Hot-Pack and 1-MHz Ultrasound Treatments Have an Additive Effect on Muscle Temperature Increase

Therapeutic ultrasound is an effective deep heating modality commonly applied alone or after cooling or heating of the treatment area. The purpose of this study was to examine the tissue temperature rise in the human triceps surae muscle group after ultrasound with prior heating via a silicate gel hot pack.

This study was designed as a 2 x 2 x 3 factorial with repeated measures on two factors (depth and time). Independent variables were temperature of pack (hot and room temperature), depth of measurement (1 cm and 3 cm), and time (beginning, after pack application, and after ultrasound). The dependent variable was tissue temperature. Subjects were assigned to one of two treatment groups: ultrasound preceded by a 15-minute hot pack treatment or ultrasound preceded by a 15-minute application with a silicate gel pack at room temperature. Measurements were taken while subjects were treated in a university training room.

Twenty-one uninjured male and female college student volunteers were randomly assigned to one of the two pack groups.

The hot packs were stored in 75 degrees C water. A 1-MHz ultrasound treatment was administered for 10 minutes at an intensity of 1.5 W/cm(2). Tissue temperature was measured every 30 seconds using 23-gauge hypodermic microprobes interfaced with a telethermometer and inserted 1 and 3 cm below the surface of anesthetized triceps surae muscle.

At both tissue depths, there was a 0.8 degrees C greater increase in tissue temperature with hot packs and ultrasound. At 1 cm, ultrasound increased temperature 3.5 degrees C after a 0.5 degrees C rise during the room temperature-pack application, but only 0.6 degrees C after a 3.8 degrees C increase during hot-pack application. At 3 cm, ultrasound increased temperature 3.85 degrees C following a slight (-0.26 degrees C) decrease during the room temperature-pack application and 3.68 degrees C after a 0.74 degrees C increase during hot-pack application.

Vigorous increases in deep muscle temperature (>/=4 degrees C) can be reached with 2 to 3 minutes less total sonation time when preheated with a hot pack. Thus, ultrasound and hot packs have an additive effect on intramuscular temperature, but the characteristics of the additive effect are different, primarily because there appears to be a tissue temperature plateau.