How Cold and Heat Affect the Body

You strain your hamstring doing a penche, so you put ice on it. Your muscles are achy when you’re lifting your sousaphone over your head after the first week of band camp, so you use a heating pack to soothe them. You know that ice and heat help, but how do they help?

Cold application (cryotherapy) and heat application (thermotherapy) are often used in athletic medicine at all levels, from weekend warriors to professional athletes. But why? Changes in temperature result in predictable physiological changes within the body. As you would expect, cold temperatures often result in different changes than warm temperatures do, but there are some changes, such as reduction in pain, that can be caused by either cold or heat application.

Cold Application

In athletic medicine, cryotherapy is used locally in the form of an ice bag or ice cup, regionally by putting your legs in a cold whirlpool, or globally by sitting with most of your body submerged in a cold whirlpool or ice bath. When cold therapy is first applied, the body goes through a series of sensations referred to as CoBAN: Cold, Burning, Aching, and Numbness. CoBAN lasts for the first 3-5 minutes of cold application. This is why it’s critically important that you don’t keep taking your feet out of that post-rehearsal ice bath every 30 seconds – you make the CoBAN period last longer by doing that, and you reduce the overall effectiveness of the cryotherapy, too.

Cold application has numerous effects on the body, including:

Vasoconstriction

The diameter of the arterioles and venules in the area of the cold application become narrower. This decreases blood flow in the area, which helps limit the release of fluid that contributes to swelling.

Reduction of cell metabolic rate

Lowering the rate of cell metabolism leads to a reduction in tissue injury and death associated with acute trauma (when the metabolic rate is lowered within the first 24-48 hours after the injury). Most of the tissue damage that is associated with an acute injury is due to hypoxic cell death. This means that cells die due to a lack of oxygen. This hypoxia happens because the cells can’t get enough of the necessary oxygen from the blood vessels in the area because the normal blood flow has been disrupted either at the time of injury (the supplying blood vessels are torn) or as a result of swelling (the excess fluid pinches the blood vessels). Slower metabolism means that the cells require less oxygen. Since they require less oxygen, there is less of a chance that these cells will die due to the reduced blood flow. By reducing the amount of cells that die from not getting enough oxygen after an injury, the extent of the injury is reduced.

Decrease of muscle spasm

Cell metabolism creates waste products that can cause muscle spasm through irritating the muscle tissue. Since cold application slows down cell metabolism, it limits the amount of waste products created by the cell, thereby decreasing the intensity of muscle spasm.

Suppression of the acute inflammatory response

Cold application decreases the extent of the inflammatory response through several mechanisms. These mechanisms include reducing the release of chemical indicators that initiate the inflammatory process and decreasing the permeability of the capillary walls, which decreases the amount of swelling produced following an injury. Decreasing the cell’s metabolic rate also helps to decrease the acute inflammatory response.

Pain reduction

Cold application decreases the sensitivity of nerve endings, raising the threshold of the nerve. This means that it takes a greater stimulus (more pain, more irritation) to cause the nerve to send pain signals to the brain. This change in the nerve’s sensitivity is also why you’ll feel a little numb after you get done with cold application. Another way that cold application decreases pain perception is through reducing the swelling that causes pain by putting mechanical pressure on nerve endings.

Increase in joint stiffness

Cold application increases collagen tissue stiffness. Collagen tissues are found throughout the body and are prevalent in ligaments and joint capsules. This effect of cold application is why you shouldn’t perform maximal effort or high-risk activities immediately after cold application to a joint (your joints are at risk of sustaining a significant ligamentous injury immediately after cryotherapy).

 

Heat Application

Thermotherapy is applied locally with a moist heat pack or dry heating pad and regionally or globally through a warm whirlpool. The body’s exact response to heat depends on the type of heat energy applied, the intensity of the heat energy, how long the thermotherapy lasts, and which type of tissue is absorbing the heat energy (different tissue types respond differently to heat application). The heat application needs to be sufficient enough to be absorbed by the target tissue and increase molecular activity in order for the treatment to be clinically effective. Unfortunately, predicting the effectiveness of various types of thermotherapy can be difficult because there are still many unknown factors surrounding how heat application causes therapeutic changes.

Just like cryotherapy, thermotherapy has many physiological effects.

Vasodilation

Heat application causes the area’s blood vessels to dilate. You can see this for yourself the next time you exercise – look at the backs of your hands when you finish your workout. You’ll likely notice the veins in the back of your hand are much larger and sticking out more. This is due to vasodilation (in this case, it’s caused by exercising, rather than through heat application). Vasodilation increases blood flow, accelerates inflammation at the site of heat application, and facilitates healing and some of the other physiological effects of heat described below. Increased blood flow improves the rate of removal of cellular debris (what the body does to optimize healing) and increases the rate of delivery of nutrients to the healing site.

Increases cellular metabolic rate

The increased blood flow associated with thermotherapy brings more oxygen to the injured area. This is critical to facilitate the healing process. The larger amount of oxygen in the area allows the nearby cells to increase their metabolic rate. This helps these cells do all of the things that they need to do to help you heal properly, like breakdown the waste products in the area that are leftover from the initial injury or are a result of swelling. Additionally, the increased availability of oxygen allows the cells that are brought into the area to clean up after an injury to increase their metabolism as well, helping them clear out the damaged cells and waste products leftover from the initial injury. This increased cellular activity is important during the subacute stages of healing (after the first 72 hours following an injury).

Relieving muscle spasm

Like cryotherapy, heat application can relieve muscle spasm, but it does it in a very different way. Muscle spasms can be caused by a lack of proper blood flow to the area (ischemia). These types of spasms are relieved by heat because heat dilates blood vessels and increases blood flow.

Pain reduction

While both cold therapy and heat application help reduce the perception of pain, they use different mechanisms to do so. It is thought that heat helps reduce pain perception through something called a “pain gate mechanism” or a counterirritant effect. The basic theory behind this concept is that the sensation of heat gives the nerves another signal to send back to the brain instead of just sending pain signals. Unfortunately, as a result of this gating mechanism, the pain-relieving effects of heat tend to go away when the heat is removed.
Heat application indirectly decreases pain by decreasing muscle spasm (described above) and by decreasing mechanical pressure on the nerve endings (through increased metabolism facilitating the body’s healing process as described above).

Decrease in joint stiffness

Heat increases the extensibility of collagen tissues, which can help decrease the stiffness of a joint. However, the heat therapy needs to be used in conjunction with stretching or range of motion exercises if therapy is going to take advantage of this tissue property change (think of it this way: how can you tell how much stretchiness a rubber band has if you don’t touch or use it?).

Physiological Effects of Cold and Heat Application

Be sure to check out the guidelines for using ice and heat.