Muscle tone or tonus is the slight contraction of muscle even when it is at rest. Muscle tone comes from having strong muscles. Athletes who look incredibly toned are that way because they are strong.
It has long been assumed that that the best way to increase muscle tone is to do lots of repetitions with light weights – not true. The best way to increase muscle tone is to increase muscle strength, and the only way to increase strength is to place demands on the muscles greater than what they are used to handling. When this occurs the body responds by building bigger, stronger, more toned muscles. This is body’s form of self-protection.
When the body is presented with demands well within the parameters of what the body can handle there is no need to increase strength. Without an increase in strength there is no increase in tone.
With an increase in tone comes a slight increase in metabolism. Constant partial contraction of muscle requires additional calories. You can be stronger, more toned, more flexible and less prone to injuries in as little as one or two 30 minute sessions a week with a high intensity training program, HIT. One HIT study - A Ninefold Improvement in Fat Burning.