Study design: Clinical measurement, concurrent validity criterion standard.
Objective: To determine the concurrent validity of digital nylon calipers in comparison to ultrasound imaging (USI) for the measurement of interrecti distance (IRD).
Background: Diastasis rectus abdominis is the abnormal increase in the width of the linea alba, measured as IRD. A diastasis rectus abdominis can compromise mechanical trunk function in both genders. IRD has been accurately measured with USI but requires costly equipment and extensive examiner training. Digital nylon calipers are inexpensive and easy to use, but their use to measure IRD has not been validated.
Methods: A sample of convenience of 56 individuals (11 men, 45 women) was measured. A single examiner was assigned to each tool (calipers or USI), and IRD was measured at 2 locations (above and below the umbilicus) under 2 conditions (abdominal muscles at rest and abdominal muscles contracted). All measurements were made during a single session, and examiners were blinded to measurements with the other tool.
Results: Above the umbilicus, the measurements of IRD with calipers were similar to those made with USI, with intraclass correlation coefficients (model 3,2) of 0.79 with abdominal muscles at rest and 0.71 with abdominal muscles contracted. The absolute mean difference between the caliper and USI measurements of IRD above the umbilicus was 0.03 cm larger with the calipers when the abdominal muscles were at rest and 0.03 cm smaller when the abdominal muscles were contracted. The values of IRD obtained with the caliper and USI techniques were not comparable when obtained below the umbilicus.
Conclusion: The calipers are a valid tool for measuring IRD above the umbilicus in males and females. Measuring IRD with calipers below the umbilicus should not be considered valid, using USI as the criterion standard. This may reflect anatomical variation of the linea alba or a limitation of the calipers to assess IRD at the same depth as USI.