Founder/developer of http://www.gympro.com. Gympro is a fun but classy website so it deserves a matching twitter page design.
Fitness professionals, people getting coached by fitness professionals. Should be enthusiastic, easy on the eye and interesting.
The only requirement is that the Gympro brand is easily identifiable so colours should match the website design (black, grey, yellow, white). Use of the logo and slogan which you can find on the website are highly encouraged. The rest is up to you.