The effects of ultrasound stimulation on passive muscle stiffness of triceps surae

Passive muscle stiffness is a critical mechanical property that influences muscle function and the risk of strain injury. This study aimed to investigate the acute effects of 1  MHz ultrasound stimulation on the passive muscle stiffness of the triceps surae. One leg of 20 healthy adults was assigned to the ultrasound-stimulated leg, while the other served as a control leg. The ultrasound-stimulated leg received continuous ultrasound (1  MHz, 2.0  W/cm²) for 10  min, while the control leg did not. Shear wave velocity (SWV), an index of passive muscle stiffness, was measured using shear wave elastography in the superficial medial gastrocnemius (MG) and the deep soleus (SOL) before and immediately after the intervention. A significant interaction of intervention and time was observed for the SWV of SOL, indicating a significant decrease in the SWV of SOL at post-intervention in the ultrasound-stimulated leg (p < 0.001, d = 0.873), but not in the control leg. Meanwhile, no significant changes were observed in the SWV of MG. Furthermore, a significant negative correlation was found between the pre-intervention SWV of SOL and its changes after intervention in the ultrasound-stimulated leg (p < 0.001, r =  - 0.758). This study demonstrated that continuous 1  MHz ultrasound stimulation acutely decreased passive stiffness in the deep SOL but not in the superficial MG. This muscle-specific response may be related to differences in anatomical depth and tissue characteristics between the MG and SOL. These findings suggest that ultrasound stimulation is an effective strategy for managing stiffness in deep muscles.