<P> The word tone derives from the Latin "tonus" (meaning "tension"). In anatomy and physiology, as well as medicine, the term "muscle tone" refers to the continuous and passive partial contraction of the muscles, or the muscles' resistance to passive stretching during resting state as determined by a deep tendon reflex . Muscle tonus is dependent on neurological input into the muscle . In medicine, observations of changes in muscle tonus can be used to determine normal or abnormal states which can be indicative of pathology . The common strength training term "tone" is derived from this use . </P> <P> What muscle builders refer to as a toned physique or "muscle firmness" is one that combines reasonable muscular size with moderate levels of body fat, qualities that may result from a combination of diet and exercise . </P> <P> Muscle tone or firmness is derived from the increase in actin and myosin cross filaments in the sarcomere . When this occurs the same amount of neurological input creates a greater firmness or tone in the resting continuous and passive partial contraction in the muscle . </P> <P> Exercises of 6--12 reps cause hypertrophy of the sarcoplasm in slow - twitch and high - twitch muscle fibers, contributing to overall increased muscle bulk . This is not to be confused with myofibril hypertrophy which leads to strength gains . Both however can occur to an extent during this rep range . Even though most are of the opinion that higher repetitions are best for producing the desired effect of muscle firmness or tone, it is not . Low volume strength training of 5 repetitions or fewer will increase strength by increasing actin and myosin cross filaments thereby increasing muscle firmness or tone . The low volume of this training will inhibit the hypertrophy effect . </P>

Muscle parts of the body being strengthened by circuit training