Cold helps cut down on swelling by shrinking the blood vessels. This reduces bleeding and therefore swelling in the affected area. Cold also helps keep the muscles from spasming (contracting uncontrollably) and relieves pain. But too much cold can damage the skin, which is why you should apply ice for a while, take a break, then apply it again. This allows the skin to return to normal temperature between treatments.

Generally speaking, you should apply ice for 10 to 30 minutes, depending on the body part and your own comfort level. Areas with little body fat (like the ankle, knee, and elbow) cannot handle cold as well as fatty areas (like the thigh and buttocks). A bag of frozen peas can make an excellent substitute for an ice bag, since the peas will better mold to the shape of your ankle (or knee or whatever).

Cold gel packs that can be frozen and refrozen cool the skin faster than an ordinary ice bag and should only be applied for short periods. Chemical cold bags (which stay at room temperature until squeezing mixes the chemicals inside) don't get as cold as an icebag, and can stay on the skin for longer periods of time. Cold therapy may not be recommended for people who are very sensitive to cold, those with blood vessels too near the skin, and diabetics and others who have problems with diminished blood flow. If you're one of those people, you should talk to your doctor about applying ice to an injury.

Heat can also relax muscles and relieve pain, but it makes swelling worse by stimulating blood flow. That's why cold is best immediately following an injury and for at least 48 hours afterward, or until the swelling goes down. Then heat can help, as the increased blood flow "cleans" out the damaged area.