1. Application

1.1. Direct Contact

Moist heat therapy has been believed to be more effective at warming tissues than dry heat, because water transfers heat more quickly than air. Clinical studies do not support the popular belief that moist heat is more effective than dry heat. Moist heat results in the perception that the tissue is heated more deeply. In fact, recent studies indicate that vasodilation, the expansion of the blood capillaries (vessels) to allow more blood flow, is improved with dry heat therapy. Expansion of the blood capillaries is the primary objective of heat therapy. Heat therapy increases the effect on muscles, joints, and soft tissue. Heat is typically applied by placing a warming device on the relevant body part.

Newer breeds of heat therapy devices combine a carbon fiber heater with a cordless rechargeable lithium battery and are built into the specific body wrap (i.e., shoulder wrap or back wrap) for targeted heat therapy. Such devices can be used as alternatives to chemical or plugged-in heating pads, but have not been shown to improve the clinical benefit. All devices primarily provide heat to promote vasodilation.

1.2. Infrared Radiation

Infrared radiation is a convenient system to heat parts of our body. It has the advantage over direct contact in that radiation can heat directly the area where the blood capillaries and neuron terminals are. When heat comes from a direct contact source it has to heat the external layer of the skin, and heat is transferred to the deeper layer by conduction. Since heat conduction needs a temperature gradient to proceed, and there is a maximum temperature that can be safely used (around 42 °C), this means lower temperature where warming is needed.

Infrared (IR for short) is the part of the electromagnetic radiation spectrum comprised between 0.78 μm and 1 mm wavelength. It is usually divided into three segments:

IR-A, from 0.78 to 1.4 μm.
IR-B, from 1.4 to 3 μm.
IR-C, from 3 μm to 1 mm.

IR radiation is more useful than the visible radiation for heating our body, because we absorb most of it, compared to a strong reflection of visible light. Penetration depth of infrared radiation in our skin is dependent of wavelength. IR-A is the most penetrating, and reaches some millimeters, IR-B penetrates into the dermis (about 1 mm), and IR-C is mostly absorbed in the external layer of the epidermis (stratum corneum).^[1] For this reason the infrared lamps used for therapeutic purposes produce mainly IR-A radiation.

2. Mechanism of Action, and Indications

Heat creates higher tissue temperatures, which produces vasodilation that increases the supply of oxygen and nutrients and the elimination of carbon dioxide and metabolic waste.^[2]

Heat therapy is useful for muscle spasms, myalgia, fibromyalgia, contracture, bursitis.^[2]

Moist heat can be used on abscesses to help drain the abscess faster. ^[3] A study from 2005 showed heat therapy to be effective in treating leishmaniasis, a tropical parasitic skin infection. ^[4]

Heat therapy is also sometimes used in cancer treatment to augment the effect of chemotherapy or radiotherapy, but it is not enough to kill cancer cells on its own. ^[5]

Heat therapy is contraindicated in case of acute injury and bleeding disorders (because of vasodilation), tissues with a severe lack of sensitivity, scars^[2] and in tissues with inadequate vascular supply (because of increased metabolic rate and demand which a tissue with poor blood supply may fail to meet resulting in ischemia).^[6]

The use of Heat therapy for deep-seated tissue can be treated with shortwave, microwave, and ultrasonic waves. This produces a high temperature that penetrates deeper. Shortwave produces a 27MHz current, microwaves use 915 and 2456 MHz, and ultrasound is an acoustic vibration of 1MHz. The way ultrasonic waves work is they selectively superimpose the incoming wave and increase the energy for absorption, and the significant part of the longitudinal compression gets converted into shear waves. When they are rapidly absorbed, the interface between soft tissue and bone is selectively heated. ^[7]

3. For Headaches

Heat therapy can be used for the treatment of headaches and migraines. Many people who suffer from chronic headaches also suffer from tight muscles in their neck and upper back. The application of constant heat to the back/upper back area can help to release the tension associated with headache pain. In order to achieve heat therapy for headaches, many use microwaveable pads which can often overheat, potentially leading to injury, and lose their heat after a few minutes. Some new products use heated water, running through pads, to maintain a constant temperature, allowing headache sufferers to use hands-free heat therapy in the treatment of their headache pain.

4. Therapeutic Benefits

Thermotherapy increase the extensibility of collagen tissues. Using heat, it can relieve the stiffness in joints in different cases. Shortwave and Microwave heat application may reduce muscle spasms, and selective heating with microwaves can accelerate absorption of hematomas. This will, in turn, allow the stiff muscle to stretch. Ultrasounds are not absorbed significantly in homogenous muscle. Heat therapy using hyperthermia has been used to treat cancer in combination with ionizing radiation. ^[8]