superficial vs deep heating agents
Not all heat reaches the same depth. A warm towel comforts the surface of an achy back, but it cannot warm a deeply buried muscle or a hip joint several centimetres down. Rehabilitation distinguishes between superficial heating agents, which warm only the skin and the fat and shallow tissue just beneath it, and deep heating agents, which can warm structures further inside the body.
Superficial agents include hot packs, warm paraffin baths, infrared lamps, and warm whirlpool water. Their warming effect fades within roughly one to two centimetres of the surface, because skin and fat are good insulators. Deep heating agents convert another form of energy into heat inside the tissue: therapeutic ultrasound turns high-frequency sound into warmth, and diathermy uses electromagnetic energy. These can raise the temperature of muscle and tendon at depths of three to five centimetres, reaching targets a hot pack never could.
The practical question is always: how deep is the tissue I want to warm? A tight superficial wrist or a cold, painful hand responds well to paraffin or a hot pack. A scarred, contracted deep shoulder capsule may need ultrasound to warm it enough to stretch. Choosing the wrong depth wastes the treatment: a hot pack over a deep hip does little except warm the skin. Deep heating also carries its own risks and is avoided over growth plates in children, over metal implants for certain diathermy types, and over tumours.
For a stiff, scarred deep shoulder capsule, a hot pack would only warm the skin, so the therapist uses ultrasound to heat the tissue three centimetres down and then stretches the shoulder while it is warm.
Match the heating depth to where the target tissue actually lies.
A common mistake is using a hot pack over a deep joint and expecting deep warming — superficial heat simply does not reach more than a centimetre or two.