Development of a Muscular Fatigue Model Taking Account of Motor Unit Types with Different Fatigability

Abstract

A model of muscular fatigue, which is used as part of a Hill type mechanical model, has been developed to evaluate the effects of muscular fatigue on the characteristics of force generation in motion analyses. For this purpose, we considered the states of activity of each motor unit type and the order of recruitment of motor units. The states of motor units in the model were divided into four conditions depending on their activity and fatigue. The changes of the states of each motor unit were formulated by the sets of differential equations based on the relationship among muscular fatigue and recovery, and activity of muscle. Furthermore, motor units were also categorized into three types depending on their characteristics of force generation and fatigue. The order of recruitment of motor unit types was modeled based on the size principle. The model correlates well with the response of the model of Liu et al. at the maximum voluntary contraction condition and the results of Ikai's fatigue test at the dynamic condition. The degrees of muscular fatigue were evaluated by the present model with different proportions of motor unit types. Thus the present muscular fatigue model is able to describe the basic response of muscular fatigue and to simulate the differences of characteristics of muscle.