Moist heat, or hot packs, may be applied to your body if you have an injury. The heat helps to increase circulation to the injured tissues, relax the muscles, and provide pain relief. In a physical therapy clinic, hot packs are kept in a device called a hydrocollator.
This is a large tank of hot water. The hot packs are cloth packs filled with a sand, clay and silica mixture. The hot pack absorbs the hot water, and then it is wrapped in terry cloth covers and towels before being applied to your body. The hot pack is usually kept on the injured body part for 15 to 20 minutes.