Muscle Thickness and Function of Transversus Abdominis and Gluteus Medius in Individuals with Chronic Non-Specific Low Back Pain.

Background: Non-specific low back pain (NSLBP) is associated with altered neuromuscular control of the lumbopelvic-hip complex (LPHC). However, the functional behavior of the transversus abdominis (TrA) and gluteus medius (GM) during upright postural tasks, with and without the abdominal drawing-in maneuver (ADIM), remains unclear. This study aimed to compare TrA and GM activation between individuals with NSLBP and asymptomatic controls during standing and single-leg stance using rehabilitation ultrasound imaging (RUSI). Methods: Thirty-two participants (16 with NSLBP and 16 asymptomatic controls) underwent RUSI assessment under four task conditions: standing and single-leg stance, with and without ADIM. Muscle function was quantified using thickness change derived from ultrasound measurements. A two-way mixed-model analysis of variance with Bonferroni-adjusted post hoc comparisons was performed. Results: Significant group × condition interactions were identified for TrA activation (p < 0.05). Individuals with NSLBP demonstrated reduced TrA activation during standing with ADIM and reduced GM activation during single-leg stance compared with asymptomatic controls. The effect sizes were moderate to large for TrA activation and small to moderate for GM activation. Conclusions: These findings suggest task-specific differences in neuromuscular activation patterns in individuals with NSLBP. Ultrasound-derived thickness change measures obtained during functional, weight-bearing tasks may provide clinically relevant information to support motor control rehabilitation strategies.