Relationship between shear wave velocity and muscle activation is inconsistent across different muscle types

Abstract

There is an increasing use of shear wave ultrasound elastography to quantify properties of muscles under various conditions such as muscle length, levels of activation, in healthy and pathological muscle. However, little is known about the variability in shear wave velocity (SWV) among muscles as most studies investigate one specific muscle. The purpose of this study was to assess the variability of the relationship between SWV and activation across muscles with different architecture, specifically pennation angle: biceps brachii (parallel), tibialis anterior (moderately pennate), gastrocnemius (highly pennate). We tested these muscles at matching levels of activation and similar muscle lengths, relative to optimal length, to limit differences in force across muscles. Shear wave velocity-squared increased with higher levels of muscle activation (p<0.001), but the relationship between SWV2 and activation was different across all three muscles (p<0.001) with the slope of the SWV2- EMG relationship greater in the tibialis anterior than in the medial gastrocnemius and biceps brachii (both p<0.001). This indicates that SWV2 was inconsistent across muscle types, even when considering similar levels of activation and torque production. Under passive conditions, SWV2 was different across muscles (p<0.001), even at shortest lengths, when differences due to passive force were minimal. We found that the probe-fascicle angle did not alter the relationship between muscle activation and SWV2. Across all muscles, we found no difference in predicting SWV2 with model included probe-fascicle angle (r2 = 0.90) compared to one that did not include probe-fascicle angle (r2 = 0.90, p = 0.96). Our findings suggest that the SWV varies across muscles under passive and active conditions even when relative length is considered and that SWV may be dependent on other muscle properties, other than pennation angle, that are different between the three muscles.