When ice packs are used after an injury, the brain becomes more aware of the sensation of the cold than the injury. This is called “the counter-irritant effect”. As the affected part gets colder, the sensitivity of pain nerve fibres is reduced and pain impulses are transmitted more slowly. The area becomes anaesthetised or numb. It is unwise to resume activity now in case you do more damage.
When the body is injured, muscles around the area become tense. This increased tension causes more pain. This is called the “pain-spasm-pain cycle”. Ice reduces pain and the stretch reflex and so allows the muscle to relax and break the cycle.
The blood circulation’s response to ice is first for the blood vessels near the surface to constrict and limit heat loss. This reduces blood flow and slows down the development of swelling. However, continued ice treatment longer than 20 minutes threatens the health of tissues, so a natural reflex expansion or dilation of the blood vessels follows. This increases the blood flow in the area and washes away waste products and brings in the body’s repair materials.
Although easy to apply and very effective, it does carry a small risk. The skin temperature is lowered and so there is a risk of “burning” the skin. Always make sure there is a layer of towel between the skin and the ice pack. So the general rule of thumb is to use an ice pack 3 times a day for 20 minutes, leaving at least an hour between each application.