If a muscle is tight the opposite muscle is probably weak and extended.
by Cary Raffle
If you're looking to improve your "flexibility" stretching alone may not be enough. When a muscle is tight chances are that the opposite muscle is, in a sense, all stretched out and weak. Your quadriceps and hip flexors may be tight from sitting all day long, which puts them in a constantly flexed position. The compensation is that your hamstrings and glutes are weak and extended. Your pecs and front delts tighten as they turn in to work on the computer or drive, and your rhomboids and lower traps get all stretched out. They might even feel sore. As your inner abdominal core muscles become weak, your lower back tightens and arches putting pressure on your spinal cord and disks. We think of muscles as agonists and antagonists, working in pairs. When a muscle becomes tight, we think of it as being "overactive." If you're only dong stretching of the tight and overactive antagonist muscles, you're missing half of the equation - the weak antagonists.
Many of my clients are familiar with training techniques that we use to work on these problems, especially targeting exercises that strengthen weak antagonists. In fact, many typical office workers should minimize or avoid many exercises altogether because they target overactive muscles. If you've got big, tight calves like me, you probably don't need to work them but you might want to work the tibialis in front of the leg. If your knees turn in, you don't need to work those tight inner thighs but you will want to work on your abductors, gluteus medius and maximus.
There's also a whole area of hands-on stretching that uses the principle of reciprocal inhibition, called neuromuscular stretching or PNF (proprioceptive neuromuscular facilitation) or contract-relax. In this technique, the 'stretchee' activates the antagonist muscle to resist against the stretch, then relaxes as the 'stretcher' gently extends the tight muscle. This isn't something to try on your own or with your workout partner, as there is potential for overstretching and injuring a muscle without proper training.
Two final points on stretching: You should always warm up before stretching. Your muscles respond better when warm, there are actually enzymes released that make the stretching (as well as your workout) more effective. Second, stretch your tight muscles don't waste time on the muscles that aren't tight. It may feel nice but if your quads are tight as could be, chances are that your hamstrings aren't and you don't need to waste your time.