Muscle tone.

Muscle is never completely at rest; it may appear to be, but it is always in a condition of muscle tone, which means ready to respond to stimuli. For instance, the knee-jerk obtained by sharply tapping the patellar tendon results in contraction of the quadriceps extensor of the thigh and slight extension of the knee joint. This is reflex produced by stimulation of the nerves. Posture is determined by the degree of muscle tone.