System identification of mechanomyogram evoked by double-pulse stimulation

Muscles do not only generate contractile force but also behave like a spring. This spring-like property, or elasticity, has been studied by applying perturbation to the muscle and measuring its responses such as the force and the evoked mechano-myogram (MMG). Muscle elasticity increases as the muscle activity increases. The system identification technique has been applied to an evoked MMG, but the evoked MMG was measured in the resting state. Many motor units are activated synchronously or asynchronously in a voluntary contraction. Forces generated by the motor units fuse. As a pilot study, the muscle elasticity in a twitch contraction was investigated. The purpose of this study was to identify the mechanical property of the muscle in a twitch contraction, using the system identification technique applied to MMGs evoked by single- and double-pulse stimulations. In double-pulse stimulation, the first pulse was applied to stimulate the tibialis anterior muscle, and before the muscle returned to the resting state, the second pulse was applied to the common peroneal nerve. We assumed that the MMG signal evoked by the second pulse stimulation reflects the viscoelasticity of the muscle in a twitch contraction. The MMG system was identified, then the natural frequency of the transfer function was calculated as an index of elasticity. The MMG system with single-pulse stimulation was approximated with a third-order model, and its undamped natural frequency was 4.0±0.4 Hz. On the other hand, the MMG system with double-pulse stimulation was approximated with a fourth-order model, and its high natural frequency was 8.2±0.5 Hz. The increase in natural frequency may be caused by increased elasticity in an activated muscle. These results suggest that the proposed method is a novel technique for estimating the elasticity of a muscle in twitch contraction.