The modern fitness industry's concept of "toning" muscles is specious--it might sound cool, but it lacks any tangible and definable meaning. The term "muscle tone" or tonus describes an electrophysiological phenomenon, a measure of ionic flow across muscle cell membranes. It can be thought of as the muscle's readiness to do anaerobic work. The more fit the muscle, the more electrophysiological activity it exhibits at rest. Lack of exercise leads to poor tone, aerobic exercise improves tone a little bit, low-intensity weight training improves tone more, and high-intensity training improves tone the fastest. As a test, go poke the traps or quads of an elite weightlifter at rest, if she'll let you. They'll be hard as rock. The same muscles of an elite road cyclist as rest will be firm, but not hard. Then compare the athletes' muscle tone to that of a sedentary person. The results will be quite enlightening. Most exercise programs that claim to improve muscle tone are actually lower-intensity hypertrophy programs and are only moderately effective for improving muscle tone. If "tone" is the goal, strength is the method.