Moist heat helps increase blood flow to the painful areas allowing for blood and oxygen to flow more readily to the sore areas. Moist heat can reduce muscle spasms as well, causing the muscles, ligaments, and tendons to relax.
Ice relieves pain through the stimulation of endorphins-a natural painkiller released by the body. Also, after injury, congestion in the area due to swelling inhibits oxygen flow to the cells of the healing tissue, thus causing impaired healing and cell death. Ice helps to inhibit this process to promote healing.