There are times where a little knowledge of biology can be harmful to understanding. For example, whenever the body is accosted by injury or illness, the natural response is typically inflammation (redness, heat, swelling and a few other things). By biological standards, swelling is a good thing. It increases the ability of fluid to move around near the injured area, allowing increased blood flow and enabling the body to fight off the nasty things that are damaging it faster and more effectively. Along comes the question that any runner will have run across: should I ice a swelling injury? Admittedly, my first reaction would be “well, golly, if swelling is the body fighting back, you should let it run its course!” Turns out, sports therapy disagrees with my limited understanding of the body’s innate responses to injury.
The human body, much like the human race, tends to overreact to anything that happens to it. Rolled ankles and sore joints will typically get a similar, albeit weaker, response to more severe injuries such as gashes and busted bones. Given that over time, humanity has gotten surprisingly good at adding exoskeleton systems to reinforce injured limbs, it makes sense that we reduce swelling with most injuries simply because we (by we I mean doctors) can typically help the injury more than the body can. Why let the body make clumsy efforts at repairing a bone if you could just reset it and lock the thing in place with a plaster mold until it seals together?
The long story short of the icing debate is: swelling bad, ice good.
The standard for sports medicine is to ice an acute injury and to heat a chronic one. Acute injuries are those that happen suddenly and for obvious reasons (rolled ankle is a great example; when it happens, you know it happened). Acute injuries are sudden and the body’s response is a five alarm fire of swelling and redness. Adding ice to the injury will do what ice does best: shrink down the blood vessels and reduce blood flow to the injured area. Reduced blood flow reduces swelling, which in turn will allow the area to move without discomfort or pain. Icing is done for things like sore muscles, reducing the swelling will allow use of the muscle without reducing mobility. Science says to only ice for 10 minutes at a time for every hour, otherwise you run the risk of causing limited frostbite damage to the skin (also, wrap your ice in a towel, no skin to ice contact)
Never let the ice touch the skin. It never goes well.
As for heating, sports therapy says to do that for chronic injuries. Chronic injuries are those that happen over time, things like stress fractures and grinding joints. Heating the area improves blood flow to it and increases the overall ability of the body to heal the damaged zone. The general rule of thumb is that you want moist heat to help the body the most (like a hot towel that you lay over the knee or something like that). Heating a swollen area is a no go, increasing the flow of fluid to a place already swollen can cause some types of damage or stretching to the skin.
As for what makes something chronic or acute… The technical definition is rather limited. Generally, the standard is to ice something that is less than 24 hour old, and to heat something that is more than 48 hours old. Anything in between those two is sort of the gray zone where you aren’t sure if it was a minor injury or if it is something that will stay with you for a long time.