Thermal effects:

The factors affecting thermal effects are the following.
·         Volume of tissue absorbing energy (heat)
·         Composition of tissues (proteinous are more sensitive to heat)
·         Capacity of tissue to dissipate heat
·         Rate of temperature rise

Effects of heat on body:

Cellular effects:

Metabolic activities are increase by the rise in temperature i.e. 13 % of metabolic activities is increased by 1°C rise in temperature. When the metabolic activities increase the demand for O2, nutrients also increases and more waste products are produced. Accelerated cellular metabolism can produce many beneficial therapeutic effects to treat injury or infection.
Effects on blood flow:
When the skin is heated the surface become reddens and the blood vessel dilated increasing the blood flow. For the healing if there is infection a good blood supply that beings more WBCs kill bacteria can assist n the process. Increase in metabolism leads to release to CO2 and lactic acid leading to greater acidity of the tissues which causes dilatation of the vessels. Histamine and tissue dilators substances such as bradykinin are also released by the greater heating of the tissues. They also help in the dilatation of vessel.

Effects on collagen tissues:

Some specific tissues in the body such as collagen tissues are sensitive to heat. It has been shown from previous study that above 50°C temperature (40 - 45°C) the extensibility of collagen tissues has been increased. This only occurs if the tissue is simultaneously stretched and requires temperature near the therapeutic limit.

Neurological effects of heat:

↓ muscle tone:

Heating of the tissues to therapeutic range (40 – 45°C) results in the reduction of the muscle spasm. Ia afferent of the muscle spindle have been shown to increase their firing rates with raise in temperature while most secondary to pathological changes afferent decreases firing rate with the rise in temperature.
Heating will stimulate the Golgi tendon organ. ↑firing of Golgi tendon organ resulting in increase inhibition. All these factors will reduce the muscle tone.

Relieve of pain:

Pain is relieved by the application of heat due to decreased nerve conduction velocity or elevated pain threshold. It may be due to sedative or counter irritation effect of heat. Pain relieve is associated with relieve of muscular spasm and increase in joint range of motion.