Noun
someone who spends all leisure time playing sports or working out in a gymnasium or health spa
Source: WordNetgym-rat
To put it another way, Michael Jordan was a gym rat. Frank Shorter
I am a gym rat who loves to work out, particularly running, Zumba, yoga, cycling, and kickboxing. Aprilynne Pike
I kind of became a gym rat. The more results I saw, the more dedicated I became. It is such a great feeling to see your hard work pay off. I mean, if I'm gonna skip the In-N-Out burgers, there better be light at the end of the tunnel! Jonathan Lipnicki
Actually, I'm not a gym rat. I'm not a gym person - I've never been. I've always been blessed to be thin. If I'm waiting for the kettle to boil, I'm doing 15 lunges. Paige Butcher