It is a hotly debated topic is the sport science world. The answer is, of course, it depends. Typically, we use ice for acute injuries where inflammation and pain are present. In such cases, the cold/ice can offer a reduction in inflammation+swelling, as well as pain relief. And heat for the more chronic and non-inflammatory conditions, which might benefit from an increase in vascularity/blood flow, like pesky “trigger points”, or tendosis (when the tendon has actually broken down, as opposed to tendinitis in which the tendon is merely painful due to inflammation).
What you most likely would not want to do, is apply heat to an acute injury, which will only increase pain+inflammation. Nor would it be sensible to apply heat to back pain or a “trigger point” if they are not inflammatory in nature.
Seems sensible enough, right? But what about icing an injury that is inflamed for more than a day or two? Don’t you want your body’s inflammatory processes to run their natural course? Perhaps. Or perhaps you want to create what has been referred to as “circulatory gymnastics”, in which 20min-ice-on/20min-ice-off is used, which temporarily suppresses blood flow for 20min, followed by a compensatory increase in blood flow for the next 20min, which can be therapeutic for reasons that are unclear (but could be attributed to a net change in vascularity to an area which is striving to repair itself. As we know, the only way repair can take place is from nutrients derived from blood).
Bottom line: heat vs. ice depends on the nature of the injury. Effects should be constantly ax’d/re-ax’d
Once again, PainScience has more on this topic: