In the John Carter movie (2012), he shows off some of his powers, like jumping abnormally high, but I have difficulty evaluating his strength.
On the one side, he shows great strength, as when he kills a thark warrior with one hand, but he is also quite mistreated by them. He also seems helpless when he is strangled by Tars Tarkas.
Why does the strength he shows seem so inconsistent?
In the books it's not so inconsistent. (There are a number of issues one can find with Edgar Rice Burrough's writing, but that's not one of them.)
John Carter was born on Earth, then later transported to Mars. Martian gravity is 38% of Earth's gravity, or roughly 1/3 of Earth's. Which would make him roughly three times stronger than a normal inhabitant of Barsoom.
That might affect his ability to break bones and jump, but it would not mean he could break metal objects any better than he could on Earth.
I haven't seen the movie yet, so I can't tell you how close it is to the books or in what points it totally misses, but one point that was raised over and over in the books was his great strength while on Barsoom.
I think the best demonstration of Carter's real strength in the movie is while he soloed the Warhoons. There he used plenty of his strength and consequent speed and almost killed everyone until he got buried by the bodies in such a fancy Hurin's (The Silmarillion) way. Anyway in every situation he could have escaped or avoided being beaten by its superior physical abilities and didn't may be attributed to, as previously said, psychology, or even an argumental ellipsis in the movie, maybe to give more dramatism or to make the character not to appear as a tyrant, to make the audience empathize with him.
I think the main reason would be psychological. When he is being manhandled by his care takers, he isn't really trying to resist all that much.
But yeah, there are some inconsistancies. When he is sword fighting and his opponents attempt to parry his strikes, his strikes should be so hard that they break their wrists/arms. Similarly, when he hits anyone full force, it should be enough to kill.
But truthfully, I thought the film was so much fun that I didn't care so much.
I believe he seemed "helpless" when being throttled by Tar Tarkas was mainly because he was reluctant to risk hurting his friend (and while Carter is stronger than any Thark, a blocked windpipe is still a blocked windpipe...)
