Intramuscular Temperature Rises With Topical Analgesics Used as Coupling Agents During Therapeutic Ultrasound

OBJECTIVE: To compare the effectiveness of Nature's Chemist as an ultrasound coupling agent with the effectiveness of another topical analgesic (Biofreeze), Aquasonic 100, and a sham treatment in producing intramuscular (IM) temperature increase during a typical therapeutic ultrasound treatment. DESIGN AND SETTING: Subjects were randomly assigned to 1 of 4 treatment groups (n = 10 in each group). Groups 1 through 3 received continuous ultrasound at 1.0 W/cm(2) for 10 minutes at a frequency of 3 MHz over the posterior calf. Group 4 received a sham treatment. In group 1, we used Aquasonic 100 alone; in group 2, we used a 1:1 (wt/wt) mixture of Biofreeze and Aquasonic 100; in group 3, we used a 1:1 mixture of Nature's Chemist and Aquasonic 100; and in group 4, we used a 1:1 mixture of Aquasonic 100 and Nature's Chemist. In all groups, IM temperature was recorded during the treatment and for 15 minutes posttreatment. We used a modified visual analogue scale to measure each subject's perception of heat at the treatment area during and after treatment. SUBJECTS: Forty college students (age, 22.5 +/- 2.0 years; height, 175.5 +/- 8.0 cm; weight, 71.6 +/- 13.1 kg; calf skinfold thickness, 17.8 +/- 7.2 mm) volunteered to become subjects. MEASUREMENTS: The IM temperature was recorded at 15-second intervals for 25 minutes at 1 cm below the subcutaneous fat with a thermocouple. Differences were analyzed within and among groups at the beginning of the treatment (T(0)), the end of the treatment (T(10)), and 15 minutes posttreatment (T(25)). RESULTS: The IM temperature increases in groups 1 through 3 were significantly different from those in group 4 (sham), but they were not significantly different from each other. Temperatures increased in group 1 (Aquasonic 100) by 7.47 degrees +/- 1.8 degrees C, in group 2 (Biofreeze and Aquasonic 100) by 6.52 degrees +/- 1.6 degrees C, and in group 3 (Nature's Chemist and Aquasonic 100) by 6.99 degrees +/- 1.1 degrees C. Temperatures decreased in group 4 (sham) by 0.56 degrees +/- 0.3 degrees C. There were no significant differences among groups 1 through 3 in the perception of heat at T(5) and T(10). CONCLUSIONS: Our results indicate that, at a frequency of 3 MHz and an intensity of 1 W/cm(2), Nature's Chemist and Biofreeze mixed in 1:1 ratios with Aquasonic 100 were effective coupling agents. Perceptions of heat by the patient may not indicate actual temperature increases within the muscle.