One of the most contested topics in the rehabilitation community. I personally recommend icing (well cooling, ice is too cold for the body). The reasons why are as follows:
(a) Cooling reduces inflammation – inflammation can last years not just the first 72 hours. if you have pain, chances are you have associated inflammation.
(b) Cooling reduces sensitivity thus reducing pain – particularly important with chronic hypersensitivity
(c) These effects are cumulative and will lead to a better long term outcome
Saying all of this Heat does have positive benefits also
(a) Increased blood flow / thus nutrient supply – It is quite rare to have a situation where the body cannot get enough nutrients
(b) increased relaxation of muscle tissue (this usually is only short lived)
(c) heat can increase flexibility
HOWEVER – Heat will increase inflammation, secondary sensitivity (chronic inflammation leads to hypersensitivity). Thus the long term risks outweigh the short term gains.
The only exception I make for this is getting to sleep. Sleep is so important that if you need a hot pack to get to sleep then you should use it.
Please note that cold should not be used without seeking medical advice. There are conditions and situations where the use of cold / ice is dangerous. Please consult your doctor / Physio