Therapeutic ultrasound (TU) is most widely used as biophysical agents to treat contracture in orthopedics rehabilitation. Its effect is to increase range of motion by increasing tissue extensibility. However, no studies have reported the effect of TU on human muscle stiffness (MS). Also, the optimal conditions including frequency, intensity, and duty cycle of TU remain unclear. This study aimed to clarify the optimal TU conditions on MS using ultrasonic shear-wave elastography (SWE).

This study was a randomized cross-over trial. All of the following TU conditions were applied on ten healthy males (mean±SD, age 25±2.9 y.o., height 171.2±4.7cm, weight 63.1±5.1kg); frequency (1 and 3MHz), intensity (continued mode: spatial average-temporal average (SATA) intensity of 0.5, 1.0, 1.5, and 2.0W/cm²; pulsed mode: 1.0, 2.0, and 3.0W/cm² with 50% duty cycle (i.e., 0.5, 1.0, and 1.5W/cm^{2 SATA})) for 10minutes. Outcome was the change rate of MS in medial gastrocnemius (MG) muscle by using SWE. Statistical analysis was performed using one-way analysis of variance followed by Bonferroni post-hoc test.

MS showed significant differences between the different frequency, intensity, and duty cycle. MS significantly decreased with 1.0, 1.5, and 2.0W/cm^{2 SATA} than 0.5W/cm^{2 SATA} at continued mode, and with 1.0, and 1.5W/cm^{2 SATA} than 0.5W/cm^{2 SATA} at pulsed mode at both 1MHz and 3MHz. There was no significant difference in MS between the above five SATA intensities, i.e., 1.0, 1.5, 2.0W/cm^{2 SATA} at continued mode and 1.0, 1.5W/cm^{2 SATA} at pulsed mode.

The optimal conditions of TU to decrease MS in MG were over 1.0W/cm^{2 SATA} of intensity for both continued mode and pulsed mode at both 1MHz and 3MHz of frequency.