This is an excerpt from Men's Body Sculpting-2nd Edition by Nicholas Evans.
An important variable for generating an overload effect in multiple-set training programs is the amount of rest allowed between sets. Muscle fibers experience physiological change only if they are both recruited and exhausted. If too much time passes between sets, the exhausted muscle fibers might recover and be used in the next set, thus training the same group of muscle fibers over and over.
The theory behind multiple sets is this: As long as the time between sets is not sufficient for the muscle fibers to recover from exhaustion, a different group of fibers will be recruited and overloaded during the next set. In this way, the multiple-set method might overload more muscle fibers and elicit greater adaptations than a one-set protocol.
The rest interval between sets must be specific, not random. Resting no more than two minutes results in more muscle fibers being overloaded.
Compare this system to shooting a gun. You take precise aim, making sure to hit the target with each shot. You deliver all six bullets, exhausting the gun of its firepower. Then you reload as quickly as possible to deliver the next round. Each set goes boom, boom, boom, with maximum precision and intensity.
The time taken between sets has an important relation to workout intensity. Power is the rate at which work is done. The quicker the work is performed, the more power is generated. In other words, your workout will be more intense when you complete it in less time. How much time you take to complete a workout naturally depends on the rest interval between sets. When your rest period is minimal, you complete the work in less time, and your workout is more intense. For a workout in which fat loss is a priority, shortening your rest interval is a useful way to burn more calories. An interval of 60 seconds or less allows just enough time to change the weight on the bar or relocate to the next exercise station.
Now we need to discuss the duration of your workout. This is based on your muscle's fuel tank—it is not an endless supply. Your muscle contains a store of chemical energy that is converted to mechanical energy. The muscle contracts, and movement occurs. Think of fuel stored in the gas tank of your vehicle. When burned, the chemical energy within gasoline is converted to mechanical energy. The wheels turn, and the vehicle moves along the road. When all the fuel is used up, the tank is empty, and your vehicle comes to a halt.
A similar process occurs inside your muscles. During exercise, your muscles' fuel tank begins to run low after 30 minutes; after an hour, it's empty. You are still able to exercise when your muscles' energy reserves are depleted because the muscles get energy from other sources in your body. But at this point, your muscles shift into an endurance mode, such as being put on cruise control. The problem is that you can no longer accelerate, so you can't achieve maximum muscle stimulus. Your muscles are merely surviving, not growing. If you venture into muscle cruise control, it will take days for your muscles to refill their fuel tank. Muscle building is an anaerobic process. The training stimulus should be short and intense, not long and steady. Do what's necessary to trigger the hypertrophic response, then leave the gym, go home, rest, and grow.