How Effective were Spears Against Cavalry?

[Domen]

They were usually better trained, equipped and supported than most medieval contingents.

Technological level of Ancient armor was nowhere near that of Late Medieval and Early Renaissance plate armor. Also horses used by heavy cavalry in the Middle Ages were larger and of better breed.

This had no match in the Antiquity:

The main difference that led to such tactics in later years was the vast decrease in the quality of infantry,

You would have to precise which exactly period and area of the Middle Ages do you mean.