If you're looking to focus more on the quads, an upright torso position is best.
Taking a longer stride (with a vertical tibia) is more hip-oriented version; taking a shorter stride with a slightly angled tibia is a more knee (quad) oriented version.
So, if you're really trying to hammer the quads, you'd combine an upright torso with a slightly reduced stance length that creates a tibia angle similar to one involved in a standard squat.
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