The Collaborative International Dictionary of English v.0.48:

Tonus \To"nus\, n. [L. a sound, tone. See Tone.] (Physiol.)
   Tonicity, or tone; as, muscular tonus.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tonus
    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]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "tonus":
   adaptability, bounce, bounciness, buoyance, buoyancy, elasticity,
   extensibility, flexibility, give, liveliness, rebound, resilience,
   resiliency, responsiveness, snap, spring, springiness, stretch,
   stretchability, stretchiness, tone, tonicity