In the case of sore muscles, I’ve been told to use an ice pack or a heating pad. Why would seemingly opposite treatments (cold vs. heat) both promote healing? Or don’t they?
Heat increases blood flow to the area. Cold decreases it. It depends on the injury which is better. For example, if the muscle is pulled, blood will tend to flow into the tears and coagulate. So, for a tear, cold is usually applied. OTOH, if it’s just soreness, you want more blood flow in the area to flush out lactic acid buildup.
Ice first, to prevent swelling. Heat later, after the area has stabalize and no more blood is seeping into the space between the tissues.
Your best bet for online help is Merck Manuals Consumer Version
For official medical studies, kind of tough reading: http://www.ncbi.nlm.nih.gov/