Measuring abdominal muscle function by abdominal muscle thickening on ultrasound: reproducibility, validity and normal range values

Background: Ultrasound measurements of abdominal muscle thickening fraction (TF) may be useful to monitor abdominal muscle activity and function. Aims: To establish the reproducibility of measuring maximal TF (TFmax) of the abdominal muscles and to confirm the validity of using TFmax to assess abdominal muscle function. Methods: In 20 healthy subjects, TF of right external oblique (EO), internal oblique (IO), transversus abdominis (TrA) and rectus abdominis (RA) were measured during maximal expiratory efforts against a closed airway and during coughing at functional residual capacity (FRC) and total lung capacity (TLC). The correlation between TF of each muscle and airway pressure during five forceful consecutive expiratory efforts against a closed airway of progressively increasing force was assessed. Reproducibility was assessed using a random effects model. Results: IO TFmax measurements were moderately reproducible (mean 0.78±0.46 cm, reproducibility coefficient 0.42) but TFmax measurements were only poorly reproducible for all the other muscles. TF was correlated with expiratory airway pressure for RA (p=0.03), IO (p 0.1 for all interactions). TFmax varied significantly between the 4 muscles (p Conclusions: IO TFmax showed the highest reproducibility. TF of the EO, IO and RA was correlated to expiratory force generation. TFmax measured during coughing varied between muscles but was unaffected by lung volume.