The Collaborative International Dictionary of English v.0.48:
atonicity \atonicity\ n. 1. 1 lack of normal muscular tension or tonus. Syn: atony. [WordNet 1.5]WordNet (r) 3.0 (2006):
atonicity n 1: lack of normal muscular tension or tonus [syn: atonicity, atony, atonia, amyotonia] [ant: tone, tonicity, tonus]