From the Wiki, which isn't the greatest source but the one I could access most easily;

[quote]For a human trying to apply pressure to a Servant, even one physically the size of a child, it feels like trying to bend a steel beam. Punches with all of their strength will not even cause a Servant to budge a millimeter, and even Souichirou Kuzuki, whose punch is potent enough to crush a human's skull, can only slightly move Archer's head from the impact. Stabbing them with a knife does nothing but bring about a "prickling irritation" that does not hurt at all.[/quote]