ELECTROMYOGRAPHIC AMPLITUDE AND FREQUENCY CHANGES IN THE ILIOCOSTALISLUMBORUM AND MULTIFIDUS MUSCLES DURING A TRUNK HOLDING TEST

Background and Purpose. Muscle endurance is an important variable to m easure in the assessment of back muscle function. This study investiga ted the electromyographic (EMG) activity and fatigue patterns of ilioc ostalis lumborum and multifidus muscles during a trunk holding test. S ubjects. Sixteen male subjects (mean age = 24.2 years, SD = 4.2, range = 20.6-31.9) without low back pain or known pathology were recruited in the study. Methods. Surface EMG electrodes were used to record the activity of iliocostalis lumborum and mutifidus muscles during a 60-se cond isometric contraction. To reflect the activity level and fatigue rate of the muscles, EMG amplitude (root mean square [RMS] values) and a frequency variable (median frequency [MF]) were measured. Results. The multifidus muscle displayed a higher level of activity, initial MF , and normalized MF slope than did the iliocostalis lumborum muscle. T here was no difference, however, in the normalized RMS slope between t he two muscles. The correlations between the normalized MF slope and t he RMS slope of the two muscles were nonsignificant. Conclusion and Di scussion. This study shows that monitoring frequency changes of the EM G signals may enable therapists to quantify the fatigue changes of ind ividual muscles during the trunk holding test. The higher fatigue rate shown in the multifidus muscle compared with the iliocostalis lumboru m muscle may be due to the higher activity level of the multifidus mus cle during the trunk holding contraction. This greater activity of the multifidus muscle during the contraction might be explained by the fu nctional differences between these two muscles.