Quantitative assessment of upper limb muscle fatigue depending on the conditions of repetitive task load.

The aim of this study was to discriminate fatigue of upper limb muscles depending on the external load, through the development and analysis of a muscle fatigue index. Muscle fatigue is expressed by a fatigue index based on an amplitude parameter (calculated in the time domain) and a fatigue index based on a frequency parameter (a parameter calculated in the frequency domain). The fatigue index involves a regression function that describes changes in the EMG signal parameter, time elapsing before muscle fatigue and the probability of specific trends in changes in EMG parameters for the population under study. The experimental study covered a group of 10 young men. During the study, they exerted force at a specific level and for a specific time in 12 load variants. During the study, EMG signals from four muscles of the upper limb were recorded (trapezius pars descendents, biceps brachii caput breve, extensor carpi radialis brevis, flexor carpi ulnaris). For each variant and for each examined muscles, the value of the fatigue index was calculated. Values of that index quantitatively expressed fatigue of a specific muscle in a specific load variant. A statistical analysis indicated variation in the fatigue of the biceps brachii caput breve, extensor carpi radialis brevis, and flexor carpi ulnaris muscles depending on the external load (load variant) according to the task performed with the upper limb. The study demonstrated usefulness of the fatigue index in expressing quantitatively muscle fatigue and in discriminating muscle fatigue depending on the external load.