In “Barbie and Action Man: Adult Toys for Girls and Boys, 1959-93,” Judy Attfield* discusses differences in two popular dolls, Barbie and Action Man (known in the U.S. as G.I. Joe). She points out that Barbie has very few joints–basically at her shoulders, hips, neck, and perhaps the waist) and is thus very limited in what you can do with her. Her arms are either entirely straight or permanently bent at the elbow. Some versions have slightly bendable knees.
Action Man, on the other hand, were designed with a number of joints in their arms and legs. Original models (available from the 60s until 1984) were marketed with an intentional emphasis on the movement the joints allowed:

Here’s the recent version of the character General Hawk, I believe from G.I. Joe: The Rise of Cobra (from Pulse Figures):

At least some dolls in the current G.I. Joe line have joints in their hands so it’s easier to make them hold things.
Attfield’s argument is that the design of these dolls illustrates assumptions about gender. She says, “…the Action Man figure embodies the increasing possibility of complex movement. Conversely, the simplicity and minimal number of joints in Barbie suggests the priority given to posing rather than action” (p. 82). She is, essentially, for dressing up and looking at, but that’s about all (and getting outfits on over those stiff arms was often quite the task).
The New Living Barbie, which came out in 1970, has more bendable joints and was advertised with the line “as posable as you are”:

So even when a Barbie was introduced to have more flexibility, the emphasis was still on posing–the box doesn’t say “as active as you are,” after all.
Thus, according to Attfield, “…the cliché of ‘feminine’ as passive and ‘masculine’ as active is literally embodied in the design of toys” (p. 85). (She also points out that while doll lines aimed at boys include “bad” characters that serve as the enemies of the protagonists, those marketed to girls, such as Barbie, have yet to include “bad” characters. There’s no evil Barbie who tries to steal Ken away.)
I thought of this when I saw the image Brian Brock sent in (found at Tennis Forum) of a new Barbie meant to represent tennis player Kim Clijsters and her daughter, Jada. Unlike other Barbies, this one clearly has multiple joints that would allow a wider range of motion (though she’s still on her tiptoes, of course):

In this case, some of the design elements that allow more movement in G.I. Joe-type dolls have been incorporated into a Barbie, making her more able to hold things, swing a racket, appear to be running, and so on.
As it turns out, however, this Barbie won’t be available in stores. It’s a showcase model to be shown at toy fairs and the like. So we have a Barbie designed to be less passive…but she’s not actually for kids to play with.
UPDATE: Reader Holly points out that Mattel has experimented with more “action” oriented Barbies before with the WNBA line:

They were actually available in stores. However, I don’t think this really undermines the overall point. There’s no reason that ALL Barbies couldn’t be built like this and still fit into the regular Barbie clothes. But the increased flexibility is incorporated only into “special” Barbies that seem to be sports-related, not the main Barbie line.
* From The Gendered Object, edited by Pat Kirkham. 1996. NY: Manchester University Press, p. 80-89.
(View original at http://contexts.org/socimages)
