Well a bit late to answer this question but no one else did so here it goes...

I took a step forward and I can definately say it wasn't my leg that moved first, but I'm not fully sure what moved first. I did notice the first thing I did was shift my weight to the leg I wasn't lifting and in order to do this I would suspect that I shifted my hip a bit sideways, however I'm very, very, very unsure about this, so someone else please answer!

Oh and one more thing, I think this is relevant to Judo in the sense that you need be able to control the opponents weight when you're throwing, so knowing what moves first to shift the weight would be useful to know.