Research On The Subject-specific Maximum Endurance Time Model Based On Muscle Fatigability

Maximum endurance time(MET)is an essential measure to evaluate worker's capacity in ergonomics field.It is widely used in task design and work-rest schedule to prevent excessive fatigue in occupational tasks and reduce work-related musculoskeletal disorders.The substantial individual variation in traditional MET model raises practical problems in using them to predict individual performance in the fatiguing task.Following the stages of proposing indicator,deriving model,and application exploration,this study proposed a subject-specific MET model based on the theoretical muscle fatigue model and validated it in different fatiguing tasks.Firstly,the present study selected the fatigue rate k in the exponential muscle fatigue model to depict the individual muscle fatigability characteristics and validated it in a hand grip muscle fatigue experiment.The good model fitting results indicated the validity of using the fatigue rate(k)as fatigability indicator.Secondly,the present study proposed and validated a theoretical subject-specific MET model based on the exponential muscle fatigue model and the fatigue rate.Experiment results showed that the subject-specific MET model could depict the intensity-endurance time relationship well on the individual level among four joints(i.e.wrist,elbow,shoulder,and knee)in the experiment.In order to further validate the subject-specific MET model in multi-joint fatigue task,this study conducted an upper limb fatigue experiment,in which the fingers,wrist,elbow,and shoulder were involved.The results found that the subject-specific MET model had a better goodness-of-fit in multi-joint fatigue task than former single joint fatigue tasks.This might be explained by the finding that the intensity level did not influence upper limb fatigue rate significantly,while single joint fatigue rates were significantly correlated with intensity level.To explore rapid measurement of fatigue rate,this study verified the sustained maximum voluntary contraction method(sMVC method).The experimental results showed that the sMVC method could measure individual muscle fatigability in sustained MVC task accurately,and the test-retest reliability was good.Furthermore,the fatigue rate measured in the sMVC task was highly correlated with the fatigue rate measured in the MET task,and the difference between them was not significant,indicating that the fatigue rate measured by the sMVC method could be used to predict individual MET performance.The present study is a further investigation of the area of MET model research field.The proposed subject-specific MET model could explain the substantial individual differences among MET models,and provides a theoretical basis for individual-level ergonomic fatigue management,and can be applied to the individual level functional capacity evaluation,task design and work-rest schedule.