Thermotherapy involves the application of Heat. The most common heating agent used in rehabilitation is a hot pack. Hot packs transfer their heat energy to the body by conduction. Superficial heat usually produces temperature elevation in the underlying tissues to a depth of up to 1cm. Adipose tissue acts as an insulating layer, which decreases the depth of heat. Commercial hot packs are canvas, usually filled with a hydrophilic substance, that is immersed in 170°F (774°C), water in a thermostatically controlled heater. The packs can retain heat up to 30 minutes. With superficial heat, local metabolism is increased and local vasodilatation with hyperemia occurs. Initial vasoconstriction is produced in deep tissue layers, followed by vasodilatation. Hot packs also promote muscle relaxation as well as sedation of sensory nerve endings.