Most exercise programs for conditioning and rehabilitation are oriented to strength development, aerobic (cardiovascular) fitness, or a combination of the 2. Because the 2 types of exercise are located at the opposite extremes of a muscular power continuum, the design of a program must be highly specific with regard to the exercise to be undertaken, as well as the intensity, duration, and frequency, in order to attain optimal results. Strength exercise programs involve weight training or the use of high-resistance machines with exercise that is limited to a few repetitions (generally less than 20) before exhaustion. Aerobic exercise involves exercise performed for extended periods (e.g., 10-40 minutes) with large muscle activity involving hundreds of consecutive repetitions that challenge the delivery of oxygen to the active muscles. The chronic physiological adaptations and the variables in program design are highly specific to the type of exercise performed.