Here is a surprising piece of research:
The best runners were the least flexible. PubMed
The study is from a group of physiologists at Nebraska Wesleyan University. They found that the tightest muscles were the most efficient, both for men and women.
In fact, the latest science suggests that extremely loose muscles and tendons are generally unnecessary (unless you aspire to join a gymnastics squad), may be undesirable and are, for the most part, unachievable, anyway. “To a large degree, flexibility is genetic,” says Dr. Malachy McHugh, the director of research for the Nicholas Institute of Sports Medicine and Athletic Trauma at Lenox Hill Hospital in New York and an expert on flexibility. You’re born stretchy or not. “Some small portion” of each person’s flexibility “is adaptable,” McHugh adds, “but it takes a long time and a lot of work to get even that small adaptation. It’s a bit depressing, really.” How Necessary Is Stretching?
As someone who has always had tight hamstrings and very poor flexibility, I can agree with that last sentiment: It is a bit depressing. Nearly a decade of daily martial arts training increased my flexibility greatly (considering where I started), but never to the extent I wished. I'm just not a stretchy person, and never will be. And, just as this study suggests, a decline in my daily practice led to a decrease in flexibility -- though I will disagree with their sentiments to extent of saying that I did experience a significant change in flexibility, but it took a lot of work. The payoff for inflexibility seems to be more efficient use of muscles and greater stamina. Certainly, the genetic aspect seems to be borne out by my experience. In general, Asian martial artists tend to be more flexible than their American counterparts and some adaption of techniques and warm-up exercises seems appropriate for different body types. Please note that extreme flexibility, while impressive in some techniques, may make the athlete more vulnerable to injury.
For the casual athlete, the goal has to be avoiding injury.
If, on the other hand, “you can’t get anywhere near your toes, and the lower part of your back is practically pointing backward” as you reach, then you might need to try to increase your hamstring flexibility, Dr. Knudson says, to avoid injuring yourself while running, cycling or otherwise exercising. You can find multiple hamstring stretches on YouTube, although you should consult with a physical therapist before replicating them at home; proper technique is important to avoid injury. “You won’t get a lot of change,” Dr. Knudson says, ” but a little may be all you need.”
But, what about those athletes -- such as gymnasts and ballet dancers -- who really do need more flexibility? Can acupuncture help? In my acupuncture practice, the most effective technique I've found has been shallow needling into the muscle sheaths and micro-current electric stimulation. Simple daily stretching seems to be necessary, but not sufficient for a significant change in flexibility. Both from personal experience and from the experience of my patients, I have found herbal formulas featuring San Qi, to be helpful.