How often does the hip start extending during the sprint cycle?
It only STARTS extending once in a sprint cycle….however in light of my other question (regarding how you define leg) and what you've written previously in this thread I think the question was perfectly valid.
Should the [i](lower)[/i] leg swing forward just prior to the hip extending?
It's not a matter of whether it should or not because it does. This is unquestionable. In all sprinters the knee begins to extend (or swing out as you say) slightly prior to the hip extending. Should it extend out as in a "B drill?" No. But does it extend? YES.