It is difficult to understand how one set of resistance exercise can be as productive as three sets of resistance exercise for building muscle strength. After all, performing three sets of bar dips requires much more work than performing one set of bar dips. Likewise, running three miles requires much more work than running one mile, and the longer exercise duration is related to better cardiovascular conditioning. However, such does not appear to be the case with strength training. It seems that exercise intensity, rather than exercise duration is the essential stimulus for strength development.
While performing several sets of exercise uses more energy and provides a good muscle pump it may not produce a greater strength-building stimulus. Consider how muscle fibers are recruited during a given exercise set. Because they have the greatest endurance, the slow-twitch (Type 1) muscle fibers are recruited first. These are joined by the fast-twitch (Type 11A) muscle fibers, which have much less endurance. As the Type 11A fast-twitch muscle fibers fatigue, they are replaced by Type 11B fast-twitch muscle fibers, which have even less endurance. Typically, when the Type 11B fast-twitch muscle fibers fatigue, the muscle is no longer able to life the resistance and the exercise set is terminated.
After a one to two minute recovery period a second set of the exercise may be performed. However, the same muscle fibers used in the first set are again recruited in the same activation pattern. That is, you work the same muscle fibers in the same manner a second time, but you can't really push any harder on your second set than on your first set. Therefore, it is simply the same stimulus a second time.
Let me explain it another way. If you perform an exercise set with 75 percent of your maximum resistance, you must stop when your strength drops below 75 percent of maximum. That is, when you fatigue 25 percent of your muscle fibers you can no longer lift 75 percent of your maximum resistance. If you perform a second set with the same resistance, you must again stop when you fatigue 25 percent of your muscle fibers. Because you fatigue the same muscle fibers in the same order each successive set, the basic training stimulus does not change.
Based on the research studies reviewed, it appears that one good set of resistance exercise is as effective as two or three sets for providing a sufficient strength stimulus and producing significant strength gains. Our research with hundreds of men and women show that single-set strength training is also effective for developing muscle tissue. In one study (Westcott 1995), 313 adults added 3.0 pounds of lean (muscle) weight after eight weeks of strength training (one set of 12 Nautilus machines, three days pert week).
Perhaps just as important as training effectiveness is training efficiency. The vast majority of non-exercising adults give the same reason for not performing fitness activities, namely, they don't have time. This may be a valid concern if you perform three sets of 12 different exercises. At one minute per set and two minutes between sets, such a workout would require over 1 ½ hours each training session.
On the other hand, a single set of 12 different exercises would take about ½ hour each training session. This represents a reasonable exercise commitment for many time-pressured adults, which encourages them to begin a strength training program. Although it may not be the preferred method of muscle-building for competitive bodybuilders, single-set strength training is an effective and efficient means for attaining a high level of muscular fitness. Remember, it is the exercise intensity rather than the exercise duration that is most important for stimulating strength development.