Several limitations in this study warrant discussion. Firstly, since this study was done on healthy subjects, care should be taken when applying the findings in our study to practical situations. Secondly, we used the same suppress strength in the assessment of passive ROM to maintain the objectivity of this experiment; however, passive ROM could be assessed by passive torque tissue resistance at a consistent angle instead in order to examine the effects of ultrasound. Therefore, we are now planning to quantitatively evaluate changes in tissue resistance by using a toque machine.