I get this question almost daily in my practice. The short answer is ice. Why you ask? Well, it all boils down to neuroanatomy and how cold and heat are delivered to the brain. It also has to do with inflammation and inflammatory enzymes which is a routine and primary cause of pain. First lets discuss how inflammation works on a simple level.
First, Injury= damaged tissue and inflammatory enzymes (cytokines) are released at the site of injury. This opens blood vessels to rush healing oxygen and nutrition to the site. This reaction also causes swelling and pain!
Second, temperature and pain both use the spinothalamic tract to communicate both types of sensations(temperature/pain) to the brain. Using heat or ice essentially “masks” the pain in delivery to the brain. Ice however will cause your vessels to constrict at least initially to stop swelling. The flushing sensation of dilation to constriction will flush the injury and many protein causing pain chemicals will be removed. Cold also with take your pain level up until numbness takes hold. This makes it much harder to over ice your injury.Not so with heat.
Using a heating pad increases the vasodilation and swelling to the injury site- causing more damage. It feels ‘good’ because you are over-riding or confusing your brain to feel only heat and not pain due to the same neurologic pathway.
So I always recommend my patients to ice until numb (about 20 mins) twice daily with injury. Heating does work usually after day or two post injury. You may use hot epsom salt bath or hot shower.