Deep Trunk Muscle Contraction Responses Across Different Postural and Task Conditions in Chronic Nonspecific Low Back Pain: A Cross-Sectional Ultrasound Study

Background: Chronic nonspecific low back pain (CNLBP) is associated with altered deep trunk muscle function, but contraction responses across different postural and task conditions remain unclear. Objective: This study compared contraction ratios of the transversus abdominis (TrA), internal oblique (IO), and lumbar multifidus (LM) muscles between individuals with CNLBP and healthy controls across two postural and task conditions differing in surface stability and contraction task. Methods: Thirty-one individuals with CNLBP and 31 healthy controls were included. TrA, IO, and LM thicknesses were assessed ultrasonographically at rest and during contraction under static (supine/prone with limb elevation) and dynamic (exercise-ball sitting with unilateral foot elevation) conditions. Contraction ratios were calculated as contracted/resting thickness. Results: Resting muscle thicknesses were similar between groups. The IO contraction ratio was higher under the dynamic condition than the static condition in both groups (CNLBP, p < 0.001; control, p = 0.030); no such differences were observed for TrA or LM. The static TrA contraction ratio was nominally lower in the CNLBP group [1.10 (0.25) vs. 1.20 (0.28), p = 0.025] but did not reach the Bonferroni-corrected threshold (p < 0.0083). No other between-group differences were observed. Conclusions: The static TrA contraction ratio was nominally lower in individuals with CNLBP but did not reach the Bonferroni-corrected threshold; the IO contraction ratio was higher under the dynamic condition in both groups. These exploratory findings suggest muscle-specific contraction responses to differing postural and task demands, and may inform future confirmatory studies on trunk stabilization in CNLBP. Simultaneous variation in body position, task demands, and support-surface stability limits causal inference regarding the independent effect of surface instability.