-twist but he had bendable legs (the old three-click knees) made of rubbery plastic. Below you have three bodies marked 1968 and a new Fashionistas Ken with a whole new type, marked 2016. Arms and hands differs between the three first types here, although they are all straight arm types.

"Edwardian"/Bendable legs Ken from late 60's, 80's Malibu Ken Hispanic, 90's Groom Ken, 2017 Fashionistas Ken.

Stamps:

1968 stamp "Edwardian" Ken (the original 1968)

1968 stamp on Malibu Ken Hispanic

1968 stamp on 90's Groom Ken

The mid 70's gave Ken bent arms with the Superstar dolls, but I don't have a nude "Superstar" to show you so the 90's dude here will have to do until then :) The bodies differs, the 70's superstar and the 90's boys. The newer body has molded underwear and another leg joint in the boxers. Besides that, the bodies are still the same. Even though most of the Ken bodies from the 90's have a new twist waist, the markings are still the same with 1968.

1968 twist waist on 90's bent arm/twist waist body

Midge's Dad, Happy Family (Neighborhood) Grandpa has a new body. He shares face sculpt with Harley Ken - with a (unknown) change but he has a brand new body with love handles. Grandpa's arms are not as strong as they used to be - they are thinner. Below a comparison with McDreamy, or rather Robert Phillip from Enchanted, Patrick Dempsey - in the flesh. Grandpa seems shorter but I think that is just an illusion! He has way longer arms, but that is due to the chimpanzee arms. I should really have had a Ken doll with twist waist but I had none readily available when I decided to take these photos, so McDreamy will have to do. At least he has the 1968 Ken torso.

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Happy Family Grandpa, marked 2003

These bodies are made of all hard plastic and came post Fashionistas. The first versions of the Fashionistas had articulated arms and legs, a feature that is missed! These model guys can't sit. At least not properly. They are just made to be clothes hangers - or to stand in a corner, looking cool. "Nobody puts Baby in a corner" - but dance? Not so much.
Some of these guys can stand well on their own but with their rounded feet and leaned posture it's sometimes hard to get them to be still. Ryan below tended to fall over, repeatedly. Steven stood as I put him and Basics made a rather good statue.
The two bodies to the right, here represented by a beach Steven and a Fashionistas Ryan are similarly built. Stevens torso is straight, while Ryan has that stanse - to the opposite direction from Basics Model #15. They all have sculpted abs and a hint of a six pack. Fashionistas have molded underwear, the beach bum not. Neither carries their own package even if they are somewhat hinting a bulge.

Back and front pics show the hip/leg joint angles. The play dolls (well, technically, they all are play dolls but the Basics are semi collector items) have slanted hips as the legs protrude from the "underwear line", while the Basics have a straight line/joint. Doesn't add much to articulation though. Ryan is the only one that slightly can move his legs apart but just by a fraction. The others just go back/forward and hardly even that.

Side view of the model types. Arms and legs are very well defined muscular wise. The hands on the cheaper Fashionistas are more detailed than the Basics. And harder to dress with the divided pointy fingers. They are close to the same height, hard to get exact height compare as their stanse is different and Ryan can't stand properly on his two feet. Basics have longer legs though, which makes him a fraction taller.

Adding Steven to that compare. They all have different arms!

The hand finish is better on Basics but the details are similar on all arms and hands.

Basics stamp:

Straight beach body (Steven) stamp:

Fashionistas model stamp:

In 2017 we saw a change in Ken. He went through a body change, just like Barbie did the year before. Ken only got three new bodies, while Barbie had four (both counting/including the new original body type). When I type this, I only have the Broad and Original bodies on hand, still waiting to get a Slim Ken.

Pictured from left is a Broad body/Fashionistas Ken #8, Original body/Fashionistas Ken #10, Broad body/Fashionistas Ken #13

Original stamp:

Broad stamp:

Slim stamp:

Added the slim body to the group, on the right. He is less buff than the other two, on left.
Now I need to find an old "original" to see the difference to them and the new "original" body type.

From left: Broad/Fashionistas Ken #12, Original/Fashionistas Ken #10, Slim/Fashionistas Ken #9

In the mid 90's we saw a new Ken type. He was a sports man. Gymnast, Skater, Skier etc. I think he arrived in time for the Winter Olympis in Nagano, but I am not nailing that fact down. These athletes were made with a slightly remade body from Michael, a guy from a 70's doll series by Mattel. Michael had articulated feet, while the athletes didn't. I added Malibu Ken Hispanic to the photo to show the difference in height - and also a Max Steel that was somewhat Mattel's answer to the Hasbro Action Man. As it happened here, the only Max Steel I had on hand was this guy with yellow torso, but he has been made with a plain skin toned regular upper torso as well. That body was used for a Ken doll in a Gift Set with Barbie. Max Steel is a bit shorter than Ken so they had to display the Ken in a new way in the box as he is shorter than Barbie as well. Anyway...

Professor Higgins, Fashionistas Ken, Malibu Ken, Max Steel

The articulated Gymnast is marked 1975, as inherited from Michael

Articulated Fashinistas Ken vs My Scene. The only difference here is the arm length (and head size of course).

These articulated bodies above have the same year markings; 1975/1999 - could be different set, but both years are present.

In 2016 we got a couple of new pivotal body sculpts, the Made-to-Move body for Barbie and this one for Ken. It was seen in the Superman and Batman dolls. They have different type of articulations and bending points. The Superman body have more points of articulation and is quite muscular.

More photos can be seen here.

Ken Pivotal Model Muse on left and Superman on right.

I have one Takara Ken on hand and while cleaning some doll mess, I decided to compare him too, with a regular 1968 Ken body. There are some differences. Takara Ken has longer legs, narrower waist and hips, smaller hands and no package bulk at all. Further, the Ken I have is all stiff plastic, so he can't bend his legs.

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Takara Ken is marked 1986.

His feet have noticeable features with pronounced toes. His feet are "squarer", than 1968 Ken's.

Well, not only Skipper's bfs but other dolls as well. (Ricky,) Scott and Kevin are were for a long time the only ones produced in the Skipper line of dolls and both boys have the same stamp as Ken - 1968, although their bodies are way smaller. They are just resized Ken bodies (Ricky share body with Skipper, so he is not included here right now). I had a Rebelde Miguel at hand to show off Ken's original size body. He will do for now. I also need to find the High School Musical guys and add one of them here. 2017 gave us a new boy in the Skipper line. I still haven't deboxed him, so no photos yet. 

Scott, Rebelde Miguel, Kevin

Added Spock to the lot to show an articulated body. Need to dig out one of those old gymnast bod types too!

Body markings from top left:
Scott | Kevin
Rebelde Miguel | Spock