Reliability of an EMG Fatigue Test for Erector Spinae Muscles D. Gordon E. Robertson 1, Heidi Sveistrup 1,2 and Cécile Réal 3 School of Human Kinetics 1 School of Rehabilitation Sciences 2 Département de Génie Biologique, Université de Technologie de Compiègne 3, Compiègne, France Biomechanics Laboratory, School of Human Kinetics, University of Ottawa, Ottawa, CANADA

Introduction The purpose of the study was to evaluate the reliability of an isometric fatigue test of the low- back muscles. Both same day, test- retest and day-to-day reliability were assessed.

Methods Four males aged >35 years, four males aged 35 years volunteered as subjects. Surface electrodes were placed bilaterally over multifidus at L5, iliocostalis lumborum and longissimus thoracis (see Figure 1). Electrodes were placed on the subject’s back and a permanent ink mark made to maximize consistency of placement from test to test. EMG data were collected at 400 Hz per channel. During each test the subject was strapped in the prone position at the calf and pelvic regions onto one table. Another table under the chest was pulled away and the subject required to hold the trunk horizontally aligned to a visual cue (see Figure 2).

Electrode Placement Figure 1. Electrode locations

For each test session the subject was asked to: i) hold their trunk in a horizontal position for 1 minute; ii) rest for 2 minutes (with trunk supported by the second table); and iii) hold their trunk in a horizontal position for a second 1 minute isometric contraction. Muscle activity was recorded during each contraction. Following the two 1 minute isometric contractions, EMG activity was recorded at 30 second intervals over a period of 5 minutes by asking the subject to hold the trunk in a horizontal position for 2 seconds. These series of contractions quantified the recovery. The subjects were given a 5 minute break and then the second series was begun. A second test session on the same day 1 but at least 6 hours later. Test day 2 was scheduled between 3 and 7 days after the first.

Figure 1. Experimental setup Figure 2. Experimental setup Experimental Setup

Statistics The value extracted from the raw EMG data was the median frequency (using BioProc). Five second bins of data were analyzed and the median frequencies were plotted for the entire 1 minute window of data (i.e., 20 median frequency values per 1 minute trial). Slopes of the line of best fit for the 20 data were determined. A negative slope determined if muscles were becoming fatigued (see Figure 3).

Results and Discussion Fatigue within each 1 minute trial: Consistent fatigue was observed in all but two subjects. One was retested and exhibited fatigue after 2 minutes. There were no significant differences within a test session or within a day; furthermore, no significant differences were observed between tests done on different days. Significant differences occurred between the rate of fatigue induced in men and women with men demonstrating greater fatigue than the women, possibly because men have a heavier upper body.

Fatigue Analysis Fatigue analysis: Median frequencies of multifidus were higher than longissimus thoracis which was higher than iliocostalis. Slopes of median frequencies had the same decreasing pattern for all muscles demonstrating the onset of fatigue. The slope of the curve for the multifidus was steepest.

Generalizability Analysis Generalizability analysis was used to determine the best values of the independent variables (day, subject, trials) that maximize the variability accounted. The data show i) iliocostalis lumborum was extremely variable; ii) decreasing the number of days did not effect the variability accounted for, suggesting that the variability in the data between days was small enough so that a single day was representative of the subjects performance; iii) decreasing the number of trials decreases the “variability accounted for” suggesting that the intertrial variability was great enough to require more than four trials for representative sampling.

Median Frequency of Right Multifidus Figure 3. Example of fatigue reducing median frequency during exercise and recovery.

References Biedermann, H.J., Shanks, G.L. and Inglis, J. Median frequency estimates of paraspinal muscles: reliability analysis. Electromyo. Clinical Neurophys., 30, 83-88, De Luca, C.J., Roy, S.H. and Casavant, D.A. Lumbar muscle fatigue and chronic lower back pain. Spine, 14(9), , De Luca, C.J. Use of the surface EMG signal for performance evaluation of back muscles. Muscle & Nerve, 16, , Ilkka, K. Restitution of EMG spectrum after muscular fatigue, European Journal of Applied Physiology, 57, , Seidel, H., Beyer, H. and Bräuer, D. Electromographic evaluation of back muscle fatigue with repeated sustained contractions of different strengths. European Journal of Applied Physiology, 56, , Winter,D.A. Biomechanics of human movement, Wiley-Interscience, Toronto, 1979.

Acknowledgements This work was supported in part by the Faculty of Health Sciences Research Development Fund and the University of Ottawa’s, Rectors Fund. Thanks are extended to Dr. Frank Johnson and Dr. David Gibbons for their support.