Background Changes in function of lumbar multifidus (LM) muscle have been observed in patients with low back pain (LBP). These changes seem to contribute to the development of pain and dysfunction in the lumbopelvic region.The aim of this study was to determine changes in thickness of LM measured by ultrasound imaging during straight leg raise test (SLR) in chronic LBP patients and asymptomatic subjects.

Methods The percentage of change in thickness of LM muscle during SLR and after maintaining a 5 s SLR using ultrasound imaging was compared. Observed changes were analysed as contraction percentage (contraction thickness-resting thickness/contraction thickness x 100).

Results Thirty-nine patients, 18 with chronic LBP (mean age: 43±9 years) and 21 asymptomatic subjects (mean age: 42±1 years) were recruited. The analysis of variance showed significant differences between both groups for ultrasound imaging measures. We observed statistically significant differences in contraction percentage during SLR comparing LBP patients with asymptomatic subjects in right LM (t=3.353; p=0.002) and in left LM muscle (t=3.889; p<0,001), with a percentage difference between groups of 8.00063% y 6.653%, respectively. The analysis of changes in contraction percentages after maintaining a SLR during 5 s revealed significant differences in both right and left LM (6.37029% y 8.51805% respectively).

Conclusions There are changes in thickness of LM muscle in LBP patients during SLR, which suggests that patients with chronic LBP exhibit lower activation of this muscle during the SLR test. Ultrasound imaging measures during SLR seems to represent a non-invasive and clinically useful method to register changes in LM function.