Well, since no one else wants to tackle this one... i'll do my best to answer. Truth be told, I think you need to be a bit more specific in your query. Opportunities could be defined as commercial (endorsements for example), amateur or professional (fitness competitions, powerlifting, etc.) just to name a few. Not really an area i'm knowledgeable in, but check out the following link at your leisure. It provides country specific fitness and bodybuilding links with about a dozen for Japan (and half of those in English). It may or may not be of use to you.
Generally speaking, I think the Japanese are a very health-conscious people (as demonstrated by their long lifespans and natural diets). Of course, the flip side is that smoking and alcohol consumption are pervasive as well among adults, go figure.
Speaking from personal experience only, most Japanese I came into contact with were very fit. This starts at an early age I believe. The majority of students I worked with in the schools either walked or rode their bikes over long distances each way (whereas here in the U.S. almost everyone takes the bus if it's more than a half-mile).
Many of my adult colleagues as well were all involved in either Kendo, Taiko, or some other activity that required physical exertion and I can't think of anyone offhand who was out of shape or morbidly obese. Not to say that such persons are non-existant, but they are certainly a minority I would think (compared to the U.S. for instance).