Ultrasound assessment of transversus abdominis muscle contraction ratio during abdominal hollowing: a useful tool to distinguish between patients with chronic low back pain and healthy controls?

Abstract

Spine stabilisation exercises, in which patients are taught to preferentially activate the transversus abdominus (TrA) during "abdominal hollowing" (AH), are a popular treatment for chronic low back pain (cLBP). The present study investigated whether performance during AH differed between cLBP patients and controls to an extent that would render it useful diagnostic tool. 50 patients with cLBP (46.3 ± 12.5 years) and 50 healthy controls (43.6 ± 12.7 years) participated in this case-control study. They performed AH in hook-lying. Using M-mode ultrasound, thicknesses of TrA, and obliquus internus and externus were determined at rest and during 5 s AH (5 measures each body side). The TrA contraction-ratio (TrA-CR) (TrA contracted/rest) and the ability to sustain the contraction [standard deviation (SD) of TrA thickness during the stable phase of the hold] were investigated. There were no significant group differences for the absolute muscle thicknesses at rest or during AH, or for the SD of TrA thickness. There was a small but significant difference between the groups for TrA-CR: cLBP 1.35 ± 0.14, controls 1.44 ± 0.24 (p < 0.05). However, Receiver Operator Characteristics (ROC) analysis revealed a poor and non-significant ability of TrA-CR to discriminate between cLBP patients and controls on an individual basis (ROC area under the curve, 0.60 [95% CI 0.495; 0.695], p = 0.08). In the patient group, TrA-CR showed a low but significant correlation with Roland Morris score (Spearman Rho = 0.328; p = 0.02). In conclusion, the difference in group mean values for TrA-CR was small and of uncertain clinical relevance. Moreover, TrA-CR showed a poor ability to discriminate between control and cLBP subjects on an individual basis. We conclude that the TrA-CR during abdominal hollowing does not distinguish well between patients with chronic low back pain and healthy controls.

Fig. 1

M-mode ultrasound image of the abdominal hollowing manoeuvre. The distances between fascial borders were derived by means of a semi-automatic approach, based on manually selected control points (white dotted vertical bars) plus tissue Doppler velocity information to track the borders between adjacent control points (shown here for TrA, transversus abdominis, as thick white lines bordering the muscle). Note: for clarity, all markings are shown with thicker line-widths than those used for the actual analysis process. No time or depth scales were displayed on the M-mode image during digitization; however, the image represents approximately 4 s worth of data (x-axis) (~1.5 s of rest and ~2.5 s of abdominal hollowing) with a total scan depth of ~37 mm (y-axis). ST subcutaneous tissue; OE obliquus externus; OI obliquus internus; AC abdominal contents

Fig. 2

Muscle thickness, given by the difference in depth of the upper and lower fascial borders of the transversus abdominis (TrA), obliquus internus (OI) and obliquus externus muscle (OE). The maximal thickness of TrA over any given 3 s period during the voluntary contraction was automatically determined (see dotted vertical bars)

Fig. 3

Receiver operator characteristic (ROC) curve for the mean TrA contraction ratio (mean of all trials, both body sides). The ROC area under the curve = 0.598 (SE 0.057) [95% CI 0.495; 0.695], p = 0.08). The solid line indicates the ROC curve (and 95% CI) and the dotted line joining the points at 0,0 and 100,100 represents the 0.5 reference line

Fig. 4

The association between the Roland Morris disability questionnaire score and the mean TrA contraction ratio for all trials (both body sides) of each patient