Post injury immediately requires ice or cold. When you initially injure yourself, your body floods the area with everything needed to fix the injury which also produces heat. Cold helps to decrease the temperature and also decreases nerve conduction and circulation (less pain and swelling). After the first 48 to 72 hours (acute phase) some people do better with heat. Heat is an analgesic, it feels good. If pain returns 30 minutes later you need to ice. Ice is an anesthetic, it numbs the pain. Generally you are always safe to ice. If pain persists for more than three days, contrast cold 10 minutes, hot 10 minutes, cold 10 minutes. This will decrease circulation, increase circulation and decrease circulation. This physiologically flushes the area and decreases the inflammation and increases oxygen.
After the acute phase my general rule of thumb is as follows:
• Small localized pain that you can touch responds better to ice.
• Large diffused cramping or itching type of pain responds better to heat.
• When in doubt, ice or contrast with cold-hot-cold or hot-cold-hot.