METHODS: Fifty-two patients with LBP and two examiners (one experience and one novice) participated in this study. 18 B-mode images at L4-L5 or L5-S1 level (both sides, 3 at rest and 6 during ASLR) were collected. For assessing test-retest reliability, the experienced examiner repeated the procedure after 7 days. Intraclass correlation coefficients (ICC), standard error of measurements (SEM) and minimal detectable changes (MDC) were calculated.
RESULTS: Inter-examiner agreement was good-to-excellent (ICC3,2=0.71-0.92) and test-retest reliability was excellent (ICC3,1=0.91-0.98). Mean average of multiple measurements improved the agreement. Greater LM thickness at rest (p<0.05) and greater thickness change after 3 seconds (p<0.01) and 10 seconds (homolateral side, p<0.01; contralateral side, p<0.05) were associated with less pain intensity.
CONCLUSIONS: US is a reliable method to assess the LM thickness at rest and contracted during the ASLR in patients with LBP. The measurement at 3 seconds after maintaining ASLR, as well as the use of the mean of 3 measurements, has been shown to be the most reliable method for measuring LM muscle thickness during ASLR.