The Collaborative International Dictionary of English v.0.48:

Tonicity \To*nic"i*ty\, n. (Physiol.)
   The state of healthy tension or partial contraction of muscle
   fibers while at rest; tone; tonus.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tonicity
    n 1: the elastic tension of living muscles, arteries, etc. that
         facilitate response to stimuli; "the doctor tested my
         tonicity" [syn: tonicity, tonus, tone] [ant:
         amyotonia, atonia, atonicity, atony]