Elbow joint angle dependency of biceps brachii muscle stiffness with an aid of system identification of mechanomyogram

The purpose of this study is to investigate the relation between stiffness and the elbow joint angle. A mechanomyogram (MMG) was recorded from biceps brachii muscle at three different elbow angles: 90, 120, and 150 degrees. The biceps brachii muscle was electrically stimulated and its mechanomyogram was measured with an acceleration sensor attached to the skin of the belly of the biceps brachii muscle. The transfer functions from the stimulation to the MMG were identified by the singular value decomposition method, and the natural frequency was derived from the transfer function. As a result, the MMG was approximated with a fourth-order model. The lowest natural frequency increased as the elbow joint angle increased. It was concluded that biceps brachii muscle stiffness increased as the elbow joint angle increased.