Ice is for injuries, and heat is for muscles. Roughly.
Ice is for injuries – calming down damaged superficial tissues that are inflamed, red, hot and swollen. The inflammatory process is a healthy, normal, natural process However, inflammation may result in pain. Icing is mostly just a mild, drugless way of dulling the pain of inflammation.
Examples: pulled muscle, joint sprain, tendinitis
Heat is for muscles, chronic pain, and stress – taking the edge off the pain of whole muscle spasms and trigger points, or conditions that are often dominated by them, like back pain and neck pain, for soothing the nervous system and the mind (stress and fear are major factors in many chronic pain problems).
How to use Ice and Heat
Heat can make inflammation worse, and ice can make muscle tension and spasms worse, so they have the potential to do some mild harm when mixed up.
Both ice and heat are pointless or worse when unwanted: icing when you’re already shivering, or heating when you’re already sweating. The brain may interpret an excess of either one as a threat. This signal may increase pain.
Heat and inflammation may be a bad combination. If you add heat to a fresh injury, the pain may increase.
If you ice painful muscles, be careful. Ice may aggravate sensations of muscle pain and stiffness, which are often present in low back and neck pain. Trigger points (painfully sensitive spots) can be surprisingly intense and easily mistaken for “iceable” injury and inflammation. Icing trigger points may increase pain more acutely.
What about injured muscle?
If you’re supposed to ice injuries, but not muscle pain, what do you with injured muscles (a muscle tear or muscle strain)? That can be a tough call, but ice usually wins. A true muscle injury usually involves obvious trauma during intense effort, causing severe pain suddenly. If the muscle is truly torn, then use ice to take the edge off the inflammation at first. Once the worst is over, switch to heat.
Which is better?
Ice packs and heating pads are not especially powerful medicine: some experiments have shown that both have only mild benefits, and those benefits are roughly equal in treating back pain.
The bottom line
Use whatever feels best to you. Your own preference is the tie-breaker and probably the most important consideration. For instance, heat cannot help if you already feel unpleasantly flushed and don’t want to be heated. And ice is unlikely to be effective if you have a chill and hate the idea of being iced. If you start to use one and you don’t like the feel of it, switch to the other.