Ice an acute injury, such as a sprained ankle, until the swelling and pain subside, which usually takes two to three days.  Icing constricts blood vessels so there’s less internal bleeding and less leakage of fluids into tissues, thus lessening swelling and pain. It also acts as a local anesthetic and reduces muscle spasms.

Icing can help some chronic overuse injuries as well, including tennis elbow (and other types of tendinitis), certain types of knee pain, bursitis, as well as persistent trigger point muscle pain. If you are an athlete or otherwise engage in regular vigorous exercise, icing the affected areas after workouts (your elbow after tennis, for example, or your shoulder after golf) may help reduce inflammation.

You can begin heat after swelling has subsided.  Applying heat too soon may worsen initial inflammation.  Heat dilates blood vessels and increases blood flow, which helps promote healing by removing waste products from the injured area. Heal also relaxes tight muscles, relieves spasms, reduces joint stiffness, lessens chronic inflammation, and has general pain-relieving effect-and thus may be helpful for osteoarthritis that continuous low-level heat helps low-back, neck, and wrist pain.

In general, ice what’s swollen, and apply heat to what feels sore and achy.