Heat has various physiological effects including vasodilation, reduced blood viscosity, increased nerve stimulation and metabolic rate, elevated pain threshold, and changes in muscle strength. Therapeutically, heat encourages healing by boosting blood flow, relieves pain, reduces muscle spasm, and increases tissue extensibility. Proper application of heat modalities can accelerate wound healing, relieve inflammation and edema, and improve joint range of motion. However, temperatures above 45°C can damage tissues.

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PHYSIOLOGIC AND THERAPEUTIC EFFECTS OF HEAT
PHYSIOLOGICAL EFFECTS OF HEAT
1. Vasodilatiation
2. Viscosity
3. Increased nerve stimulation
4. Increased pain threshold
5. Changes in muscle strength
6. Increased metabolic rate
7. Increased tissue extensibility/ effects on Collagenous tissue
8. Increased sweating
9. Effect on pulse rate & blood pressure
10. Increased rate of breathing
1. Vasodilatiation
Heat causes vasodilatation and thus an increase in the rate of blood flow. With heating the skin
surface erythema or redness is produced due to Vasodilatation.Vasodilatation occurs not only to
distribute the additional heat around the body i.e. compensatory heat loss, but also to protect the
heated skin. Superficial therapeutic heating agents produce more pronounced vasodilation in the
local cutaneous blood vessels where they cause the greatest change in temperature and less
pronounced dilation in the deeper vessels such as those running through muscles.
Thermotherapy causes vasodilatation by variety of mechanisms such as;
• Direct reflex activation of the smooth muscles of the blood vessels by cutaneous thermo
receptors,
• Indirect activation of local spinal cord reflexes by cutaneous thermo receptors and
• Increasing the local release of chemical mediators of inflammation.
Superficial heating agents stimulate the cutaneous thermo receptors via their axons directly to
nearby cutaneous blood vessels causes the release of bradykinin and nitrous oxide. Bradykinin
and nitrous oxide act as vasoactive mediators stimulating relaxation of the smooth muscles of the
vessel walls to cause vasodilatation. This vasodilatation occurs locally, in the area where the
heat is applied.
Cutaneous thermo receptors project via the dorsal root ganglion to synapse with interneurons in
the dorsal horn of the gray matter of the spinal cord. These interneurons synapse with
sympathetic neurons in the lateral gray horn of the thoracolumbar segments of the spinal cord to
inhibit their firing and thus decrease sympathetic output. This decrease in sympathetic activity
causes a reduction in smooth muscle contraction, resulting in vasodilation both at the site of heat

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application and in the cutaneous vessels of the distal extremities. This distant vasodilative effect
of thermotherapy may be used to increase cutaneous blood flow to an area where it is difficult or
unsafe to apply a heating agent directly.
Increase in blood flow that occurs in response to increased tissue temperature, acts to protect the
body from excessive heating and tissue damage. The increased rate of blood flow increases the
rate at which an area is cooled by convection.
2. Viscosity
The resistance to flow in a blood vessel depends directly on the viscosity of the fluid & inversely
on the radius of the vessels.
• It is temperature dependent, so raising the temperature in the liquids lowers the viscosity.
• Viscosity changes affect not only the fluids in narrow vessels (Blood & Lymph), but also
fluid movement within & throughout the tissue spaces.
• Viscosity is a measure of the resistance of a fluid. It describes a fluid's internal resistance
to flow.
↑ Temperature
Inflammation
Cutaneous
Thermo receptors
Smooth Muscle
relaxation
Release of ↑Vasodilators
like histamine &
prostaglandin
Spinal Dorsal
Root Ganglia
↓ Sympathetic
adrenergic
activation
Vasodilatation

