If you've ever done any serious running, you're probably familiar with the runners' nod. When two runners run past each other, there is a sort of ritualized greeting. It's like you're both members of some club and thus have to greet each other with the secret handshake, even if you've never met before. While two strangers walking past each other on the street hardly feel the need to greet each other, runners generally do.
In the U.S., this is the runners' nod; you meet eyes with the other runner and just nod at them briefly, with a brief smile being optional. (We'll often do the same to cyclists as well. I wonder if they call this the cyclists' nod.) A nod, as opposed to a vocal greeting, is used by necessity; often, on a run, you don't want to say anything because that could screw up your breathing, potentially resulting in cramps, side stitches, etc. A nod is subtle enough to let you greet the passerby and maintain your form at the same time.
The nod, however, is not universal. Nevertheless, some mystical runners' bond does seem to be somewhat universal, so that even in places where strangers would rarely greet each other runners still seem to do so. What changes is not the fact that there is a greeting, but rather how it is done.
In Japan, for instance, the runners' nod becomes the runners' bow. Instead of a brief bending at the neck, you do a very brief bow, bending instead at the waist. In order to avoid breaking your running stride, this bow is generally very limited; even in cases where politeness would require a somewhat deeper bow, runners don't bother. Here the typical rules of politeness take a bow (lame pun fully intended) to the rules of good running form and allow you to minimize the bowing.
If you do the nod in Japan, people will probably get your drift, but it's like speaking with an accent; they understand you, but you're clearly not quite there yet in the "language". Yup, I'm saying that body language is part of learning a language, and it's one that's very rarely covered in any book. So be sure to not just listen and read, but to watch as well; watch how native speakers move when they're talking. Mastering the body language used by native speakers will go a long way to making you seem more like one.