The short answer

Ice for the first two days after something new and sharp; heat for the stiff, achy back that has been there for a week or more. Twenty minutes at a time, a towel between you and the pack, and never on skin you cannot feel properly.

Why the timing matters

A fresh strain is inflamed: warm, swollen, guarded. Cold narrows the blood vessels and takes the edge off the pain so you can keep moving, which is the part that actually speeds things up. Once the heat and swelling have settled, the problem is usually muscles holding on too tight. Warmth relaxes them and makes the stretch you were avoiding feel possible.

When neither is the point

If you need the pack to get through every day for more than two weeks, the pack is managing a symptom that something else is causing. That is the moment to have the joint looked at rather than reaching for a bigger bag of ice. Numbness, weakness in a leg or pain that wakes you at night are reasons to call sooner.