If something hurts, one of the things that people tell you is to "Put ice on it" or likewise "Put heat on it". A common question that my clients ask me is "Which one should I use?" Both have their benefits, and either one can be used - depending on what is causing the problem.
Let's look at Heat
First of all, what does applying heat do? When heat is applied to an area of the body, it causes your muscle fibers to soften, loosen, and relax. It also attracts red blood cells to the area which help increase your circulation and those red blood cells then bring nutrients and good stuff to the area. Also, as the heat makes those fibers relax, any nasty stuff like lactic acid and toxins that were hanging around in the area or tangled in the muscle fibers are then carried away by those same red blood cells.
When to use heat
Heat is best used when muscles are tight or in the case of an injury such as a muscle pull or an overuse / repetition injury. Lifting too much weight or being a weekend warrior and being sore the next day could use heat to relax muscles and take away the lactic acid that is causing the soreness. Heat also helps muscle spasms relax. If you sit in front of a computer for work too long and constantly have neck or low back issues, some heat can help those stiff and 'stuck' muscles soften.
How to apply heat
There are a number of ways to safely apply heat. You can use an electric heating pad, a hot water bottle, or one of those popular rice-filled pillows that you can microwave to get warm. However the best type of heat to use is moist heat. Since your skin is porous, adding a little moisture lets the heat soak into your muscles a little more for a deeper, more effective treatment. You can add moisture to your heat by sprinkling a little water on your rice pillow before microwaving it, and some electric heating pads have a thin sponge that you can dampen and slide in under the cover. Another method is to make a simple hot towel: Take a regular hand or dish towel, soak it in water, wrap it up and put it in the microwave for about 60 seconds. Take a dry towel and lay that on your sore area, then lay the hot towel on top of it, then either take another dry towel and lay it on top, or if the bottom dry towel is long enough, you can wrap it up over the hot one, then just let it sit until it cools off. They key for using any type of heat application is to make sure that the actual hot object is not touching your skin directly and there is some sort of barrier (A towel, a cover, etc) between the heat element and your skin to prevent burns. Most doctors recommend either heat or cold treatments be done in increments of 20 minutes on the sore spot, then 20 minutes off, then repeat.
Now about cold
Cold is usually recommended when you need the opposite effect of heat. Cold makes things condense or shrink so it is used to make things smaller, decreasing things like inflammation, swelling, and bleeding. You may have heard of R.I.C.E. in first aid - Rest, Ice, Compression, and Elevation as a first step for an acute injury. The ice keeps the swelling following the injury from getting too bad so that a proper diagnosis can be made, then it can help calm down the side effects of that injury.
When to use cold
Cold is best used if there is swelling or inflammation that you want to decrease. Conditions such as tendonitis, bursitis, or a sprain or strain on a joint where there is swelling or irritation can be calmed down by cold. Occasionally, even migraines are advised to use cold to numb the nerves and decrease pain levels. Cold is also useful if there is a chance that an injury could leave a bruise since cold can help slow internal bleeding, therefore making a bruise less severe.
How to apply cold
Just like with heat, whenever using an application of cold, you want to make sure there is a barrier such as a towel between the cold object and your skin. You don't want to risk frostbite or the cold object sticking to your skin because that will just make things worse. Ice or an ice pack is a quick and easy way to get cold applied. In a pinch, you can always use the "mom trick" of grabbing a package of frozen vegetables for a short term fix. You can also freeze that same rice pillow that you have on hand for heat applications as well. Physical therapists often have a mixture of water, salt, and rubbing alcohol that freezes very cold but stays pliable so it can be wrapped around an area such as a joint to keep it cold for a long time.
My personal thoughts on heat vs. cold
Over the years, I've sustained many injuries and while I try to stick to what the research and doctors tell me, when it comes to heat vs. ice I have found that truly, the best thing to use is whatever feels right for your body. Yes when I roll over my ankle I put ice on it, but personally, when my hip flexor tendonitis flares up, I will usually use heat on it because if feels good and I know my body will respond positively to it. When my hands are swollen at the end of a marathon day of massages, I dip them in a hot paraffin bath - cold really bothers my hands so I usually turn towards heat even if the research says that this condition should have a cold application. So ultimately, there is research, there is recommendations, but if you really want to know which you should use for your pain - ice or heat, check to see what your doctor says and try it, but if it doesn't feel like it is working, or if you are more comfortable with another method, don't be afraid to use whichever temperature feels best for you.