Influence of subjects' characteristics and technical variables on muscle stiffness measured by shear wave elastosonography

Purpose: The aim of this study is to quantitatively assess lower limbs muscle elasticity in a court of healthy subjects and to evaluate the influence of technical variables (e.g., diameter of the ROI—region of interest) and examined subjects’ characteristics (e.g., sex, levels of physical activity, side evaluated) on muscle stiffness. Materials and methods: 54 healthy subjects (48 men, 6 women) were evaluated for a total of 108 lower limbs. Shear wave elastography was performed with a multifrequency linear probe (15–4 MHz). Two radiologists performed the evaluation of lower limbs from left to right side (first calf and then thigh). The measures were taken on gastrocnemius and on femoral biceps muscle belly. We chose to place for this study two ROIs of 4 and 2 mm of diameter. Results: The mean muscle stiffness was 1.98 ± 0.48 (range between 1.89 ± 0.36 and 2.15 ± 0.57 m/s). The difference in muscle stiffness between left and right side of the body and between different levels of physical activity never became statistically significant (p value between 0.314 and 0.915). Only in one test out of eight the difference of muscle stiffness between male and female resulted statistically significant (p value 0.020). When comparing the measurement obtained with a 2 and 4 mm diameter ROIs the values were statistically different only for the left thigh (p value 0.028). Conclusion: Our study, despite its limitations (low sample and low female population), seems to give some clear advice: physiological or technical factors do not determine statistically significant differences on passive muscle stiffness.