Two-dimensional Muscle Fatigue Measure Based On Fatigue Rate And Resident Maximum Voluntary Contraction

Measuring muscle fatigue and calculating rational muscle workload are very important in ergonomics. However, due to limitations of current muscle fatigue measures,a lack of consistency is always reported by previous studies. A synthetical measure is needed for accurate evaluation of muscle fatigue degree.To solve this problem, this study proceeds in three stages. In the first stage, a comprehensive review of previous muscle fatigue measure index is made. This review investigates several main measures, such as Maximum Endurance Time, muscle fatigue model, Residual maximum voluntary contraction(Residual MVC) and Electromyography(EMG) and summarizes their application scopes and limitations. Special attention is paid to the descriptions and measurements of subject-specific fatigue attributes in previous studies.In the second stage, an accurate definition of subject-specific muscle fatigability is proposed, and a simplified measuring method is designed based on Force-Load Muscle Fatigue Model: recording the time series of muscle force during sustained maximum voluntary contraction exertion procedure and fitting the force decline with Force-Load Muscle Fatigue Model. Experiment results are as follows: the mean fatigability value is1.75 min-1and mean curvefit goodness R2 is around 0.9. This result suggests the availability of the rapid measurement method. Model reliability and subject-specific muscle fatigability stability are tested by repeating the same experiment within subjects six months later. Statistics shows that mean curvefit goodness R2 is 0.89, and significant correlation between fatigability values of the two experiments is found with a correlation coefficient 0.87. This result on the one hand shows the stability of subject-specific muscle fatigability during a certain period, on the other hand attests to the reliability of Force-Load Muscle Fatigue Model.Another experiment tests the Validity of Residual MVC using as a muscle fatigue measure. Subjects exerts their Residual MVC and tries to maintain their maximumforce for a while when their muscles were in three different fatigue states. Result shows no differences between their Residual MVC under different fatigue states while the force decline rates differ significantly. These results cast doubts on the validity for residual MVC fatigue measure.Finally, this study integrates Residual MVC with fatigability and comes up with the concept of “Two-dimensional Fatigue Measure”, which is to describe muscle fatigue state comprehensively and predict residual muscle force-exerting ability.