If you are experiencing pain or discomfort before, during, or after stretching or athletic activity, then you need to try to identify the cause. Severe pain (particularly in the joints, ligaments, or tendons) usually indicates a serious injury of some sort, and you may need to discontinue stretching and/or exercising until you have sufficiently recovered.
If you are experiencing soreness, stiffness, or some other form of muscular pain, then it may be due to one or more of the following:
Overstretching and engaging in athletic activities without a proper warm-up can cause microscopic tearing of muscle fibers or connective tissues. If the tear is not too severe, the pain will usually not appear until one or two days after the activity that caused the damage. If the pain occurs during or immediately after the activity, then it may indicate a more serious tear (which may require medical attention). If the pain is not too severe, then light, careful static stretching of the injured area is supposedly okay to perform (see section Static Stretching). It is hypothesized that torn fibers heal at a shortened length, thus decreasing flexibility in the injured muscles. Very light stretching of the injured muscles helps reduce loss of flexibility resulting from the injury. Intense stretching of any kind, however, may only make matters worse.
Overexertion and/or intense muscular activity will fatigue the muscles and cause them to accumulate lactic acid and other waste products. If this is the cause of your pain, then static stretching (see section Static Stretching), isometric stretching (see section Isometric Stretching), or a good warm-up (see section Warming Up) or cool-down (see section Cooling Down) will help alleviate some of the soreness. See section Why Bodybuilders Should Stretch. Massaging the sore muscles may also help relieve the pain (see section Massage). It has also been claimed that supplements of vitamin C will help alleviate this type of pain, but controlled tests using placebos have been unable to lend credibility to this hypothesis. The ingestion of sodium bicarbonate (baking soda) before athletic activity has been shown to help increase the body's buffering capacity and reduce the output of lactic acid. However, it can also cause urgent diarrhea.
Exercising above a certain threshold can cause a decreased flow of blood to the active muscles. This can cause pain resulting in a protective reflex which contracts the muscle isotonically (see section Types of Muscle Contractions). The reflex contraction causes further decreases in blood flow, which causes more reflex contractions, and so on, causing the muscle to spasm by repeatedly contracting. One common example of this is a painful muscle cramp. Immediate static stretching of the cramped muscle can be helpful in relieving this type of pain. However, it can sometimes make things worse by activating the stretch reflex (see section The Stretch Reflex), which may cause further muscle contractions. Massaging the cramped muscle (and trying to relax it) may prove more useful than stretching in relieving this type of pain (see section Massage).
If you are already experiencing some type of pain or discomfort before you begin stretching, then it is very important that you determine the cause of your pain (see section Common Causes of Muscular Soreness). Once you have determined the cause of the pain, you are in a better position to decide whether or not you should attempt to stretch the affected area.
Also, according to M. Alter, it is important to remember that some amount of soreness will almost always be experienced by individuals that have not stretched or exercised much in the last few months (this is the price you pay for being inactive). However, well-trained and conditioned athletes who work-out at elevated levels of intensity or difficulty can also become sore. You should cease exercising immediately if you feel or hear anything tearing or popping. Remember the acronym RICE when caring for an injured body part. RICE stands for: Rest, Ice, Compression, Elevation. This will help to minimize the pain and swelling. You should then seek appropriate professional medical advice.
If you stretch properly, you should not be sore the day after you have stretched. If you are, then it may be an indication that you are overstretching and that you need to go easier on your muscles by reducing the intensity of some (or all) of the stretches you perform. Overstretching will simply increase the time it takes for you to gain greater flexibility. This is because it takes time for the damaged muscles to repair themselves, and to offer you the same flexibility as before they were injured.
One of the easiest ways to "overstretch" is to stretch "cold" (without any warm-up). A "maximal cold stretch" is not necessarily a desirable thing. Just because a muscle can be moved to its limit without warming up doesn't mean it is ready for the strain that a workout will place on it.
Obviously, during a stretch (even when you stretch properly) you are going to feel some amount of discomfort. The difficulty is being able to discern when it is too much. In her book, Stretch and Strengthen, Judy Alter describes what she calls ouch! pain: If you feel like saying "ouch!" (or perhaps something even more explicit) then you should ease up immediately and discontinue the stretch. You should definitely feel the tension in your muscle, and perhaps even light, gradual "pins and needles", but if it becomes sudden, sharp, or uncomfortable, then you are overdoing it and are probably tearing some muscle tissue (or worse). In some cases, you may follow all of these guidelines when you stretch, feeling that you are not in any "real" pain, but still be sore the next day. If this is the case, then you will need to become accustomed to stretching with less discomfort (you might be one of those "stretching masochists" that take great pleasure in the pain that comes from stretching).
Quite frequently, the progression of sensations you feel as you reach the extreme ranges of a stretch are: localized warmth of the stretched muscles, followed by a burning (or spasm-like) sensation, followed by sharp pain (or "ouch!" pain). The localized warming will usually occur at the origin, or point of insertion, of the stretched muscles. When you begin to feel this, it is your first clue that you may need to "back off" and reduce the intensity of the stretch. If you ignore (or do not feel) the warming sensation, and you proceed to the point where you feel a definite burning sensation in the stretched muscles, then you should ease up immediately and discontinue the stretch! You may not be sore yet, but you probably will be the following day. If your stretch gets to the point where you feel sharp pain, it is quite likely that the stretch has already resulted in tissue damage which may cause immediate pain and soreness that persists for several days.