Breathless Strength: Ultrasonographic Insights into Expiratory Muscle Dysfunction in Spinal Cord Injury

<i>Background and Objectives:</i> This study aimed to evaluate the predictive value of ultrasonographic abdominal muscle thickness and thickening ratios for expiratory muscle strength in SCI patients. <i>Materials and Methods:</i> A case-controlled, cross-sectional study was conducted with 36 SCI patients and 30 age- and sex-matched healthy controls. Ultrasonographic measurements of the rectus abdominis (RA), external oblique (EO), internal oblique (IO), and transversus abdominis (TrA) were performed at rest and during forced expiration. Maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) were measured to assess respiratory muscle strength. Correlation and regression analyses were performed to determine the relationship between ultrasonography (USG) parameters and expiratory muscle function. <i>Results:</i> SCI patients exhibited significantly lower MIP (76.27 ± 29 cmH₂O vs. 91.63 ± 17.3 cmH₂O, <i>p</i> = 0.007) and MEP (64.52 ± 21.55 cmH₂O vs. 119.1 ± 26.48 cmH₂O, <i>p</i> < 0.001) compared to healthy individuals. Ultrasonographic measurements revealed a significant reduction in forced thickness and thickening ratios of EO, IO, and TrA muscles in SCI patients (<i>p</i> < 0.001). MEP was positively correlated with EO forced thickness (r = 0.333, <i>p</i> = 0.047), IO forced thickness (r = 0.501, <i>p</i> = 0.002), and TrA forced thickness (r = 0.530, <i>p</i> = 0.001). Multiple linear regression analysis identified TrA forced thickness as the strongest predictor of MEP (β = 0.49, <i>p</i> = 0.001). <i>Conclusions:</i> Ultrasonographic measurements of abdominal muscle thickness and thickening ratios provide valuable insights into expiratory muscle dysfunction in SCI patients. TrA forced thickness demonstrated the strongest association with MEP, suggesting its potential as a novel, non-invasive biomarker for expiratory muscle weakness. These results support the use of USG as a practical clinical tool for guiding respiratory assessment and rehabilitation strategies in patients with spinal cord injury.