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.