How Heat Therapy Relieve Pain

How Heat Therapy Relieve Pain

One of the oldest methods of musculoskeletal pain relief is heat therapy. The principle is simple: it is to apply heat or coolness to the painful area. Unlike medicinal methods, heat therapy effectively reduces or eliminates pain with few side effects. The heat or the cold will also have physiological effects improving the therapeutic treatment. Today, manufacturers are imagining increasingly innovative and adapted products. We tells you more about the benefits of heat therapy, its principle and how this therapeutic method can relieve pain. Pain is defined by the International Association for the Study of Pain as “an unpleasant feeling and emotional experience in response to or described in actual or potential tissue damage”. To understand how heat therapy can relieve it, it is still necessary to understand how trauma can cause a sensation of pain.

Physiological Mechanism

The human body is traversed by specialized nerve endings contributing to proprioception and the perception of touch (vibration, pressure, etc.). The signals that these endings send pass through nerve fibers of strong caliber surrounded by a sheath of myelin – a fatty substance that has the effect of increasing the speed of transmission. They thus quickly send information to the spinal cord.

The skin, muscles and viscera walls also contain nociceptors, free, unspecialized nerve endings, at a rate of 200 nociceptors / cm². Their role is to inform the body when it suffers an attack likely to endanger its integrity. When activated, these nociceptors transmit a nerve signal that will pass through different fibers, the A-Delta fibers and the C fibers. A-Delta fibers are weakly myelinated and C fibers are not. They are smaller in diameter than the previous fibers. They transmit their signals less quickly.

Cold (non-painful), light touch, and rapid, transient sharp pain (pain from a sting, for example) circulate through the A-Delta fibers. As for the C fibers, which are slower, they transmit the heat message (not painful), the acute late pain and the diffuse pain, causing a sensation that is maintained over time.

The difference in transmission speed between these fibers explains why, during an injury, we first feel a sharp, localized pain, then a vague, diffuse pain. It would also be at the origin of the anesthetic effect of the cold.

The “gate control”

In 1965, Robert Melzack and Patrick D. Wall proposed the theory of the “gate control” or “wicket” mechanism in French. The pain message sent to the brain does not get there directly. It passes through the marrow, where it is subjected to inhibitors (blockers). If it is powerful enough, it accesses the brain, generating the sensation of pain. Otherwise, it is stopped before.

Since myelinated fibers transmit information more quickly, it would suffice to activate them during a painful episode for them to send a nerve message to the spinal cord. There, their message, which came first, would “close” the door to the pain message delivered by the slower conduction C fibers. This is what explains why we rub or vigorously massage the skin after a shock or trauma: we block the message of the C fibers, relieving the pain. The same would apply for TENS (electrical stimulation) and acupuncture. Their messages pass through larger myelin sheathed fibers, preventing the “pain” message carried by the slower fibers from reaching the brain. 

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