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• Increasing the temperature in the liquids lowers the viscosity not only the fluids in narrow
vessels (Blood & Lymph, but also fluid movement within & throughout the tissue spaces.
3. Increased nerve stimulation
Increased temperature changes nerve conduction velocity and firing rate i.e decrease the
conduction latency of both sensory and motor nerves. Nerve conduction velocity has been
reported to increase by approximately 2 meters/second for every 1 °C increase in temperature.
Elevation of muscle temperature has been shown to result in a;
• decreased firing rate of type II muscle spindle efferents and gamma efferents
• an increased firing rate of type lb fibers from Golgi tendon organs
• a reduction in the firing rate of alpha motor neurons
• decrease in gamma neuron activity which causes the stretch on the muscle spindles to
decrease
• decreased alpha motor neuron activity
All these leads to relaxation of muscle contraction and thus to a reduction in muscle spasm
4. Increased pain threshold
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 sensations. The sensation of heat also alters the perception of pain. Heat increases the
activity of the cutaneous thermoreceptors which can have an immediate inhibitory gating effect
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 heating agents. It is proposed that this decrease in muscle strength is the result
of decreased firing rate of alpha motor neurons due to the changes in the firing rates of type II
muscle spindle efferent, gamma efferent, and type Ib fibers from Golgi tendon organs caused by
heating of the motor nerves. This also thought to result in augmentation of the individual's pain
threshold.
6. Increased metabolic rate
According to Van’t Hoff’s law the rate of chemical reaction increases two to three folds or each
100
rise in temperature.
Metabolism being a series of chemical reactions will increase with a rise & decrease with a fall
of temperature. Any increase in enzymatic activity will result in increase in the rate of cellular
biochemical reactions. This can increase oxygen uptake and accelerate healing but may also
increase the rate of destructive processes. This is the reason heat is not advised in arthritic
conditions like rheumatoid arthritis where heat can increase the activity of collagenase and thus

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accelerate the destruction of articular cartilage. Therefore thermotherapy should be used with
caution in patients with acute inflammatory disorders.
Increasing tissue temperature increases the oxygen-hemoglobin dissociation, resulting in more
oxygen available for tissue repair. It has been shown that hemoglobin releases twice as much
oxygen at 410
C which in conjunction with the increased rate of blood flow and the increased
enzymatic reaction rate, contribute to the acceleration of tissue healing by thermotherapy.
7. Increased tissue extensibility/ effects on Collagenous tissue
At normal tissue temperatures, collagen primarily exhibits elastic properties & only minimal
viscous flow. At temperatures within the range of 40 – 45˚C, the extensibility of collagen tissue
increase. Joint stiffness is often associated with changes in the visco-elastic properties of joints.
So heat can be used prior to treating joint stiffness.
8. Increased sweating
Sweating is mechanism of thermo regulation. Raised temperature increases 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.
9. Effect on pulse rate & blood pressure
The blood vessels of the body, such as peripheral blood vessel of skin dilates or contracts
depending on body temperature. When the temperature of the body drops the peripheral blood
vessels contracts to conserve heat. When body temperature increases above normal, the
peripheral blood vessels dilate to allow more heat to escape. The blood vessel dilation lowers the
blood pressure because the blood distribution has increased to areas which previously had less
distribution. The body now has to compensate. Because of the change in pressure, less blood is
pumped to the vital organs of the body. It does this by increasing the cardiac output by increasing
the heart rate. Thus an increase in temperature through vasodilatation of blood vessels causes a
drop in BP which is compensated for by increasing the cardiac output, which can be achieved
through increase in heart rate. Therefore heart rate increases when the temperature increases.
Lowering of blood pressure also due to decrease in sodium concentration, loss of urea & other
nitrogenous substance due to increased metabolism due to heating.
10. Increased rate of breathing
As the temperature increases there is more oxyhemoglobin dissociation. Heat also increases
metabolism. Metabolically active cells require more O2 and liberate more acids and heat. The
acids and heat, in turn, promote release of O2 from oxyhemoglobin. Due to these change in O2
demand the rate of respiration too increases to meet the demand.
1. Increased elimination through KIDNEYS.

