I know. You just want it to be simple.
Today, I can make it simple for you.
You've got pain (somewhere) and the question running through your mind is, "Should I use heat or ice"?
ICE is for INJURIES. Use it for:
Modulating INFLAMMATION after muscle trauma - think sprained ankle or a 'corkie'
Any 'micro' muscle trauma - think big workout at the gym or beefy AFL players braving the Southern Ocean in July
And it's generally used during the first few days - think FIRST AID
Heat is for, well, HEATING. Use it for:
It's kind of like a cuddle - it's nice for the body and the mind
When the wrong choice can make it worse:
Heat is generally thought to increase blood supply to an area, so if you have a muscle that is inflamed e.g. pulled, torn, etc. then using heat might make it worse.
If you have a tight muscle and it's been that way for quite a long time (but nothing is injured or torn) then chances are you'll make yourself more uncomfortable using ice. It won't cause any damage but a neck in spasm is worse than a tight one.
When the wrong choice doesn't matter a bit:
And did you notice that pain relief is mentioned in both heat and ice. A 2010 study when either a heat or ice pack was given in addition to nurofen, both were deemed equally effective (in a very minor way).
So, if it's 40 degrees outside on a hot Melbourne summer day and you start using a heat pack and realise that it feels wrong (which it no doubt would). Switch and try the ice.
Garra et al. Heat or cold packs for neck and back strain: a randomized controlled trial of efficacy. Academic Emergency Medicine. 2010
* It's therapeutic effect is quite complex (like everything to do with the body and brain) but there is no additional benefit to understanding the neurophysiology behind heat and pain (well, not at the moment).