Muscle tone is the continuous and passive partial contraction of the muscles. It helps maintain posture.
Unconscious nerve impulses maintain the muscles in a partially contracted state. If a sudden pull or stretch occurs, the body responds by automatically increasing the muscle's tension, a reflex which helps guard against danger as well as helping to maintain balance.
Usage note
Some people use the term muscle tone differently, to refer to how "in shape" a person is. Technically, what they are referring to is muscular strength or a low fat-to-muscle ratio. "Toned" in that context means "fit" or "trim".
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