There seems to be a lot of confusion lately about whether to ice or heat an injury. Years ago, medical professionals often recommended to apply ice for 24 hours and then to start using heat for the following 48 hours. There are many instances where this is no longer the case, and quite frankly, bad advice.
When you injure a body part, whether it is a sprained ankle, pulled hamstring or a broken toe, the body protects itself by sending a rush of fluid to the area to splint it. The area may remain swollen for days or weeks.
Ice is a vasoconstrictor. It slows down blood flow to limit the amount of swelling in said area. It also helps to reduce pain by numbing the tissue and decreasing muscle spasm through desensitizing the muscle spindles. Ice penetrates 5cm down into the muscle and the benefits can last for several hours, which make it very effective at reducing pain and swelling.
Heat is a vasodilator, which increases blood flow. Heat is good for helping muscles get more elastic or flexible; it does help reduce pain but not to the extent of ice. While heat penetrates 1 to 2cm into the muscle, the effects of heat dissipate as soon as the heat source is removed. Placing heat on an area that is swollen will most certainly make the swelling worse, and can set you back days or even weeks.
The basic rule of thumb should be if the area hurts and/or is swollen continue to use ice. Ice packs should be placed on the area for 20 minutes at a time, every couple of hours. Severe pain can be iced every hour. It is important to allow warm blood back to area to act as a pump to push the dead blood cells and extra fluid out. The area being iced does not get any colder the longer you do it. The tissue is virtually the same temperature at 20 minutes and 40 minutes. If using a bag of ice, it is OK to place directly on the skin because frost bite cannot occur if the ice is melting. If using a chemical ice pack, place a thin wet barrier between the skin and ice pack. The moisture will help conduct the cold. Some people may have a reaction to ice (welts, itching, red patches), if that is the case a barrier may be needed or apply the ice for a shorter period of time.
Use heat for muscle pulls, chronic joint pain, arthritis, etc. Heating pads are generally placed for 20 minutes also. If you have an injury that is no longer swollen and not that painful, then try heat. If it feels worse a couple hours later, go back to ice.
What about Biofreeze or Icy Hot? These products are considered skin irritants. They do not penetrate into the tissue like ice and heat. For the best results, use ice and heat as indicated by your medical professional, and use these types of products only when ice and heat are not readily available.