The application of ice is generally most helpful during the first 48 hours following a back-muscle strain. After this period, heat therapy is more beneficial to the healing process. If the lower back is swollen or bruised, heat should not be used; it is better to use a cold pack to reduce the inflammation or swelling in the area. It is believed that heat deactivates nerve fibers which force muscles into irritating spasms; heat also induces the release of endorphins, powerful opiate-like chemicals that block pain transmission. Heat also appears to be better than cold for loosening muscles and increasing flexibility. Strangely enough, cold therapy also can reduce muscle spasms. It is known to relieve pain and reduce swelling. The pain-killing effect of cold is caused by its "deadening" of nerve-cell activity. Cold decreases muscle spasms by making muscles less sensitive to being stretched. Bottom line: Use the treatment that works best for you. Whether you choose heat or cold, be careful not to apply it directly to the skin and not to leave it in place too long.