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Mean frequency (MNF) and median frequency (MDF) are often used to assess muscular fatigue from surface electromyography (sEMG) signals. To determine muscle fatigue at different levels of isometric and dynamic contractions, it is necessary to know the relationship between the force of contraction and the MNF/MDF methods. Despite numerous studies, no unique answer has been found. This study re-examines this relationship by investigating 19 subjects and performing isometric and dynamic contractions over a full elbow range of motion measured from the biceps brachii muscle. We obtained the following key results: (i) a strong positive relationship between MNF/MDF methods and the force of contraction was found frequently during isometric contractions at an angle less than 90° and during dynamic contractions in 3 subjects (16%); (ii) a strong negative relationship during isometric contractions at an angle greater than 90° and during dynamic contractions in 8 subjects (42%), and (iii) no strong linear relationship during isometric contractions for all the angles and during dynamic contractions in 8 subjects (42%). After possible causes for these conflicting results are discussed, the study then investigates if it is possible to use anthropometric variables of subjects as the criteria to create homogeneous subgroups of relationships. A number of anthropometric variables were used to separate the subgroups, i.e., the forearm and biceps circumferences for males and the hand and shoulder breadths for females.

The effects of the force of contraction and elbow joint angle on mean and median frequency analysis for muscle fatigue evaluation