Heat is commonly used following exercise to prevent delayed onset muscle soreness (DOMS). Most heat used in a clinical setting for DOMS are only applied for 5 to 20 minutes. This minimal heat exposure causes little, if any, change in deep tissue temperature. For this reason, long duration dry heat packs or organic hot/cold 'heat' packs used at home to slowly and safely warm tissue and reduce potential heat damage while reducing pain associated from DOMS.
Clinically, it has been shown that moist heat penetrates deep tissue faster than dry heat. Therefore, in home use heat packs along with moist heat may be more effective than dry heat to provide pain relief and reduce tissue damage following exercise DOMS. However, heat packs moistened with heat will only last for 2 hours compared to the 8 hours duration of chemical dry heat packs.
Heat has been used therapeutically for thousands of years. It offers immediate pain relief and can increase circulation to speed the healing process after injury. For this reason, it is popular for use on many types of pain including joint and muscle pain as well as soft tissue damage.
The effect of heat on pain is mediated by heat sensitive calcium channels. These channels respond to heat by increasing intracellular calcium. This generates muscle action potentials that increases stimulation of sensory nerves and causes the feeling of heat in the brain. These channels have in common their sensitivity to other substances such as vanilla and menthol. These multiple binding sites allow a few factors to activate these channels. Once activated, they can also inhibit the activity of pain receptors, and are located in the peripheral small nerve endings . For peripheral pain, for example, heat can directly inhibit pain. However, when pain is originating from deep tissue, heat stimulates peripheral pain receptors which can alter what has been termed gating in the spinal cord and reduce deep pain.
Another effect of heat is its ability to increase circulation. These receptors along with noiciceptors, increase blood flow in response to heat. The initial response to heat is mediated through sensory nerves that release substances to increase circulation. After a minute or so, nitric oxide is produced in vascular endothelial cells and is responsible for the sustained response of the circulation to heat. This increase in circulation is considered essential in tissue protection from heat and repair of damaged tissue.
Heat is used in different modalities in the treatment of back pain and muscle soreness. Dry heat can be applied through either heat packs or techniques that warm tissue such as diathermy and ultrasound Heat packs can be dry or moist. In Sports Massage Hydrocolator heat packs are usually at 165 deg F and are separated from the skin by 6 - 8 layers of towels and used only in clinical settings. Hydrotherapy (warm) uses water at 105 deg F and involves immersing a limb in the water. Hydrotherapy can include contrast baths or simply warm water immersion. A major problem with this type of heating is that it is usually used for short periods of time, for example, 5 - 20 minutes. There is also a combination of cryotherapy-heat therapy (10 min ice/10 min heat) which increases circulation and speeds muscle relaxation, generally used in minimizing pain and inflammation.
Moist heat, in most studies, appears to be advantageous in pain relief to many short duration dry heat modalities such as electric heat pads. But these heat modalities are used for short periods of time, for example, 20 minutes maximum. Many studies have shown that short duration of heat application results in poor heat transfer to deep tissues. Therefore, in deep injuries, heat application for short duration causes pain relief through the gate control theory of pain in the central nervous system and not through the peripheral nervous system. Long term application of heat, such as in chemical heat wraps, solves this issue by applying heat for hours to warm deep tissue gradually. But dry chemical heat wraps heat deep tissue much slower that pain relief is delayed by at least 30 minutes.
While moist heat penetrates deep tissues better than dry heat for warming. This is supported by research examining heat transfer from various types of heat modalities from skin to subcutaneous tissues. Moist heat modalities transfer heat much faster than do dry heat modalities and research shows that they cause much faster heat penetration than dry heat. Even air with high humidity transfers heat faster than dry air. But it is not just the type of heat but the duration as well that affects heat transfer into deep tissues.
For example, contrast baths use warm and cold water immersion that alternates within minutes in warm and cold baths. While this changes skin temperature, there is no evidence that it penetrates into deep tissues. Whirlpool heat penetrates quickly, but is used for only a short duration as are hydrocolator heat packs which provide moist heat but are left on for less than 20 minutes due to their high temperature. Someone with thick subcutaneous fat will therefore only see a small difference in deep tissue temperatures with these modalities.
To penetrate deep into tissue, lower temperature and long duration heat packs are often used. Long duration heat products (for example, chemical dry heat) offer the advantage of being safer and can be left on for hours to warm deep tissue and provide increased circulation and pain relief. But the increase in tissue temperature is slow as is the onset of pain relief. Chemical moist heat lasts for a shorter duration than dry chemical heat packs, lasting between 30 minutes and two hours. For more information refer to Moist or Dry Heat for Delayed Onset Muscle Soreness, Journal of Clinical Medicine Research - J.Petrofsky, 2013