  Using shear wave elastography, we measure the changes in the wave speed with
the stress produced by a striated muscle during isometric voluntary
contraction. To isolate the behaviour of an individual muscle from
complementary or antagonistic actions of adjacent muscles, we select the flexor
digiti minimi muscle, whose sole function is to extend the little finger. To
link the wave speed to the stiffness, we develop an acousto-elastic theory for
shear waves in homogeneous, transversely isotropic, incompressible solids
subject to uniaxial stress. We then provide measurements of the apparent shear
elastic modulus along, and transversely to, the fibre axis for six healthy
human volunteers of different age and sex. The results display a great variety
across the six subjects. We find that the slope of the apparent shear elastic
modulus along the fibre direction changes inversely to the maximum voluntary
contraction (MVC) produced by the volunteer. We propose an interpretation of
our results by introducing the S (slow) or F (fast) nature of the fibres, which
harden the muscle differently and accordingly, produce different MVCs. This
work opens the way to measuring the elastic stiffness of muscles in patients
with musculoskeletal disorders or neurodegenerative diseases.
