Development Of A Model To Predict Localized Muscle Fatigue During Hand-Intensive Work

Abstract

The purpose of this study was to develop and test a model which could predict muscle fatigue expressed as changes in wrist flexion force during fatiguing work. Industrial engineers and exercise scientists are very concerned with fatigue and cumulative trauma disorders which may have similar etiologies. This model will help to minimize fatigue. It will also provide a method for predicting the cumulative effects of a hard day's work on the hand/wrist. Ten right hand dominant males, who gave their informed consent, were recruited from the general student population at the University of Massachusetts at Amherst. Hand dominance was determined via writing preference. All subjects underwent four days of testing, and on each day performed 30 maximal isometric wrist flexions with their right wrist. All contractions were 5 seconds in duration with a 5 second rest interval between contractions. Only the data from the first day was used in this pilot study; several other papers have been published by Hilliard using the entire data set. The model has an exponential (e) fatigue parameter and exponential recovery parameter. The observed peak force decreased over the 30 contractions. The ith observed peak force usually revealed some recovery when compared to the previous observed ending force. A computer program was used to search for the fatigue and recovery parameters which rninimized the squared difference between the predicted and observed(obs) force. The model was: pred-peak(l)=obs-peak(l) pred-peak(i) = pred-end(i-l) + obs peak(l) >I- [1 e(-recovery • rest-time)] pred-end(i) = pred-peak(i) - pred-peak(i) >I- [l-e(-fatigue· work-time)] Analysis of variance determined that predicted peak forces were not significantly different from the observed peak forces (p=0.20). The r2 between predicted force and observed force ranged from a low of 0.02 to a high of 0.98, with an overall r2 Of 0.71. After viewing the subject's actual force data it was concluded that the model does not work when subjects "pace themselves." Subjects who had a lower ith peak force than a ith+t peak force could be called pacers. Subjects were told to contract maximally, they were not told how fast they should contract. Some did not reach their peak force until the middle or end of the 5 second contractions. This model will be further tested with subjects who will be instructed to contract as rapidly as possible.