Time-frequency methods applied to muscle fatigue assessment during dynamiccontractions

This paper discusses the assessment of the electrical manifestations of mus cle fatigue during dynamic contractions. In the past, the study of muscle f atigue was restricted to isometric constant force contractions because, in this contraction paradigm, the myoelectric signal may be considered as wide sense stationary over epochs lasting up to two or three seconds, and hence classic spectral estimation techniques may be applied. Recently, the avail ability of spectral estimation techniques specifically designed for nonstat ionary signal analysis made it possible to extend the employment of muscle fatigue assessment to cyclic dynamic contractions, thus increasing noticeab ly its possible clinical applications. After presenting the basics of time-frequency distributions, we introduce i nstantaneous spectral parameters well suited to tracking spectral changes d ue to muscle fatigue, discuss the issues of quasi-stationarity and quasi-cy clostationarity, and present different strategies of signal analysis to be utilized with cyclic dynamic contractions. We present preliminary results o btained by analyzing data collected from paraspinal muscles during repetiti ve lift movements, from the first dorsal interosseus during abduction-adduc tion movements of the index finger, and from knee flexors and extensors dur ing isokinetic exercise. In conclusion, data herein reported demonstrate that the described techniqu es allow for evidencing the electrical manifestations of muscle fatigue in different paradigms of cyclic dynamic contractions. We believe that the ext ension of the objective assessment of the electrical manifestations of musc le fatigue from static to dynamic contractions may increase considerably th e interest of researchers and clinicians and open new application fields, a s ergonomics and sports medicine. (C) 1999 Published by Elsevier Science Lt d. All rights reserved.