Therapeutic Heat (Thermotherapy)

Thermotherapy [Heat Therapy or Therapeutic Heat ] is the application of heat to the body for pain relief and health. It creates higher superficial tissue temperatures, which aids the healing process in some conditions.

It is used primarily to control pain, increase circulation, increase soft tissue extensibility, and accelerate healing in the rehabilitation side.

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) Micro-wave 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

 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 providing 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, hydrocollator Pack. eg- Fluidotherapy, whirlpool, moist air baths, and 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 variety of mechanism 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, lower 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 alone oxygen and nutrients that can help to speed healing and relax muscles, which can decrease pain sensation.

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

5) Changes in muscle strength-

Muscle strength and endurance have been found to decrease during the initial 30 minutes after the application of the heating agents. It is proposed that this decrease in muscles strength is the result of decreased firing rate of alpha motor neuron due to the change in the firing rates of type 2nd  muscle spindle efferent, gamma efferent, and type Ib fibers from Golgi tendon organs caused by heating of the motor nerves.

However if exercise are done after the lapse  of hours, the strength and endurance increases, as studies reveals that exercise done two hours after the application of heat increased muscle strength.

6) Increased metabolic rate –

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

Metabolism being a series of chemical reaction will increase with rise and decrease with a fall of temperature.

Any increase in enzymatic activity will result in an increase in the rate of cellular biochemical reactions. This can increase oxygen uptake and accelerate healing but may also increase the rate of the destructive process. As an example-a specific destructive enzyme such as collagenase (found to have an important role in rheumatoid arthritis) bring about an increase in collagenolysis at 36°C. This is the reason heat is not advised in arthritic conditions like rheumatoid arthritis and inpatient with acute inflammatory disorders.  

7) Effects on pulse rate and blood pressure-

The blood vessels of the body, such as peripheral blood vessels of the skin dilate or contracts 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 dilates to allow more heat to escape.

Drop in BP is compensate by increase the cardiac output, which can be achieved through increase in heart rate.

8) Increased sweating –

Sweating is the mechanism of thermo regulation. Raised temperature increase sweating owing to the stimulation of anterior hypothalamus.  The heat regulatory function of the hypothalamus is affected by inputs from temperature receptors in the skin.

The stimulation of sweat gland, secret hypotonic sweat on to the body surface so that increased evaporative cooling may take place.

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-

 As the temperature increases there is more oxyhemoglobin dissociation. Heat also increases metabolism. Metabolically active cell requires more O2 and liberate more acids and heat. The acids and heat, In turn promote the release of o2 from oxyhemoglobin.  Due to this change in o2  demand the rate of respiration to increase to meet the demand.

Therapeutic effects of heating-

1) Encouragement of healing-

  • Chronic inflammatory stages, the stages of repair, and regeneration are all appropriately treated with mild heating.
  • The application of heat to inflammatory injuries in the early stages is not beneficial.
  • Superficial heating agent increases the temperature of only the superficial tissue, so they accelerate the healing of only superficial structure,
  • It is advised to use deep heating modalities to achieve deeper effects. 

2) Relief of pain-

  • Therapeutic heat is widely used for the relief of pain.
  • Stimulation of sensory heat receptors may activate pain gate mechanism.
  • Vascular changes could also decrease local pain.
  • The increased blood flow could wash out some of the pain provoking metabolites like prostaglandins and bradykinin resulting from tissue injury.

3) Reduction of muscle spasm-

It has been suggested that heating the secondary afferent muscle spindle nerve ending and Golgi tendon organ could be a way in which an inhibitory influence is applied to the motor neuron pool to diminish muscle excitation.

Also, the pain and muscle spasm are interdependent, a reduction in one will cause a reduction in the other.

4) Sedative effect-

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

5) Increase in range of joint motion-

The analgesic effects of heat allow greater tolerance of stretching. Increased collagen extensibility occurs at higher temperature. Thus, heat can used prior to passive stretching and exercise to increase joint movement or lengthen scars or contractures.

6) Prevention of pressure sores-

Heat applied to areas of skin subjected to prolonged pressure or friction has been suggested in order to promote a greater blood flow in the skin and thus decrease the risk of skin breakdown.

7) Reduction of edema in extremities-

Heat has been recommended for the treatment of chronic oedema of the hand and foot. Vessels dilation induced by heating will allow increased rate of fluid exchange and thus may help to increase the reabsorption of exudates. This must be given with the part in elevation. Such heating arrangements coupled with exercise are valuable.

8)  Resolution of some skin disease-

Fungal infection 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