Sonoelastic determination of human skeletal muscle elasticity

It is not currently practical to directly measure viscoelastic parameters in human muscles in situ. Methods used in vitro cannot readily be applied, and motion analysis provides only a gross estimate. We report on the application of a hybrid approach, sonoelastography, which uses ultrasound to measure the propagation of shear waves induced by externally applied vibrations. Because shear waves predominate in incompressible viscoelastic media at low frequencies, sonoelastic data should be comparable to those obtained using conventional means. We recorded vibration propagation speeds as a function of applied load in the quadriceps muscles of ten volunteers as they underwent a series of static contractions. Data collection during dynamic contractions, not possible with the current equipment, will be the subject of future experimentation. Although statistically significant correlations were not uniformly obtained above 60 Hz nor for propagation perpendicular to the muscle fibers, this is felt to have resulted from deviations from the applied plane wave model. Calculated values of Young's modulus for 30 Hz propagation parallel to the muscle fibers were 7 +/- 3, 29 +/- 12 and 57 +/- 37 x 10(3) Nm-2 for applied loads of 0, 7.5 and 15 kg, respectively. The corresponding values at 60 Hz were 25 +/- 6, 75 +/- 61 and 127 +/- 65. These values were statistically significant and linearly correlated with the applied load, as expected. Our data represent the first in situ human measurements of their kind. It is anticipated that sonoelastography will provide a useful adjunct to the study of human biomechanics.