Bodybuilding and fitness can be done for 3 reasons IMO.1.Competition/intimidation- Pluto/Mars
2.Defense against threat-Moon
3.Aesthetics and Sex appeal-Venus/Sun
4.Health and body vitality- Mercury/Saturn
Most people nowadays are doing it for reasons 3 and 4. Whilst in the past, it was largely because of 1 and 2.
Mars in 1st house people would be more inclined to reason 1. They want to look "fierce".
But if Mars in 1st house has Mercury influence, the reasons are two-fold; look fierce AND be healthy or have lots of stamina for life.
If Mars in 1st has Sun/Venus influence, the reason could be to look fierce AND sexy!! To get attention from suitors.
It is the 2nd reason,with Moon reacting with Pl/Ma that the need to use the body to defend against uncertain but threatening and imminent "enemies" that I would think the most dangerous.
In the latter instance, the native keeps the body in constant anticipation of a danger or threat that could come "anywhere". It can create quite a paranoid personality.
Extreme sports or dangerous workouts could appeal with someone who has Pluto/Mars/Moon reacting with each other in signs Scorpio/ Cancer or Aries.