Muscle Tone

Except during certain stages of sleep, most of our muscles are in a state of slight contraction; this is what is known as muscle tone. When sitting upright, for example, the tone of your neck muscles keeps your head up, and the tone of your back muscles keeps your back straight. This is an important function of muscle tone for human beings, because it helps us to maintain an upright posture. In order for a muscle to remain slightly contracted, only a few of the muscle fibers in that muscle must contract. Alternate fibers contract so that the muscle as whole does not become fatigued. This is similar to a pianist continuously rippling her fingers over the keys of the piano - some notes are always sounding at any given moment, but the notes that are sounding are always changing.

Muscle fibers need the energy of ATP in order to contract. When they produce ATP in the process of cell repiration, muscle fibers also produce hear. The heat generated by normal muscle tone is approximately 25% of the total body hear at rest. During exercise, of course, heat production increases significantly.