This comes up more than any other question, usually with a note of suspicion that there is a correct answer being withheld. There is not much mystery to it.
The short version: ice for something new and angry, heat for something old and stiff. If you take nothing else from this page, that will serve you well most of the time.
Ice
Best in the first couple of days, while an area is swollen, hot and painful to touch. It numbs the area and takes the edge off, which is worth having.
What it is genuinely good at is pain relief. What it is not clearly good at is speeding up healing — that reputation is stronger than the evidence behind it, and there is a reasonable argument that heavy, prolonged icing gets in the way of the inflammatory process that repairs the tissue. So use it to be comfortable, and do not feel you have failed if you skip it.
How: fifteen to twenty minutes, a bag of frozen peas or a gel pack wrapped in a damp tea towel, never straight onto skin. Leave at least an hour before repeating. Take it off if the skin goes white or numb rather than just cold.
Heat
Better for the long-standing stuff: a stiff neck, an aching lower back, a calf that has been tight for weeks, general soreness the day after training. Heat relaxes muscle and increases blood flow to the area, and it makes stretching and movement afterwards more productive.
How: a hot water bottle, wheat bag or a warm shower, twenty minutes or so, warm rather than hot. Then move the area while it is still warm — the heat on its own does very little, it is what you do next that counts.
Neither, in these cases
- Over a fresh, swollen injury — no heat, for the first two to three days at least.
- On skin with no normal sensation. If you cannot feel temperature properly you cannot tell when it is doing damage. This includes areas affected by diabetes or nerve problems.
- On broken, infected or recently scarred skin.
- While asleep. Almost every ice burn and heat burn happens this way.
- If you have a circulatory condition, or Raynaud’s, check with your GP before using ice at all.
Honestly, it is not the main event
Both are comfort measures. Neither fixes the reason the tissue got irritated in the first place, and if a problem has lasted more than a couple of weeks, the ice-or-heat question is not the one that will change it.
What changes it is finding out why the tissue is being overloaded — which is what an assessment is for — and then doing something about the load. Ice and heat make the next few hours easier. They are not treatment.