The cornerstone of complete physical fitness includes muscular strength and endurance; cardiovascular fitness; and flexibility. You develop increased muscular strength and endurance by engaging in resistance exercise, typified by weight training. Muscular and cardiovascular endurance can also be developed by participating in regular aerobic exercise. The best way to develop increased flexibility is through stretching movements. Of the three major constituents of physical fitness, the flexibility portion is the most controversial. Some people suggest that additional stretching exercises are superfluous if you maintain a complete range of exercise motion in all your weight exercises. Indeed, starting each exercise in a pre-stretched position is not only known to promote muscle flexibility but also to activate mechanisms in the muscle that promote the release of insulin-like growth factor-1 (IGF-1), a major intramuscular anabolic hormone. This effect is so potent that animal studies have shown that you can induce muscular growth in animals, such as birds, merely by inducing a stretch in the animals. Providing additional resistance isn't even required. Of course, this wouldn't work in humans. That is, if all you did were various stretching movements the likelihood of attaining massive muscles would be nil.
Stretching is most familiar to athletes, who are often advised to perform stretching prior to engaging in any physical activity. The ostensible reason for this common advice is that stretching will induce greater flexibility in muscles and serve to warm up muscles. This, in turn, is thought to offer injury prevention insurance. While it certainly is a good idea to warm up muscles before engaging in any form of exercise, the notion that stretching will prevent such injuries has been largely discarded. This is based on various published studies that examined the effects of stretching prior to activity in the prevention of injuries. What the studies showed was little or no effect of stretching in this regard. What does work is starting out slow and letting the involved muscles warm up through low-level activity. For example, rather than starting out with an all-out sprint, you would be better served in terms of injury prevention by starting with a walk, then progressing to a slow jog, then after a few minutes progressing to the higher intensity running or sprinting. These minutes of low-level activity will increase blood flow to the muscle and warm them up sufficiently. With weight training, rather than start a set with the heaviest weight, it would be far more prudent to start with relatively light weights, doing a set or two of higher repetitions in the same exercise that you plan to lift heavier weights with. Even high-intensity weight-training advocates, such as Mike Mentzer and Dorian Yates, always began each exercise by lifting a lighter weight for higher reps, about 12 to 15, before progressing to lifting far heavier weights to complete failure. Interestingly, they never counted these "warm-up sets" as true sets in . . .