A comparison of human muscle temperature increases during 3-MHz continuous and pulsed ultrasound with equivalent temporal average intensities.

STUDY DESIGN A repeated-measure crossover design was used. The independent variable was the type of ultrasound (pulsed or continuous) and the dependent variable was intramuscular temperature. OBJECTIVE To compare changes in intramuscular temperature resulting from the use of pulsed ultrasound versus continuous ultrasound with an equivalent spatial average temporal average (SATA) intensity. BACKGROUND There is a lack of research on the heat-generating capabilities of pulsed ultrasound within human muscle. METHODS AND MEASURES The subjects were 16 healthy volunteers (mean age +/- SD, 21.3 +/- 2.5 years). Each subject was treated with pulsed ultrasound (3 MHz, 1.0 W/cm2, 50% duty cycle, for 10 minutes) and continuous ultrasound (3 MHz, 0.5 W/cm2, for 10 minutes) during a single testing session. Tissue temperature returned to baseline and stabilized between treatments and treatment order was randomized. Tissue temperature was measured every 30 seconds using a 26-gauge needle microprobe inserted at a depth of 2 cm in the left medial gastrocnemius muscle. Data were analyzed using a linear mixed model. RESULTS Treatment with continuous ultrasound produced a mean (+/-SD) temperature increase of 2.8 degrees C +/- 0.8 degrees C above baseline. Treatment with pulsed ultrasound produced a mean (+/-SD) temperature increase of 2.8 degrees C +/- 0.7 degrees C above baseline. Statistical analysis revealed no significant differences in either the extent or rate of temperature increases between the 2 modes of ultrasound application. CONCLUSION Pulsed ultrasound (3 MHz, 1.0 W/cm2, 50% duty cycle, for 10 minutes) produces similar intramuscular temperature increases as continuous ultrasound (3 MHz, 0.5 W/cm2, for 10 minutes) at a 2-cm depth in the human gastrocnemius. Spatial average temporal average intensity is an important consideration when selecting pulsed ultrasound parameters intended to deliver nonthermal effects.