The usual rule is cold for a new injury, heat for a stiff one. That rule is roughly right and it leaves out the cases where it matters most: the ache with no obvious cause, the pain that has been there a month, and the ones where the answer is neither.
Start with the warning signs. If any of them apply, the tool stops and tells you so, which is the correct output rather than a failure.
Read these first. Any one of them means the question is not heat or cold.
What cold actually does
It reduces pain and makes a swollen area easier to rest. That is a real and useful effect. What it does not do is speed up healing, and the evidence for icing shortening recovery from a soft tissue injury is thin enough that several long-standing protocols have quietly dropped the R for rest and softened the I for ice. Use it for comfort in the first day or two, and do not expect more.
Twenty minutes at a time, a thin cloth between the pack and the skin, at least an hour before the next go. Longer is not better and can burn.
What heat actually does
It makes tissue more comfortable to move for a short window. That is why the timing matters: heat before you move, not after you have finished for the day. Used as a way to start moving, it earns its place. Used as a treatment on its own, it is a warm feeling for twenty minutes.
Never on skin with reduced sensation, and never overnight. Low-temperature burns from a heat pad left on all night are one of the more common self-treatment injuries, and they happen at temperatures that feel entirely comfortable at the time.
Why movement keeps appearing in the answers
Because for most of what people bring to a page like this, it is the treatment and the pack is the preparation. Stiffness that eases once you get going is telling you what it needs. Full rest beyond the first couple of days tends to make it worse.
When to stop reading and go
Something that will not take weight, numbness or weakness that is new, a joint that looks wrong after an impact, fever with spreading redness, or pain that has not shifted at all in six weeks. None of that is a heat or cold question. See our health disclaimer: this site is not medical advice and does not replace seeing someone.