Interrater and intrarater reliability of transverse abdominal and lumbar multifidus muscle thickness in subjects with and without low back pain.

STUDY DESIGN: Two-group, repeated-measures reliability study. OBJECTIVES: To determine interrater and intrarater reliability of ultrasound measurements of transverse abdominal (TrA) and lumbar multifidus (LM) muscle thickness, during rest and contraction, in subjects with low back pain (LBP) and healthy subjects over 3 consecutive days, performed by an experienced and a novice rater. BACKGROUND: Previous reliability studies of TrA or LM thickness did not simultaneously account for muscle state, rater experience, and multiday assessment in large subject samples. METHODS: The 2 raters measured TrA and LM thickness on 3 consecutive.days in 42 healthy subjects and 56 subjects with LBP, during rest and contraction, and calculated the percent thickness change from rest to contraction. Intraclass correlation coefficients (ICC2,k) and 95% minimal detectable change in thickness were derived for a single measure (day 1) and an average measure (days 1-3). RESULTS: The interrater ICC2,1 values for singlemeasure thickness (LBP group, 0.71-0.87; healthy group, 0.94-1.00) were similar to those for average-measure thickness (LBP group, 0.73-0.84; healthy group, 0.93-1.00). Both interrater ICC2,1 and ICC2,3 were lower for the relative thickness. change (0.61-0.96). lntrarater ICC2,1 values 3 consecutive days were high for both raters across the 2 groups (LBP group, 0.95-1.00; healthy group, 0.93-1.00), albeit lower for the relative thickness change (0.79-0.99). The 95% minimal detectable changes were less than 0.3 mm for the TrA and less than 2 mm for the LM (but, in most cases, less than 10% of average thickness). CONCLUSION: Both experienced and trained novice raters provided reliable measurements of TrA and LM thickness in participants with LBP and healthy participants, during rest and contraction. One-time measurements were-similar to averaged measurements. Small absolute errors were observed.