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NOTE: - 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, oedema, 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.
THERAPEUTIC EFFECTS OF HEAT
1. Encouragement of healing
It is evident that any condition in which increased metabolic rate, cell activity & local blood flow
were beneficial could be appropriately treated by mild heating. But the application of heat to
inflammatory injuries on the early stages is not beneficial.
Chronic inflammatory stages, the stages of repair & regeneration are all appropriately treated
with mild heating.
All forms of therapeutic heating are applied to a wide range of chronic & post-traumatic
conditions including the arthroses, soft tissue lesions & post surgical heating.
Because superficial heating agents increase the temperature of only the superficial few
millimeters of tissue, they are most likely to accelerate the healing of only superficial structures
such as the skin, or deeper tissue layers exposed due to skin ulceration. Deeper effects may also
occur as the result of consensual vasodilatation in areas distant from or deep to the area of
increased temperature. 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. It was found that heat is the most effective
non-analgesic method of pain control. Much therapeutic heating is of the skin. Stimulation of
sensory heat receptors may activate pain gate mechanism. Vascular changes could also decrease
local pain. The increased blood flow that has been observed could wash out some of the pain
provoking metabolites like Prostaglandins & Bradykinin resulting from tissue injury.
3. Reduction of Muscle Spasm
It has been suggested that heating the secondary afferent muscle spindle nerve endings & Golgi
tendon organs could be a way in which an inhibitory influence is applied to the motor neuron
pool to diminish muscle excitation. Also the pain & 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.

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5. Increase in Range of Joint Motion
The analgesic effect of heat allows greater tolerance of stretching. The viscosity of tissues will be
decreased which partly account for the reduction of joint stiffness that occurs with heating.
Increased collagen extensibility occurs at higher temperatures. Thus heat can used prior to
passive stretching & exercise to increase joint movement or lengthen scars or contractures.
6. Prophylaxis (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 & thus decrease the risk of skin breakdown.
7. Reduction of Oedema in Extremities
Heat has been recommended for the treatment of chronic oedema of the hand & foot. Vessel
dilatation induced by heating will allow increased rates of fluid exchange & thus may help to
increase the reabsorption of exudates. This must be given with the part in elevation since the
application of superficial heating will tend to increase oedema if the part is dependent. Such
heating arrangements coupled with exercises are valuable.
8. Resolution of Some Skin Diseases
Fungal infections which are difficult to control & thrive in moist conditions are sometimes
treated with regular infrared therapy.
Important Facts to Note in Thermotherapy Treatment Techniques
1. Due to heat there is increased capillary permeability & Increased capillary dilatation causes –
Erythema or redness.
2. The skin temperatures over 45˚C causes tissue damage. Further rise in temperature will lead
to denaturation & death of cells & 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 oedema should be treated with any heat modality until the reasons for the
oedema are determined.
Degrees of heat sensation can be categorized as follows;
a) MINIMAL WARMTH: - Threshold value, Gentle comforting warmth.
b) MEDIUM WARMTH: - Distinct feeling of agreeable warmth.
c) MAXIMUM WARMTH: - Intense feeling of heat, maximum heat tolerance is exercised.
d) DANGER LEVEL: - Intolerable heat, Burning sensation.
References

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1. Introduction to Physiotherapy & Electrotherapy – Unit – 1.
http://faculty.mu.edu.sa/download.php?fid=8915
2. Cameron MH. Physical Agents in Rehabilitation: From Research to Practice. Second
Edition. 2003, Elsevier.
3. Singh J. Textbook of Electrotherapy. 2 edition. Jaypee. 2012.
4. Forster A, Palastanga N. Clayton’s Electotherapy, Theory and Practic. 9th
Ed. W B
Saunders; 2006.
5. http://en.wikipedia.org/wiki/Perspiration
6. http://wiki.answers.com/Q/How_does_temperature_increase_heart_rate?#slide=14