Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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