Ultrasound imaging is a reliable and reproducible method for evaluation of muscle structure, function, and activity15).
This method allows assessment of muscle activity by measuring changes in muscle geometry during contraction16).
In this study, there was high intra-tester reliability for the ultrasound measurements of the MF and TrA muscle
thicknesses at rest and during contraction (ICCs= 0.87 to 0.96). To measure the thicknesses of the MF and TrA muscles,
a B-mode ultrasound apparatus (MyLab 50 XVision, ESAOTE S.p.A, Genova Italy) was used. Measurement of
the thicknesses of the TrA and MF muscles was performed in a resting position and during the tasks with submaximal
muscle contraction on both sides17). To record the thicknesses of the TrA and MF muscles, we used an LA523 linear
probe (set to 12 MHz) and a CA431 convex array probe (set to 7.5 MHz). In order to measure TrA muscle thickness, the
subjects were set in crook-lying position18). The ultrasound probe was placed midway between the iliac crest and costal
margin, on the midaxillary line, about 10 cm off the midline of body at the level of umbilicus19). TrA muscle thickness
was measured in millimeters between the fascial lines, one centimeter away from the muscle junction in the direction
of the thoracolumbar fascia18). The two submaximal tasks were performed for the TrA muscle, the abdominal draw-in
maneuver (ADIM) and active straight leg raising (ASLR)20). MF muscle thickness measurements were performed in the
prone position with a pillow under the abdomen. The probe was placed along the spine, such that the midpoint of the
probe was in line with the spinous process of the fourth lumbar vertebra. Then it was moved so that the facet joint
between the fourth and fifth lumbar vertebrae was visible. This point is located directly on the MF muscle. The muscle
thickness was measured from this point to the plane between the subcutaneous tissue and muscle16). The submaximal
task for this muscle was elevation of the contralateral arm in a prone position with a small weight (0.5 kg) on the
arm, the elbow at a right angle, and the glenohumeral joint at 120 degrees of abduction21).