Reproducibility of lumbar paraspinal surface electromyogram power spectra

OBJECTIVE: To establish the reproducibility of surface recordings of electromyograms from lumbar erector spinae muscles. DESIGN: Repeated measurements in 10 male volunteers under controlled conditions. METHODS: While isometric load was held constant at (2/3) and (1/3) of maximum voluntary contraction, surface electromyograms were recorded from the 4th/5th lumbar interspace on three occasions separated by at least a day. Fresh surface electrodes were applied on each test occasion. The raw electromyographic signal was filtered and digitized and the signal processed by fast Fourier transformation to give median frequency and total amplitude spectra. Linear regression lines of median frequency and amplitude against time were calculated. The 'spectral halfwidth" was defined as the full width at half maximum of the composite amplitude spectrum. Reproducibility was assessed by calculation of the intraclass correlation coefficient. RESULTS: Reproducibility of initial median frequency and spectral halfwidth were found to be excellent at both (2/3) (ICCC 0.91 and 0.88) and (1/3) (ICCC 0.84 and 0.83) maximum voluntary contraction. Initial power and regression slopes of power and median frequency were not reproducible. CONCLUSIONS: The reproducibility of two electromyographic variables, the initial median frequency and the spectral halfwidth, were found to be satisfactory.