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Passive muscle force adversely affects joint movement and the motion control system. Relaxing passive muscle force can substantially improve movement impairment and also relieve muscle pain and cramping caused by high-intensity exercise. We intended to study the effect of ultrasound on passive muscle force relaxation. The results showed that the passive force of the specimens in the ultrasound group decreased by 21.9% compared with the initial value. Experimental data were fitted and analyzed using the standard linear solid model of biological tissue—the viscosity coefficient and elastic coefficient of the model group were higher than those of the control group, and the relaxation time increased significantly. We propose that ultrasonic mechanical stimulation can change the viscoelasticity of muscle and decrease passive muscle force, which provides an important theoretical basis for research on the effect of ultrasound on muscle tissue, its use in the treatment of disease and in the rehabilitation of function, and new methods of maintaining muscle function.

Ultrasound levitation technology has been widely applied in various fields. Two main kinds of ultrasound levitation configurations are the standing-wave system and the phased array system. Depending on the principle of the phased array and the focusing transducers, we proposed and designed a levitation system consists of two pairs of focusing ultrasound transducers fitting in confocal mode. When paired transducers are excited in the reverse phases, the acoustic pressure at the geometric focus will be constant 0 Pa, which means a potential well exists here. Depending on the finite element method, we simulated the field distribution and demonstrated the designed potential well. By modulating the phase sequences of the paired transducer, we obtained three kinds of acoustic field distributions with potential well. We optimized the included angle between the axis of the paired transducer to reduce the secondary potential well and improve the accuracy of the levitation. Moreover, we carried out an experiment in water with ultrasound of 2 MHz. The quartz sand with a diameter of 0.2 mm was captured by our levitation system stably.

The phased-array ultrasonic levitation is a convenient technique to manipulate particles. In order to provide convenience for both professional and nonprofessional users to use the ultrasonic phase array levitation technique, we designed a man-machine interactive levitation system. It consists of a man-machine interaction module, a multi-channel transmitting circuit module, and an ultrasonic transducer array module. The man-machine interaction module can calculate the corresponding phase sequence according to user-defined position and angle. The transmitter module provides electrical excitation for the transducer array according to the calculated phase sequence. They can consequently generate the required sound field. Then, the man-machine interaction levitation module can control the particle position and rotation angle in real-time. In this paper, we described the man-machine interaction module and levitation module in detail and also demonstrated an example of realtime particle manipulation using the system.

Overtraining under muscle fatigue will greatly increase the risk of injury, therefore objective and accurate assessment of muscle fatigue is of great significance for exercise guidance. Muscle fatigue embodies the reduction of muscle force and the changes in muscle microstructure. The changes in muscle microstructure will affect its acoustic transmission characteristics. In this paper, we study the ultrasonic attenuation coefficient as an indicator for assessing muscle fatigue. Eight healthy young males participated in this experiment, none of them had a history of neuromuscular disease. We collected the B-mode ultrasound images and the corresponding reflected ultrasonic signals of the biceps in the rest, contraction, and fatigue state. The muscle thickness increases to the maximum in the contraction state and decreases in the fatigue state but remains higher than the initial state. The ultrasonic attenuation coefficient of the biceps increases to the maximum in the contraction state and decreases to a lower value in the fatigue state than that in the initial state. The results demonstrated that the variation of attenuation coefficient is a feasible scheme for the assessment of muscle fatigue.

Ultrasound sources of an appropriate size and controllable power are crucial for extensive investigations and applications, especially in exploratory experiments needing precise ultrasound stimulations. We designed an acoustic wave funnel (AWF) that changes the ultrasound source to a suitable size and acts as a small ultrasound source suited for widespread experiments and applications. The AWF consists of three parts—two cladded waveguides of different radii connected by a hollow cone. The larger waveguide has the same radius as the transducer, whereas the radius of the smaller waveguide is set for application requirements. Using the cone, the AWF squeezes the acoustic wave from the large transducer into the small output port and produces a small intense acoustic source. Both theoretical analyses and numerical simulations show that AWF works efficiently and exhibits a pretty amplification factor. The AWF is effective and practical in providing a miniaturized acoustic wave source with high intensity. These results indicate that, when requiring a miniaturized ultrasound source in experiments and applications, the AWF is a practical and reliable device.