anyone who tells you an image isn't "stock" hasn't sold stock photography. ANYTHING sells - I think this is a great shot - would love to see a new ball, but it would still sell. I'm amazed at what sells - I send it ALL in, whether I think it's stock or not.
A solid effort and as a statement of fitness, works on a couple of levels. The use of DOF is different and it might have been better to focus on a sharper foreground with a gentle fade into the dark background the author's choice is perfectly understandable.
Very nice shot, I think I would have turned the ball so the brand name wasn't right there if this were really going to a stock photography site, but I love the perspective on this and think it's a job well done.
I am not voting on this image but if I did I would more than likely give it a 4 or lower.
Let me explain, I "personally" don't think you met the challenge's request to take an image of a "stock" photo of fitness. Your photo isn't bad but I just don't think it's "stock" or "fitness".