The Biology:

Between the ages of 20 and 50 the average individual loses approximately 15 pounds of muscle and gains over 45 pounds of fat (for a net gain of 30 pounds) as a result of the decrease in metabolism associated with muscle loss. Strength training can quickly and safely stimulate the body to produce additional muscle tissue and improvements in cardiovascular and other support systems. In addition to improved strength and endurance, regaining or building muscle tissue also increases metabolism, and improves body shape, bone density, cardiovascular efficiency, and joint stability. It is important to note that training is the stimulus, and the physical improvements are produced by the body as a response to that stimulus. The human body is an economical organism, which resists change unless stimulated by a demand exceeding its current ability. Intense exercise quickly fatigues the working muscles, and therefore must be brief. As intensity increases, so does the stimulus, and the larger the stimulus, the greater and longer lasting the results.