Superficial heat affects the skin and subcutaneous soft tissues and stimulates a reflex relaxation in deeper muscles. Treatment with superficial heat elevates the pain threshold and thereby causes a decrease in the pain response. It may also cause a sedative effect and aid in muscle relaxation. Heat-induced changes in the viscoelastic characteristics of connective tissue aid in stretching activities. Superficial heat increases metabolism and oxygen consumption of the tissues. The metabolic rate increases two to three times that of normal for every 10°C increase in skin temperature. This heat modality also causes vasodilatation, which may aggravate swelling, edema, and superficial or subcutaneous bleeding. The therapeutic range of heating is between 40°C and 45°C. Anything above this range becomes painful. The length of treatment time is important. After approximately 8 minutes, superficial heating has extended only 0.5 cm into tissues. After approximately 15 to 30 minutes, the heating has extended 1 to 2 cm deep into muscles. After the application of heat, the area may be passively and actively stretched to improve flexibility and range of motion.

Like superficial heating modalities, deep heating modalities aid in improving flexibility of muscles and joint range of motion. Deep heating modalities work below the skin and superficial layers of tissue to a depth of approximately 3 to 5 cm.

Ultrasound is used to treat multiple soft tissue injuries such as tendinitis, bursitis, myositis, joint contractures, and scar tissue. Ultrasound uses a frequency between 0.8 MHz and 3 MHz to deliver sound waves through the tissues. The sound waves are emitted by a piezoelectric crystal that creates vibration and produces mechanical waveforms, which are transmitted from the ultrasound unit’s sound head (transducer) through a coupling medium such as gel into the tissues. The sound waves travel through the soft tissues and are reflected off the bones in an irregular pattern, which is absorbed in the connective tissues as heat. As with superficial heat, ultrasound increases local metabolism, blood flow, and nerve conduction velocity. Heating of the collagen fibers in the connective tissues elevates the pain threshold and allows for greater tissue distensibility. Scar tissue may be denatured and become more amenable to stretching using this modality. Ultrasound increases tissue temperature up to 43.5°C.

Acute localized inflammatory processes such as tendinitis, tenosynovitis, and epicondylitis can be treated with a corticosteroid solution (e.g., 1% hydrocortisone cream or a local anesthetic such as 1% lidocaine) coupled with ultrasound. This technique is known as phonophoresis. The ultrasound transducer allows deeper penetration of these medications than by topical application alone. After phonophoresis, the area is further treated with passive and active range of motion, manipulative medicine procedures, and if indicated, deep friction massage to break up scar tissue.

Short wave diathermy is used to treat acute and chronic pain complaints such as myositis, tendinitis, bursitis, and joint contractures. It uses radio waves at 27.12 MHz to heat the subcutaneous tissues. The unit uses a capacitor or inductor to transmit radio waves, which are absorbed in and heat the tissues. The temperature in subcutaneous fat is raised approximately 15°C while muscle is raised 4°C to 6°C by either continuous or pulsed waveforms from the unit. After short wave diathermy, the area is treated with passive and active range of motion or osteopathic manipulative medicine (OMM) to improve flexibility and range of motion.

Microwave diathermy uses electromagnetic radiation at frequencies of 9.15 MHz and 24.50 MHz to heat subcutaneous tissues. Its effects are similar to short wave diathermy including raising subcutaneous fat temperature 10°C to 12°C and subcutaneous muscle temperature 3°C to 4°C. It has a lower depth of penetrance than short wave diathermy and therefore should not be used on deeper structures, such as hips or deep low-back musculature. It is primarily used around superficial muscles and joints. The area is treated for 15 to 30 minutes and is followed by passive and active range of motion and manual medicine procedures.

Contraindications are the same as with short wave diathermy (1,2,3,4,5).