Heat feels good on sore muscles. That's why a soak in a hot tub or steaming shower is such a relief. However, lengthy exposure to heat -- especially heating pads -- is bad news on minor injuries. Even though heat treatment is a common part of physical therapy, you should avoid leaving a heating pad on sore muscles.
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Causes of Soreness
Muscle soreness from exercise comes from microscopic injury to the muscle fibers, which become inflamed as part of your body's natural response to injury. Sore muscles from tension, also frequently accompanied by small injuries and inflammation, are similar to a small cramp and the result of muscle fatigue.
Effects of Heat
When you apply heat to a muscle, it opens up the blood vessels in that area. This increases blood flow, creating two effects. It relaxes the muscles, which is the source of how soothing heat treatment can feel. It also accelerates some of your healing, as increased blood flow brings more cells that help repair the area faster.
Here's the problem with increasing blood flow: it also increases inflammation. Since inflammation of your muscles is not just a symptom, but a cause, of muscle soreness, anything you do that increases inflammation will slow your healing and cause additional pain once you take the heat off.
Although less common that other heat pad problems, it's possible to sustain first-, second- and even third-degree burns by leaving a heating pad on the same area for too long. This is an especially serious risk for people with diabetes or other conditions that reduce sensitivity or those who fall asleep while under a heating pad. It doesn't have to hurt immediately to burn. Just as you can get a sunburn without feeling anything until it's too late, the long application of slightly too-warm heating pads can cook your skin.
Common Sense Caution
Follow your doctor's instructions if if he gives you contradictory advice to normal use of heating pads for simple, sore muscles. For any injury or soreness, you should always check with your medical professional before beginning or discontinuing any treatment program.