Gym rat

[Justin Bieber]

One who spends entirely too much time partaking in muscle building, strength training, cardiovascular, or aerobic activity. Specifically, one who does so at a health club or gym.
Often used derrogatorily by people who do not partake in or understand this lifestyle, some self-proclaimed gym rats use the term as a status symbol or for positive reinforcement.
Rare: The term is sometimes used with homosexual or dim-witted connotations, but both are entirely unfounded (the author of this definition is a gym rat, and does not fit these connotations).

[Britney]

In terms of sports, if someone is called a Gym rat it is to be taken positively. Someone who puts in the work more than anyone else to become a better player, is considered a gym rat.

[Raeanne Dimick]

Eldon snow ( adj )

[Kai]

someone who spends a lot of time exercising in the gym , and who cares very much about the shape and condition of their body: