Most of today's household heating pads are what have become known as 'electrical pads' (as opposed to chemical heating pads, or the now-antiquated hot water bottle). In the case of an electrical pad, the power is provided via an outlet. Once you turn it on, you can choose a heat setting (most blankets have somewhere between 5 and 10 settings). The electricity generates heat that is distributed by way of nichrome coils that lie beneath at least one layer of plastic or fabric safely separating those coils from the skin. Still, you should strictly regulate your electrical heating pad to ensure it doesn't get too hot. Most pads also come with some type of automatic shutoff function to prevent them from becoming fire hazards.
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The primary purpose of a heating pad is to relieve tension. Muscle tension constricts blood flow, resulting in pain. Applying heat causes a person's blood vessels to dilate, creating increased circulation and stimulating the affected area. Repetitive use of heat therapy will more than likely result in decreased inflammation, temporary pain relief, increased range of motion, and long-term wellness (among other benefits).
Assuming you're on the go, there are several non-electrical heating pads that you can warm up by placing in a microwave, before either wrapping them around the affected muscle, or strapping them across the back with the use of some kind of harness.