Heat – The therapeutic temperature range is 40° to 45° C. Heat should be maintained for 5 to 30 minutes. Superficial heat is considered to penetrate 1 to 2 cm; deep heat, 3.5 to 8 cm.

Conduction is transfer of heat by contact, for instance, paraffin baths and hot packs (e.g., Hydrocollator packs). Although paraffin bath temperatures are ≈52° to 54° C, poor heat conductivity allows tolerance. Convection involves the flow of heat, e.g., fluidotherapy, whirlpool, and moist air. Examples of conversion (where nonthermal energy is converted to heat) include infrared, US, SWD, and MWD. Infrared has a 2-cm depth penetration. US penetrates 3.5 to 8 cm; the greatest heating is at the bone-tissue interface. US parameters include a frequency of 0.8 to 1.1 MHz; intensity of 0.5 to 4 W/cm²; treatment area of 100 cm²; and duration of 5 to 8 minutes. SWD penetrates 4 to 5 cm; fat is heated more than muscle; the most commonly used frequency is 27.12 MHz. MWD is rarely used. Penetration is not as deep as SWD; the 915 MHz frequency penetrates deeper than 2,450 MHz.