System identification of mechanomyogram for the construction of a single motor unit mechanical model

A mechanomyogram (MMG) reflects the viscoelasticity of the muscle. The MMG evoked with single pulse (SP) stimulation dominantly reflects the mechanical properties of the slow muscle because approximately 80% of the fibers in the tibial anterior muscle are slow muscle fibers. In this study, MMGs of the muscle are evoked with double pulse (DP) stimulation. With the second evoked pulse, fast muscle fibers dominantly contract because the contractile velocity of these fibers is faster than that of slow muscle fibers. The MMG system with SP stimulation was approximated with a third-order model. On the other hand, the MMG system with double pulse stimulation was approximated with a fifth-order model because the MMG amplitude of the slow muscle was relatively suppressed with the short stimulation interval. This means that the proposed method is a novel technique for estimating the stiffness of fast muscle.