Lots of experts talk about how great it is to stretch regularly...but why?

Well, there's a short answer, and a few longer ones.

One of the core concepts around stretching is that it generally keeps muscles, tendons and other tissues limber, both in the short term for active use and in the long term, where people who stretch and generally exercise are going to be in better shape later in life than those who do not.

Another reason for stretching involves a specific role in the fitness routine: some call it "post-workout stretching".  Now, a lot of runners and other trainers stretch before the run to loosen up muscles a little, for a lower chance of injury.  However, for many, the benefits of post-routine stretching are more important.  In fact, some fitness leaders say the pre-workout stretching shouldn't be done at all!  Basically, in addition to "cooling down" muscles, the post-routine stretch will work against the kind of lactic acid buildups and any muscular contortion that may occur during periods of "hard exercise".  That means that post-routine stretching is key to comfort for those who sometimes experience pain and discomfort after training.

Then there are other "side factors" that stretching can improve: a simple stretch program, while working key muscles, can promote de-stressing, agility, and dexterity.  There's also the role of stretching in each specific athletic capacity.  This is sometimes called a "functional range of motion" for an activity.  

The idea of a functional range of motion is this: every fitness activity places different demands on the body, and uses a different set of optimized conditions.  For example, you don't need to be as flexible as a gymnast to perform well in a range of contact sports.  That means it can be counter-productive to train up to a super flexibility.  A functional range of motion scheme is a plan to build to just the flexibility you need, without over-working.  A potential result of over-working muscles is something that some experts call "compromised joint stability" - the idea that a joint has been worked so loose that it doesn't have the stability a body might need for the contact sports mentioned earlier.

That's why, in a stretching program, it's important to identify the goals.  This is true if your stretching includes more than just a simple warm-up/cool-down, which is pretty universal.  A good example is to look at some of the meticulous sessions that professional athletes do for sports like football, basketball and tennis.  Looking at these in detail will show how the demands of each sport differ, and how different kinds of stretches and joint work get the results athletes need.

But make no mistake, stretching is not just for athletes.  In the case of a "regular" routine, stretching generally consists of a few simple activities before and after a run or other exercise period: this will address the "functional range of motion" needed for running, ellipticals, weight training, etc.  Other strength training and workout activities may need some specialized stretching, and some new trainers will want to get tools like balance boards or isometric balls or rings to work with.  The general guideline is to start slow, both in terms of the time spent in stretching and the range of motion that you use.  This will avoid high incidents of injury.  Easing into a stretching routine, and keeping it up, is the best way to ensure that muscles and limbs will be up to the tasks they are put to in any workout session.