Tonus :  (noun)

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

adaptability, bounce, bounciness, buoyance, buoyancy, elasticity, extensibility, flexibility, give, liveliness, rebound, resilience, resiliency, responsiveness, snap, spring, springiness, stretch, stretchability, stretchiness, tone, tonicity,