Background: Activation of the deep stabilizing trunk muscle transversus abdominis (TrA) is important for trunk stabilization and spine stability.

Sling exercises are used for the activation of trunk muscles, therefore we determined the thickness of the TrA in a standardized sling exercise in comparison to
rest and abdominal press. Furthermore we propose a standardized measurement method, which can be used to compare relative muscle thickness levels in different exercises.

Methods: The main objective of the study was to assess and to compare the thickness of the TrA during different conditions; resting condition, sling exercise condition (non-voluntary contraction), and abdominal press condition (voluntary contraction) using a non-invasive ultrasound-based measurement method. Ultrasound measurement (USM; 8.9 MHz, B-mode) was employed to measure the thickness of the TrA in twenty healthy volunteers (13 m, 7 f), each one measured three times with breaks of 48 h. On each measurement day, the subjects were measured on three different conditions: resting condition (RC), sling condition (SC), and abdominal press condition (APC).

The USM images were analyzed using a custom-made MatLab script, to determine the thickness of the TrA.  AA A two-way repeated-measurements ANOVA was performed with a significant effect of the factor condition [F(2,38) = 47.82, p < 0.0001, η2 = 0.72], no significant effect of the factor time [F(2.38) = 2.45, p =0.1, η2 = 0.11], and no significant interaction effect [F(4,76) = 0.315, p = 0.867, η2 = 0.02].

Statistically corrected post-hoc t-tests revealed significant differences in TrA thickness showing that RC < SC (p < 0.001; η2 = 0.19; d = 0.96), SC < APC (p <0.0001; η2 = 0.23; d = 1.10), RC < APC (p <0.0001; η2 = 0.53; d = 2.11). As for the test-retest reliability the intra-class correlation coefficient (ICC) yielded a value of 0.71, 0.54, and 0.29, on the conditions RC, SC, and APC, respectively.

Conclusions: We showed that the investigated sling exercise can be used to significantly increase the TrA thickness, and that the TrA thickness was significantly different on the three conditions (RC,SC, APC) using the ultrasound-based method.

Keywords: Transversus abdominis, Sling system, Ultrasound method, Measurement method