Therapeutic Heat (Thermotherapy or Heat therapy)

Thermotherapy (Heat Therapy or Therapeutic Heat ) is a therapeutic technique that utilizes heat to promote healing and alleviate discomfort.

Applying heat to a sore area increases tissue temperature and can aid in healing. Heat therapy is mainly used to help manage pain, increase the flow of blood and oxygen to the affected area, make your muscles and other soft tissues more flexible, and speed up the healing process.

Types of Therapeutic heating modalities:

Two types of Therapeutic heating modalities – 1) Superficial heating modalities; 2) Deep heating modalities.

Superficial heating modalities Deep heating modalities
Paraffin Wax Therapy (PWB) Short Wave Diathermy (SWD)
Moist Heat (Hydrocollator Pack) Microwave Diathermy (MWD)
Contrast Bath Ultrasound Therapy (UST)
Fluidotherapy Infrared Radiation (IRR)
Whirlpool Bath Ultra-violet Radiation (UVR)
Electrical heating pad LASER (light amplification by stimulated emission of radiation)

Transfer of heating

Transfer of heating

In general, the transfer of heat (whether the purpose is heating or cooling) often is classified into 3 general types of heat transfer: 

  1. Conduction, 
  2. Convection, and
  3.  Conversion.

They are characterized as follows:

Conductive heating Convective heating Conversion heating
It is the movement of heat from one object to another one that has a different temperature when they are touching each other. This form of heating is produced by the movement of the transferring heating medium, usually air or a fluid.  This involves the conversion of one energy form (eg, light, sound) into another (heat).
Superficial heat is usually conductive heat It also provides superficial heat. Deep heating modalities are usually produced by the conversion of applied energy into heat as the energy penetrates tissue
eg- hot water baths, hot packs, electric heating pads, paraffin wax bath, and hydro collator Pack. eg- Fluidotherapy, whirlpool, moist air baths, hot-air baths, contrast baths. eg- deep heating modalities

Physiological Effects of Heating

Heat therapy is most commonly used for rehabilitation purposes. The therapeutic effects of heat include increasing the extensibility of collagen tissues; decreasing joint stiffness; reducing pain; relieving muscle spasms; reducing inflammation, edema, and aids in the post-acute phase of healing; and increasing blood flow. The increased blood flow to the affected area provides proteins, nutrients, and oxygen for better healing.

1) Vasodilation-

Heat causes Vasodilation and thus an increase in the rate of blood flow. With heating the skin surface erythema or redness is produced due to vasodilation.

Thermotherapy causes vasodilation by a variety of mechanisms such as-

  • Increasing the local release of chemical mediators of inflammation.
  • Indirect Activation of local spinal cord reflexes by cutaneous thermoreceptors
  • Direct reflex activation of the smooth muscles of the blood vessels by cutaneous thermoreceptors.

2) Viscosity –

The resistance to flow in a blood vessel depends directly on the viscosity of the fluid and inversely on the radius of the vessels.

It is temperature dependent, so raising the temperature in the liquid, lowers the viscosity.

3) Increased nerve stimulation –

Increased temperature changes nerve conduction velocity and firing rate. Nerve conduction velocity has been reported to increase by approximately 2 meter/sec for every 1°C increase in temperature.

4) Decrease pain sensation-

Heat causes more blood to flow into the area. When blood flow increases to an area, it brings along oxygen and nutrients that can help to speed healing and relax muscles, which can decrease pain sensation.

The sensation of heat also alters the perception of pain .heat increase the activity of the cutaneous thermoreceptors which can have immediate inhibitory gating effects on the transmission of the sensation of pain at the spinal cord level.

5) Changes in muscle strength-

Heat therapy can temporarily decrease muscle strength and endurance by changing the way certain nerve fibers firing rate. This effect lasts for about 30 minutes after heating. However, studies show that doing exercise a few hours after heating can increase strength and endurance.

6) Increased metabolic rate –

According to Van’tv Hoff’s law, the rate of chemical reaction increases by two or three folds or each 10° rise in temperature.

Metabolism is a series of chemical reactions that will increase with a rise and decrease with a fall in temperature.

