Body mass normalisation for ultrasound measurements of lumbar multifidus and abdominal muscle size

The purpose of this study was to determine if ratio scaling or allometric scaling is the more appropriate method for normalising ultrasound measurements of lumbar multifidus and abdominal muscle size to body mass. In a convenience sample of 62 male career firefighters, cross-sectional area and thickness of the lumbar multifidus, as well as, thicknesses of the external oblique, internal oblique, and transverse abdominal muscles were assessed with ultrasonography. Ratio scaling entailed dividing muscle size by body mass, while allometric scaling entailed dividing muscle size by body mass raised to a power. Significant positive correlations (r = 0.25 to 0.49, p < 0.05) existed between body mass and all muscle size measurements, except for transverse abdominal thickness (r = 0.21, p = 0.100). Ratio scaling was deemed inappropriate for normalising the muscle size measurements, because it merely reversed the direction of the correlations between body mass and the muscle size measurements (r = −0.31 to −0.50, p < 0.05), with external oblique abdominal thickness representing the only exception (r = −0.17, p = 0.192). Allometric scaling with derived allometric parameters was deemed appropriate for normalising muscle size measurements, because it caused the correlations between body mass and muscle size to become insignificant and near to zero (r = −0.06 to 0.00, p > 0.05). The current study provides allometric parameters that can be used to normalise muscle size measurements to body mass in male firefighters. Future research is needed to establish reference databases of population-specific allometric parameters in other groups.