Monitoring of muscle fatigue during isokinetic exercise.

The aim of this study was to evaluate and clarify the usefulness of different spectral characteristics of myoelectrical activity in the follow-up of development of muscle fatigue. Vastus lateralis (part of quadriceps) muscle loaded in a simple isokinetic exertion test was used as a model. Twelve, well trained athletes served as study subjective to minimize the inter-individual variations. They went through one-minute test with isokinetic device at the range of 90 degrees extension and flexion at the level of their maximal force. The myoelectrical signals were registered from middle of the vastus lateralis muscle by a computerized fast Fourier transform analyzing system based on 80286 and 80287 microprocessors and using surface electrodes. Mean power frequency (MPF) was 65.6 (SE=Standard Error, 1.6) Hz, median frequency (MF) 55.8 Hz and zero crossing rate (ZCR) 92.5 (SE 1.5) during first performance. All these parameters decreased apparently linearly and significantly from the beginning of the test (P<0.001). The decrease rate of ZCR was fastest and MF slowest. The area of power spectrum (PSA) was 1753 (SE 320) units during first performance, and it increased markedly with slightly accelerating rate during the test. In conclusion, for the evaluation of the muscular performance during the development of fatigue it is beneficial to monitor simultaneously MPF, MF and ZCR.