It's hard to know what to do when injured to relieve pain and improve healing. There are two main factors to consider to help you make a decision: the type of injury and the age of the injury. Ice is the best choice for injuries that are less than a day old, or for injuries that cause prolonged swelling [source: Humphries]. The reason ice works for acute injuries is that it narrows blood vessels and slows flow of blood to the injury, reducing swelling. An example is a deep muscle bruise, also called a contusion. The discoloration you see is blood that collects around the injured area. In most circumstances, experts consider ice a safer immediate method of treatment than heat. Frostbite is one injury that you should not use ice to treat.
While ice can restrict blood flow, heat can increase circulation. Heat is best for relaxing muscles that are spastic or sore, and it is most effective on lower-back, mid-back and neck muscles. People who have osteoarthritis can use heat to reduce pain and increase their range of motion. Heat also can help prevent injuries by loosening up muscles, helping them move and stretch more easily [source: Karu].
Neither heat nor ice should be used for more than 30 minutes at a time. It's best to apply ice for 20 minutes at a time for the first day or two following an injury, then remove it for 20 minutes. The sooner you apply ice after your injury -- within 20 minutes of injuring yourself if possible -- the better it will work [source: Stanten]. After 48 hours have passed, you might consider alternating ice and heat for a short time. Use ice or cold water for a few minutes and then switch to hot water; the ice reduces some of the remaining swelling, while the heat brings oxygen to the injured area for healing [source: Stanten].
A thin, clean towel between the ice and your skin helps make applying ice more comfortable, and wrapping the ice tightly makes it work better. The cloth also can protect your skin from any leaks that might occur in chemical ice packs, which can burn unprotected skin [source: Karu]. It's not good to press the ice down with body weight, however. Carefully and slowly running frozen water that's been frozen inside a paper cup over a painful area such as a shin splint can serve to massage and soothe the area.
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