This is an excerpt from Fitness Illustrated by Brian J. Sharkey.
You may come across other so-called facts about training, but you should be aware that some of them are actually fallacies or misconceptions. These oft-quoted statements are not true and have no basis in medical or scientific research.
Fallacy 1: No Pain, No Gain
Although serious training is often difficult and sometimes unpleasant, it shouldn't actually hurt. Here is an important distinction: Pain is not a natural consequence of exercise or training; it signals a problem that you need to address. In fact, well-prepared athletes sometimes perform in a state of euphoria, free of pain and oblivious to discomfort. Think about it: You've probably seen the end of a long-distance race where the winner finishes full of life even though the rest of the field appears wasted. This is made possible by the fact that when you exercise, your body produces natural opiates (endorphins) that can mask discomfort of the effort. But if you suffer real pain while training, back off. And if the pain persists, get it evaluated.
All of this notwithstanding, discomfort can accompany difficult training such as heavy lifting, intense interval training, and long-distance work. This discomfort (as distinct from pain) results naturally from the lactic acid that accompanies the anaerobic effort of lifting or doing intense intervals—and of the muscle fatigue, microscopic muscle damage, and soreness that come with long-distance training. Thus, whereas I reject the "no pain, no gain" mantra, I accept the following statement: No discomfort, no excellence. Overload is necessary for adaptation, and it sometimes requires you to work at your limit of strength, intensity, or endurance, which certainly can be uncomfortable. But if your exercise results in outright pain, it is probably excessive.
Fallacy 2: You Must Break Down Muscle to Improve
Neither pain nor injury is a normal result of training—whether for muscular endurance or for strength—and you can avoid both of these outcomes. Weightlifters can traumatize their muscles by using excessive weight or doing excessive repetitions, but such trauma is not a necessary stage in the development of strength. Similarly, some people who train and compete vigorously experience microtrauma in their muscles, but this too is an unnecessary (and undesirable!) outcome of training. Some runners, for example, experience microtrauma at the end of a marathon that includes long downhill stretches requiring eccentric muscular contractions (i.e., contractions of a lengthening muscle). Such contractions are a major cause of muscle soreness, which is associated with muscle trauma, reduced force output, and a protracted recovery period (4 to 6 weeks). Thus, "breaking down" muscle does not help you train. It brings your training to a standstill.
Read more about Fitness Illustrated.