Some are stiffer than others, that is, they develop more force during a given amount of elongation. In this figure of muscles' passive stress-strain curves , the stiffer the muscle, the steeper its curves' slope. The distinctive stiffness properties of any soft tissue are illustrated by its length-tension curve. The knee's anterior cruciate ligament has a distinct length-tension curve. In addition to displaying the property of elasticity, many tissues display viscosity, a resistance to flow.
Viscous materials resist elongation more at higher rates of elongation.
The passive length-tension curve of a viscous material has a slope that changes, increasing as the velocity of elongation increases. The faster the rate of elongation, the stiffer the material's behavior. A problem of neurologic or muscular impairment. Evaluation of and methods to change muscle tone.