For the use of the term "tone" in weight training and bodybuilding, see Toning exercises.
In physiology, medicine, and anatomy, muscle tone (aka residual muscle tension or tonus) is the continuous and passive partial contraction of the muscles. It helps maintain posture, and it declines during REM sleep. It is not to be confused with the concept of toning in physical exercise.
In physics, tension is the magnitude of the pulling force exerted by a string, cable, chain, or similar object on another object. It is the opposite of compression. As tension is a force, it is measured in newtons (or sometimes pounds-force) and is always measured parallel to the string on which it applies. There are two basic possibilities for systems of objects held by strings. Either acceleration is zero and the system is therefore in equilibrium or there is acceleration and therefore a net force is present. Note that a string is assumed to have negligible mass.