EXTRAPOLATION OF TIME-SERIES OF EMG POWER SPECTRUM PARAMETERS IN ISOMETRIC ENDURANCE TESTS OF TRUNK EXTENSOR MUSCLES

The aim of the present study was to test the viability of using short isometric contractions of trunk extensor muscles to perform an assessm ent of their endurance capacity. To this aim two types of analysis wer e performed. First, electromyographic (EMG) mean power frequency (MPF) slopes with respect to time as estimated over shorter fixed periods w ere compared to slopes estimated over the full contraction period of a contraction sustained until the endurance time. Second, the relations hip between MPF slope estimates as estimated over various periods and the endurance time of the muscle group was evaluated. Five subjects pe rformed three isometric trunk endurance tests at 25%, 50% and 75% of t heir maximum voluntary contraction (MVC), respectively. EMG signals of the left and right multifidus, iliocostalis and longissimus muscles w ere continuously recorded and spectral parameters were calculated. The MPF appeared to decrease consistently during all endurance tests. The extrapolation from a MPF time series of half the estimated contractio n period to the time series of the complete contraction period gave re asonable results at all force levels, when data from several electrode locations were incorporated in a single slope estimate (mean or steep st slope). The accuracy of the prediction of trunk extensor endurance on the basis of these parameters describing the MPF time series over h alf the estimated contraction period was satisfactory. Endurance time predictions from yet shorter periods were unreliable. (C) 1998 Elsevie r Science Ltd. All rights reserved.