Abstract
Electrical muscle stimulation is limited to surface muscle activation; however, focused ultrasound (fUS) has the potential to stimulate deep skeletal muscles. This study aims to evaluate the effects of fUS peripheral nerve stimulation on muscle mechanical properties in an in vivo murine model. The sciatic nerve of 19 Sprague-Dawley rats was insonified with 3.68-MHz fUS (p+ = 6.4 MPa, p− = 5 MPa) using 10-ms pulses (0.9-ms on/0.1-ms off, 10 times) repeated at 0.5-Hz for 30, 90, or 180 severy second day for two weeks. Stimulation was monitored using electromyography (EMG) with indwelling electrodes inserted in the gastrocnemius muscle belly. Bilateral muscles were harvested for tensile mechanical testing (n = 15) and histology (n = 4). Muscle contraction was confirmed via EMG in 16/19 rats. Elastic moduli of stimulated muscles (133.2 ± 42.1 kPa) and controls (101.4 ± 39.2 kPa) were not different (ANOVA, p = 0.053). A trend toward increased elastic modulus with number of muscle stimulations was observed until 450 stimulations (not significant: ANOVA, p = 0.437), after which elastic modulus began to decrease. No tissue damage was observed histologically. The results of this pilot study demonstrate that fUS can induce repeatable muscle contraction that may improve muscle strength and mechanical properties over time. [Work supported by PSU College of Engineering Seed Grant 2021.]
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