by: Kelly Baggett
A: Too much flexibility is just as important as lack of flexibility and here's why: Flexibility is defined as the range of movement for a given bodypart or joint. Before you can perform a movement optimally you have to be able to get into the proper position to apply tension with the right muscles at the right time.
For any given joint movement there are at least 2 muscles involved in moving the joint through it's ROM. One is called an agonist and one is called an antagonist. One of them contracts, or "tightens" to provide the movement, while the other "relaxes" or loosens, to allow the movement. Think of 2 movements that you can do with your lower leg from where you're seated right now. You can extend your lower leg out, which is done by "tightening" or flexing your quadricep muscle while relaxing the hamstring. You can also pull your lower leg back under your chair towards your butt, which is done by tightening or "flexing" your hamstring muscle whilst relaxing the quadricep. Generally speaking, strengthening a muscle makes that muscle more proficient at "tightening" and weakening a muscle makes that muscle more apt to "loosen", or relax.
That's not to say that getting stronger causes tightness because it doesn't. What causes tightness and inhibits range of motion is when the opposing muscle groups responsible for a particuluar movement get out of balance. As long as they are kept in balance all the strength training in the world won't cause problems. It's kind've like a pulley or a seesaw. You can tighten or add weight to one side or loosen or remove weight from the other but the end result is the same. Ideally for every joint movement, you want a balance between the muscles that move the joint in one direction and the muscles that provide movement in the opposite direction. When you don't, you have problems that are normally attributed to either lack of "flexibility" or too much flexibility. Either one can throw the seesaw off kilter.
Now, for a variety of reasons, including posture and activity, "flexibility" problems typically result in one of the muscles that provide movement for a bodypart or bodyparts being chronically tense, or chronically tight, and the other being loose. If one is too tight the range of motion of a joint will "lean" towards dominance of the tight muscle and "away" from the loose muscle. Now go back to our example of you moving your foot out vs back towards your butt. Now what if your quadricep is tighter then your hamstring? Then you'll have a natural propensity to be proficient at extending your lower leg out and a natural propensity away from pulling your lower leg back. If the hamstring is weak or loose the result is the same.
Here's another more obvious example that most people experience to a certain extent: Right now as you sit there reading this screen you're probably leaning forward slightly. In order to hold your head up in that posture the muscles on the back of the neck have to fire and the muscles on the front of your neck have to relax. If you maintain that posture long enough (by spending enough time at a desk) the posture will become chronic. You'll walk around with your head hung slightly forward with the muscles on the back of your neck tight and chronically flexed while the muscles on the front of your neck chronically weak and loose. Eventually you will start to get pains in the back of your neck. Now, in order to correct the problem and bring the resting range of motion of your neck back to center you would need to loosen, or stretch, the muscles on the back of your neck and tighten, or strengthen the muscles on the front of the neck. So you can see there's a balance here between strengthening and stretching.
Now stand up and lift one foot up off the ground a couple of inches. Engage the hip flexors by moving the foot forward in front of you about 6 inches, back to center, and then engage the glutes by moving your foot back behind you about 6 inches. In most people, the tendency is for the hip flexors to be tighter, thus the tendency is to be less proficient at extending your hips (moving your leg back) vs engaging your hip flexors (moving your leg forward). Now, lift that same foot off the ground and, from the center, rotate the toes out 45 degrees by engaging the external hip rotators, back to center, and then rotate the toes inward slightly. Again, in many athletes the external hip rotators tend to be more active then the internal hip rotators thus they walk (and run) with the foot turned out instead of straight or inward slightly.
So, it's not as simple as saying "stretch the hell out of everything" Muscles need to be strengthened so that things remain in balance and stretched so that things either remain in balance or are brought back into balance. If you start off with things in balance, then for every muscle that you strengthen you should strengthen its opposing muscle group too. One problem with bodybuilders is they strengthen the muscles that pull their shoulders forward moreso then they strengthen the muscles that pull their shoulders backward. The result is rotator cuff problems etc.
To figure out which muscles to stretch and all that I recommend you either go to a knowledgeable physical therapist or structural guru or learn how to do it on your own by reading books such as Flexitest, Athletic Body in Balance, or "Muscles: Testing and Function".
Having said all that, here are a few common stretches most people cam really benefit from.