After an acute injury
, use ice to decrease local swelling for the first 48-72 hours. After that you may wish to alternate ice with heat to help blood flow to the area and aid in the healing process. Consult with your doctor if you have any questions.
For chronic conditions, it is up to the individual. Many times heating a local joint will allow the tissues to relax and allow for improved flexibility. Some people report that cooling makes them feel better. Generally, heat can be more effective in chronic conditions, but you should do what feels best.