Heiss DG, Hitt J, Henry SM, Navalgund A; Ohio State University, Columbus, OH and University of Vermont, Burlington, VT. Heiss.8@osu.edu.

PURPOSE: Multifidus muscle impairments associated with low back pain are difficult to characterize due to the muscle’s deep location. A recent study (Stokes et al. 2003) found that surface electromyography (EMG) did not accurately reflect the activity of the multifidus, based on wire and surface EMG at the L2 and L4 spinal levels. Given that patients often have pain at the L5/S1 level, it is important to characterize the muscle impairments at this level also. The purpose of this study was to determine whether surface EMG at the S1 level accurately reflects the activity of the underlying multifidus. SUBJECT: The subject was a 55 y.o. male (85 kg, 1.83 m) in good health. METHODS: Following the Stokes et al. protocol (2003), intra-muscular wire electrodes were inserted into the right and left multifidus at the level of S1 (WMUL). Surface electrodes (20 mm interspace) were placed: on the skin above the multifidus (SMUL) bilaterally adjacent to the insertion points of the WMUL electrodes, and bilaterally over the longissimus, 30 mm lateral to the L4 spinous process (SLON). The subject stood with a foot placed on separate force plates, which were mounted on a moveable platform. The platform was translated unexpectedly for 5 trials in each of three directions: posterior, leftward, and rightward. The EMG signals were collected for 5 sec at 1000 Hz, preamplified, and band passed filtered (10 Hz - 400 Hz). ANALYSIS: Fourier analyses indicated the power of the EMG signal fell between 70 – 100 Hz. The signals were bandpass filtered 10 – 100 Hz (Chebyshev type II, no-lag), rectified and lowpass filtered at 10 Hz (Butterworth 2^nd order, no-lag). To further smooth the data, a 100 ms RMS filter was applied. Cross-correlational analysis in MatLab compared the 1) WMUL and SMUL EMG; and 2) WMUL and SLON EMG. Paired t-tests calculations (Excel, Microsoft) tested that the R² values for the WMUL-SMUL correlations were greater than the SMUL-SLON correlations. RESULTS: In 83% (25/30) of the comparisons, the SMUL-SMUL correlations were greater than the SMUL-SLON correlations, and the t-test results supported that the mean R² for the WMUL-SMUL correlations were greater (p < .05) than the SMUL-SLON correlations. CONCLUSIONS: In this case example, surface EMG at S1 more accurately represented the underlying multifidus activity rather than cross-talk from the longissimus muscle. Methods to accurately and non-invasively measure the activity of the multifidus are important for understanding its role in lumbar stabilization. FUNDING SOURCE: None A; Ohio State University, Columbus, OH and University of Vermont, Burlington, VT. Heiss.8@osu.edu. FUNDING SOURCE: None.

Copyright 2004 by the American Physical Therapy Association

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