Here's my work in progress.
I like the gesture/pose.
That right lower leg is looking odd and bulky, and I also think that either the back leg is too small/short or the front leg is too large/bulky. In other words, the perspective on the legs is not agreeing with the perspective on the upper body.
I have to say that I don't like the pose at all, it's silhouette is not clear and it does not really look like running in the way that he'd be actually running, more like someone trying to pose still as if he was running. Like, I can't say that I run too much, but I don't think you need to lift your knee that high (might be more understandable if he were tilted more towards the ground overall) and it doesn't really come off very dynamic looking either and since this is Flash, having both of his feet off the ground would work in my opinion.
Also look out for the angle of the foot, right now the knee is facing more towards us, but his foot is in profile.