Establishing measurement properties in the assessment of inter-recti distance of the abdominal muscles in a postnatal women

Background: Diastasis of the rectus abdominis muscle is often evaluated in clinical practice but it is unknown which clinical method has the best measurement properties. Objective: The aim of this study was to determine: (i) the criterion validity of the callipers, tape measure and finger-width to evaluate the inter-recti distance of the abdominal muscles compared to ultrasound; and (ii) inter-rater and retest reliability of these methods. Design: Measurement study. Methods: Two physiotherapists assessed the inter-recti distance of rectus abdominis on 50 women postnatally using the three clinical methods. These measurements were compared to ultrasound and assessed for inter-rater and retest reliability. Results: Callipers had very good positive correlations with ultrasound (r = 0.85 to 0.99) and narrow limits of agreement (LOA) (<6 mm). Finger-width and tape measure had moderate to very good correlation with ultrasound (tape measure r = 0.82 to 0.98; finger-width r = 0.75 to 0.98) with wider limits of agreement (tape measure LOA <8 mm; finger-width LOA <18 mm). Callipers demonstrated excellent inter-rater (ICC = 0.80 to 0.99) and retest (ICC = 1.00) reliability. Inter-rater reliability testing was very good for tape measure (ICC = 0.80 to 0.97) and moderate to very good (ICC = 0.44 to 0.85) for finger-width. Retest reliability demonstrated very good reliability (ICC = 0.99 to 1.00) for both finger-width and tape measure. The largest inter-recti distances, strongest correlations and reliability were found at the level of the lower umbilicus. Conclusion: Callipers, tape measure and finger-width are valid and reliable methods of measuring inter-recti distance in postnatal women. Validity, and relative and absolute reliability were found to be the strongest using the callipers.