Ultrasound Strain Zero-Crossing for Assessing Adult Hypertonic Muscles: A Feasibility Study.

Objectives: To assess the feasibility of ultrasound strain zero-crossing (ZC) to assess adult hypertonic muscles.

Methods: After receiving Institutional Review Board (IRB) approval and informed consent, we prospectively performed free-hand compression ultrasound strain imaging (USI) on normal and hypertonic upper trapezius muscles (UTM) and subcutaneous tissues using a linear array transducer (10L4, bandwidth 4-10 MHz). Ultrasound data containing tissue deformation and relaxation produced by axial compression and decompression were processed using 2D speckle tracking software. Using the ZC point on the reference strain curve as the reference, we determined a negative ZC if the muscle strain curve returned to zero strain after or at the same time as that of the reference curve. We determined a positive ZC when the muscle strain curve returned to zero strain ahead of the reference curve. Modified Ashworth Scale (MAS) was used to assess muscle tonicity performed by clinicians. The difference in ZC between normal and hypertonic muscles and the diagnostic performance of ZC in determining ≥mild hypertonic muscles were analyzed using a paired t-test and the receiver operating characteristic curve (ROC), respectively.

Results: From October 2023 to August 2024, we performed USI on 28 subjects with normal muscles and 56 with hypertonic muscles (38 men and 46 women, mean age: 52 years). The difference in ZC between normal and hypertonic muscles was statistically significant (P < .001). The area under the ROC of ZC for determining ≥mild hypertonic muscles was 0.92.

Conclusions: The study results suggest that ultrasound strain ZC is feasible for determining ≥mild hypertonic muscles with good diagnostic performance.