The mechanism by which heat relieves pain is not exactly known, although it is believed that heat inactivates nerve fibers which can force muscles into irritating spasms, and that heat may induce the release of endorphins, powerful opiate-like chemicals which block pain transmission. Heat appears to be better than cold for loosening muscles and increasing overall flexibility. Strangely enough, cold therapy also can reduce muscle spasms, and cold is noted for killing pain, reducing swelling, and lowering metabolic activity. The pain-killing effect of cold is caused by its "deadening" of nerve-cell activity; hospital studies show that patients who use cold therapy on injuries tend to require much less pain medication. Cold decreases muscle spasms by making muscles less sensitive to being stretched. The bottom line is that you should use whatever works best for you. Whether you choose heat or cold, be careful not to apply it directly to the skin and do not leave it in place too long; ice applications should be left in place no longer than 10 to 15 minutes at a time and heat no longer than 15 to 30 minutes.