Mechanical strain-wave propagation through human muscle measured using B-mode ultrasound.

Mass is a fundamental physical property of all tissues in the human body, including skeletal muscle. Surprisingly intramuscular mass is almost ubiquitously ignored in models of muscle contraction. Here we demonstrate that that muscle mass plays an important role in the dynamics of the tissue deformation in the human biceps brachii and brachialis. Twenty-one subjects held a vibrating dumbbell that generated 10 Hz oscillations within the muscle. Because of tissue inertia, strain-wave propagations developed that we tracked with B-mode ultrasonography: the velocity of these waves increased with increased tension in the muscle. A complementary study in ten participants measured the myoelectric activity in this experimental paradigm and determined that the tonic vibration reflex was minimal for this low frequency set-up, and thus were not the cause of the strain-wave propagations. These experimental results show that muscle mass has a significant effect on the internal dynamics of contraction in human muscle. It is suggested that models of human muscle contraction should re-evaluate whether the inclusion of muscle mass is important for the accurate prediction of muscle force.