When you think of a film featuring a strong musclebound man, your first thought will probably be something with Arnold Schwarzenegger or Sylvester Stallone. Indeed, both men have something of a trademark in showing off their bodies, and you certainly would not be unjustified in thinking of them right away. Schwarzenegger certainly has a body to be admired, which comes from him getting his start as a weightlifter. In fact, the film that first brought Arnold to fame and made him a star was Pumping Iron, a documentary which followed him and several other weightlifters through a series of championships.
Indeed, the whole profession of bodybuilding in general is essentially built around making a spectacle of one's own body. Back in the olden days it was not so unusual for circuses to have a "strong man" act, which consisted of a huge muscular man to step on stage and demonstrate his "incredible" strength. The idea of using a man's body to create a spectacle is one that goes back a lot farther than one could think, and it it hardly confined to action movies.
One of the earliest examples of such an act comes very early in the history of film, back when film was little more than a novelty. Here we meet Thomas Edison, one of the biggest jerks in American history. In addition to inventing the electric light bulb, Edison also had been working with one D.W.L. Dickson in developing the phonograph, and was interested in the idea of giving it a visual accompaniment (these two are responsible for one of the earliest experiments in synchronized sound). This led to the discovery of celluloid as a way of recording moving images (prior to this, the closes thing to "film" was series photography, along with other pieces of equipment such as the zoetrope and the "magic lantern").
Unlike many other filmmakers, Edison saw a clever way to make lots of money off this new discovery in fairly cheap manner. Edison was not fond of anything beyond making lots of money off a simple fad, and in fact he went on to single-handedly hold cinema back from its full potential by accusing other filmmakers of "patent infringement". Later on after a ton of lawsuits he formed an alliance with other contemporary filmmakers called "The Trust" that would hold an oligopoly over the global film market and demanded anyone interested in making movies pay them a licensing fee. These same people also forbid the production of feature films because they preferred to extort money out of people paying to see individual pieces of a serial and shorts than to allow them to purchase one whole movie at once.
What Edison did with his discovery was to create a machine known as a "Kinetoscope". Kintetoscopes would often be found in public environments, and worked as small contraptions where a customer could place some change in a slot, and look through a lens that would allow them to briefly see a moving image. Many of these films can still be seen for free today, and compared to modern standards they usually were not anything particularly spectacular. One film in particular, "The Kiss" sparked a huge controversy... because it consisted of a kiss between a man and a woman.
Edison was working in an era of "Cinema of Attractions", when the mere idea of a moving picture was enough of novelty to attract viewers. The images themselves, less than a minute and generally depicting very little, were fascinating to viewers who had never seen anything like it before. There is one film in particular that I would like to bring up, Sandow (The Strong Man). This particular film consists of nothing more than 20 seconds of a muscular man, wearing nothing but underwear, posing for the camera.
This particular film is somewhat unique among the early Kinetoscope works in that it isn't just the spectacle of a moving image that is meant to interest the viewer. It also clearly intends to impress spectators with the impressive body that is displayed. The man seen here, Eugen Sandow, (1867-1925) was in fact a real bodybuilder. In fact he was one of the first to really make a career out of it and was promoted as "the father of modern bodybuilding", which makes his single appearance in Edison's Kinetoscopes the first film to use a celebrity as a means of attracting attention; essentially an early form of stardom.
In later years, muscular display would not be as popular an idea, especially when early viewers became bored with paying to see a brief moving image and films shifted focus to telling stories. Story began to come above all else, and everything had to in some way serve to advance the story. Spectacle was still used in sorts, as with slapstick comedy, but in later years studios would prefer to attract viewers with elaborate sets and costumes.
Still, there was at least one major studio-era actor who was able to make a spectacle of his body. Kirk Douglas didn't do it all the time, and when he did it had the same effect as Sandow. Spartacus would naturally be one of the best examples of Douglas displaying his body. In fact it's literally put on display at one point, when a slavemaster paints on his abs to show his gladiators in training where they should strike.
Much like Schwarzenegger and Stallone later on, Kirk also exposes his body for many of his action scenes, and even when he is clothed his muscles are still exposed. This is the most extreme example, but it can be seen in some of his other films as well. In 20,000 Leagues Under the Sea Kirk Douglas never appears shirtless, but does spend most of it in t-shirts that expose his muscles and there's no doubt he gets to show off his strength.
Martial arts films were very much about muscles, and this was a big part of the stars that appeared in them. People like Bruce Lee were experienced in the field and were purported to be capable in real life of actually doing the same types of things they do in their movies. A lot of them were also trained in dance as well, which plays into the choreography of the fight scenes (as hard as it may be to imagine now, Jean-Claude Van Damme was actually trained as a ballerina). Naturally muscular display becomes an important part of martial arts films, where the display comes from a mix of the impressive body and seeing it in motion. It leads to a concept known as "muscular sympathy", where the viewer experiences a bodily feeling as though they are part of the action.
Enter The Dragon demonstrates this quite clearly, especially during the climax when Bruce Lee is forced to pursue the villain through a hall of mirrors. It allows us to see him from all angles, and he spends his whole time shirtless, with injuries also being visible on his chest. By contrast the villains are generally clothed and don't rely so much on their fists, especially the villain who replaces his hand with a gigantic claw. In a genre that believes there is honor to be found in beating people up, a display of muscles is naturally to be expected.
That brings us to the more famous incarnations in the form of the "hardbodied heroes" of the 1980's. Among other things, hardbodied heroes are known for displaying their impressive bulk, and often contriving situations to allow it to be seen. This is why Rambo is always running around shirtless. Commando has a scene where Arnold Schwarzenegger rows a boat ashore without a shirt for no apparent reason beyond giving the audience a chance to admire his body. When he begins suiting up he just puts on a vest that still allows us to see his gigantic biceps.
This time round, it's a woman who gets to show off her body for once. By the events of Terminator 2: Judgement Day, Sarah Connor has developed into a far more "masculine" figure compared to her previous appearance in The Terminator. In keeping with the film's challenging of gender conventions, Sarah develops some very noticeable muscles in the sequel, and becomes something of a spectacle herself the same way Schwarzenegger would be in any of his other films. Much like male hardbodied heroes she almost exclusively wears outfits that wholly expose her muscles, ranging from the t-shirt she wears in the psychiatric ward to the tank top she wears throughout most of the rest of the film.
Sarah Connor became crucial to breaking the trend of "macho men". After Terminator 2, we get a whole line of women displaying their muscles. Angelina Jolie would go on to do something very similar in Lara Croft: Tomb Raider. Uma Thurman also got to show hers off in certain portions of Kill Bill (though her default outfit is still the bright yellow jumpsuit). On television you also get Lucy Lawless as Xena, and Buffy Summers to a lesser extent.
The idea of muscular display still continues today. Somewhat infamously, the Twilight series attempted to create a bodily spectacle in the form of Jacob Black. To an extent it worked, as it managed to attract the relentless admiration of countless fangirls, although for everyone else it gave the series one more aspect to make fun of. Taylor Lautner's character became infamous for almost never putting on a shirt in favor of trying to impress the viewer with his muscular build. While this might not be as effective an example, the basic principle and intention is still the same as that of Sandow or Schwarzenegger.