LUMBAR PARASPINAL MUSCLE FATIGABILITY IN REPETITIVE ISOINERTIAL LOADING - EMG SPECTRAL INDEXES, BORG SCALE AND ENDURANCE TIME

The purpose of this study was to develop a submaximal repetitive isoin ertial back muscle endurance test by defining the relationships betwee n the power spectral indices of paraspinal muscle electromyographic (E MG) activities, endurance time and a subjective estimate of fatigue (B org scale). Bilateral surface EMG recordings were obtained over the lu mbar paraspinal muscles in ten individuals who were currently free fro m back pain. All subjects performed repetitive upper trunk extensions (25 degrees flexion and 5 degrees extension, 30 repetitions per min), while movement below the third lumbar vertebral body was mechanically restricted. The load level depended upon upper body mass, sex, and age . The tests continued for as long as the subjects were able to maintai n the required repetition rate (endurance time). Median (MF) and mean power frequency (MPF) slopes were calculated by performing a fast Four ier transformation after confirmation of EMG stationarity by recurrenc e quantification analysis. MF and MPF correlations with endurance time /Borg scale were measured for the first 60 s (0.60-0.88/0.42-0.86), th e first 90 s (0.62-0.89/0.52-0.90), the first 120 s (0.50-0.76/0.41-0. 73), and the entire repetitive run (0.63-0.88/0.54-0.90). To test for the reproducibility of the spectral indices, EMGs were recorded for 2 min during repetitive loading from the same subjects on two consecutiv e days. Corresponding spectral slopes of MF and MPF were correlated at 60 s (0.36-0.93), 90 s (0.58-0.92), and 120 s (0.70-0.94) at the L3-L 4 and L5-S1 levels, indicating good reproducibility of results from al ternate recording sessions at the L5-S1 level. It is concluded that pa raspinal muscle spectral indices (MF and MPF) measured before the onse t of total muscle fatigue are good predictors of endurance time and ar e closely related to the subjective perception of fatigue.