Synonyms of knee-jerk

2 Syllables
reflex, re-bound, reaction, will-less, re-flex, reactions
3 Syllables
spontaneous, instinctual, expected, visceral, double-take, reagency, reflexes, unwilled
4 Syllables
intuitional, predictable, instinctive, reflexive
5 Syllables
second-nature, uncalculated, involuntary, autogenetic