Background

Previous evidence showed altered lumbar multifidus (LM) activation in populations with chronic non-specific low back pain (LBP). We aimed to investigate the test-retest and inter-examiner reliability of ultrasound imaging (US) for assessing LM thickness at rest and activation during the active straight leg raise test (ASLR) and the association between thickness changes with clinical outcomes.

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.