This was one of the first questions that came up in physiotherapy school. And with recent research, I can’t say that the answer is much clearer all these years later. However, here are the basics:
In the initial stages of a muscle or joint injury, there is an increased activation of white blood cells in the area. These cells play a critical role in triggering the healing process by “eating up” dead/damaged tissue and releasing “cytokines” that regulate the behaviour of other important cells in the body needed to help recovery. The increase in white blood cells is what causes an inflammatory response (i.e. swelling and/or pain) with an injury.
Ice is used primarily to decrease inflammation. The application of a cold compress reduces blood flow, thereby reducing swelling and inflammation. In addition the cooling of the skin affects the sensory nerve endings, acting as a secondary pain reliever. The application of ice to treat an initial trauma (muscle strain, sprain, or contusion) is based on the notion that, aside from being painful, too much swelling can induce secondary tissue damage to healthy cells around the area of injury.
However, this must all be taken with the proverbial “grain of salt”. Animal studies have shown a potential for cold-induced tissue damage if the ice is applied for too long, or is too cold. There have been case reports of burns, nerve injury and frost bite if cold is applied for prolonged periods.
A general rule of thumb is, use ice only in the first 24-72 hrs after an injury. Apply for 10-15 minutes, then wait 90 minutes before re-applying. If a person is elderly, the duration of time to apply the cold compress should be limited (less than 10 minutes) and monitored to avoid tissue damage.
The application of heat (e.g., hot water bottles, heating pads, hot packs, hot baths) increases blood flow, metabolism, and elasticity of connective tissues. Increasing the temperature of body tissues is thought to promote healing by increasing the supply of nutrients and oxygen to the site of the tissue damage.
Research (which includes several randomized control trials) suggests that the use of heat is best for acute and subacute low back pain and/or muscle soreness from overexertion. Moist heat (versus dry heat) seems to be the most effective for reducing pain. Apply for 15-20 minutes, every 4 hours.
Again, there are some precautions associated with applying heat to an injured area. Specifically, people with altered nerve function or decreased sensation need to be closely monitored. This would include those with multiple sclerosis, poor circulation, diabetes, spinal cord injuries, or rheumatoid arthritis.
The Bottom Line:
If you have an acute injury with swelling (e.g., a ligament sprain or muscle strain), apply ice during the first 48 to 72 hours. After 72 hours, there is little demonstrated benefit. Moist heat is the preferred application for acute low back pain or muscle soreness. Use it when you feel muscles are tight or aching.