Increased enzyme activity can help speed up healing by increasing oxygen uptake, but it can also cause damage to the body. For instance, heat can activate destructive enzymes like collagenase, which breaks down important tissues in rheumatoid arthritis. Therefore, using heat is not recommended for arthritis and other acute inflammatory conditions.

7) Effects on pulse rate and blood pressure-

The blood vessels of the body, such as peripheral blood vessels of the skin dilate or contract depending on body temperature.

When the temperature of the body drops the peripheral blood vessels contract to conserve heat. When body temperature increases above normal, the peripheral blood vessels dilate to allow more heat to escape.

A drop in BP is compensated by increasing the cardiac output, which can be achieved through an increase in heart rate.

8) Increased sweating –

Sweating helps regulate your body temperature. When your body gets too hot, your brain’s “thermostat” (the Anterior hypothalamus) signals your sweat glands to produce sweat. Sweat is a watery liquid that is released through tiny pores in your skin, and it helps cool you down by evaporating into the air. The more you sweat, the more your body cools off.

Your skin also has special temperature sensors (temperature receptors) that send signals to your brain, telling it to turn on the sweating mechanism when you need it.

9) Increased tissue extensibility/ effects on collagenous tissue-

At normal tissue temperature, collagen primarily exhibits elastic properties and only minimal viscous flow at a temperature within the range of 40-45°C, the extensibility of collagen tissue increases.

Joint stiffness is often associated with changes in the visco-elastic properties of joints. So heat can be used prior to treating joint stiffness.

10) Increase rate of breathing-

  • When temperature increases, there is more separation of oxygen from hemoglobin molecules.
  • Heat also increases cell metabolism, which requires more oxygen and produces more acids and heat.
  • The acids and heat produced promote the release of oxygen from oxyhemoglobin.
  • This change in oxygen demand causes the rate of respiration to increase to meet the demand.

Therapeutic effects of heating-

1) Encouragement of healing-

  • Mild heat can be helpful in treating chronic inflammation, repair, and regeneration.
  • However, applying heat to injuries in the early inflammatory stages is not beneficial and may even make things worse.
  • Superficial heating agents only heat up the surface layers of tissue and thus are only effective for healing superficial structures.
  • To achieve deeper effects, it is recommended to use deep heating modalities.

2) Relief of pain-

  • Therapeutic heat is commonly used to relieve pain.
  • Heat can stimulate sensory heat receptors which activate the pain gate mechanism.
  • Heat can also cause vascular changes that reduce local pain.
  • Increased blood flow due to heat can wash away pain-causing substances such as prostaglandins and bradykinin resulting from tissue injury.

3) Reduction of muscle spasms-

It has been proposed that by applying heat to certain nerve endings in the muscles, specifically the secondary afferent muscle spindle nerve ending and Golgi tendon organ, we may be able to reduce the excitation of the motor neurons that control muscle movement. This could lead to a reduction in muscle activity and potentially alleviate symptoms such as pain and spasms.

Furthermore, pain and muscle spasm are related to each other, and reducing one may also lead to a reduction in the other.

4) Sedative effect-

This might be simply a consequence of pain relief. The sedative effects are evident in milder forms of heat therapy.

5) Increase in range of joint motion-

Heat makes stretching easier by reducing pain and increasing muscle flexibility, allowing for greater joint movement and scar/contracture lengthening.

6) Prevention of pressure sores-

Applying heat to areas of skin that are under prolonged pressure or friction can increase blood flow to those areas, which can help reduce the risk of skin damage or sores.

7) Reduction of edema in extremities-

Heat, combined with elevation and exercise, can be used to treat chronic swelling in the hands and feet by increasing fluid exchange and absorption.

8)  Resolution of some skin diseases-

Fungal infections which are difficult to control and thrive in moist conditions are sometimes treated with regular infrared therapy.

Important facts to note in the thermotherapy treatment techniques

  1. Due to heat there is increased capillary permeability and increased capillary dilation causes erythema or redness.
  2. The skin temperature over 45°C causes tissue damage. further rise in temperature will lead to denaturation and death of cells and tissues.
  3. The dosage of heat treatment can only be guided by the feeling of warmth on the part of the patient.
  4. No patient with edema should be treated with any heat modality until the reason for the edema is determined.  


  • Singh J textbook of Electrotherapy
  • Electrotherapy Simplified by Basant Kumar Nanda