Yevamos 102 - 103 - A Couple of Points:

1. Gemara equates Yom Kippur and Chalitzah regarding na'al (anpilya). By chalitzah the gemara defines a shoe that is fully leather (and acc. to tosafos must even be a kosher animal). This is not a special din in chalitzah but rather the definition of a shoe as learned from the pasuk of "v'enalecha tachash". Based on this only a fully leather shoe should be assur on Yom Kippur!

2. Ameimar who says that the yaveim must put his entire foot on the floor, is this l'chatchila or even bidieved? Tosafos says that the braisa which says he can be sitting or lying is only bidieved, but l'chatchila he must stand. The gemara asks on ameimar from this braisa. Based on Tosafos we should be able to answer that the braisa allows lying down only bidieved, but l'chatchila one must put their entire foot on the floor. What then is the gemara's question?! It must be that ameimar is even bidieved, that his foot must be fully on the ground to qualify as chalitzah, then the gemara has a fair question from the case of lying down. To exclusion of Ra'avad who says that ameimar is only l'chachila.

3. "Shok" in the gemara refers to the calf, not the thigh. Chason Ish (O.C. 16:8) discusses "shok b'ish ervah" - the leg of a women is an ervah, whether this refers to the calf or the thigh. He suggests it refers to the calf (as our gemara would indicate) and therefore would requires skirts to the ankles meikar hadin. He then concludes that it could mean the thigh, in which case it would be permitted to have skirts down to the knees.