RELATIONSHIP BETWEEN MYOELECTRIC AND MECHANICAL MANIFESTATIONS OF FATIGUE IN THE QUADRICEPS FEMORIS MUSCLE GROUP

Fatigue is commonly defined as ''the failure to maintain the required force''. As such, it may be argued that the use of electromyographic ( EMG) power spectral statistics to monitor muscle fatigue is inappropri ate, because, during the maintenance of a submaximal force of contract ion, EMG changes are readily observable in the absence of any decline in the muscle's mechanical output. However, it is possible that the EM G changes reflect the changing metabolic status of the muscle and henc e its inability to generate its normal maximal force. The present stud y sought to examine whether the decline in EMG median frequency, which occurs during the maintenance of a submaximal force, is correlated wi th a reduction in the muscle's maximum force-generating capacity. The maximum voluntary contraction (MVC) of the knee extensors in ten young , healthy subjects was determined. On five separate occasions, randoml y assigned forces of 20, 30, 40, 50 and 60% MVC were held to the limit of endurance. At intervals throughout the sustained contractions, sub jects were required to rapidly generate an MVC for 1-2 s, then return to the fixed submaxial target force. Surface EMG signals were recorded throughout the contractions from the rectus femoris and vastus latera lis muscles, from which the power spectrum median frequency was calcul ated. Regression analysis revealed highly significant relationships be tween the rate of decline in MF and the rate of decline in MVC, and be tween each of these parameters and endurance time to fatigue (P = 0.00 01, in each case). It is concluded that the decline in MF can be used to monitor fatigue, where fatigue is defined as the inability to gener ate the maximum force that can be produced by the muscle in its fresh state.