Final answer:
The deepest heat penetration in physical therapy is achieved through ultrasound diathermy, which requires trained application to avoid tissue damage. Microwave diathermy is also used for deep tissue heating and treating various muscular conditions.
Step-by-step explanation:
In physical rehabilitation, the treatment method that provides the deepest penetration of heat into the tissues is ultrasound diathermy. High-intensity ultrasound at intensities of 1,000 to 10,000 W/m² and frequencies of 0.8 to 1 MHz is converted to thermal energy, which is particularly effective for treating injuries or overused muscles to relieve pain and promote flexibility. However, the application requires skill to avoid complications like bone burns and other tissue damage.
Another method used for deep tissue heating is microwave diathermy, which utilizes microwave radiation to penetrate deeper than shorter wavelengths into the tissue, aiding in the treatment of muscular pains, spasms, tendonitis, and rheumatoid arthritis.