Transversus abdominis (TrA) and lumbar multifidus (LM) muscle have been proposed to play key role in stability of the lumbosacral spine. The muscles' functional and/or structural deficits have been linked to the low back pain (LBP) syndrome. Evaluation of the transversus abdominis and lumbar multifidus muscles' function in clinical practice is scarce and not well defined due to number of reasons. Surface electromyography of TrA and LM muscles, as a method of examining muscles function, is not involved in clinical routine. Rehabilitative Ultrasound Imaging (RUSI) of the TrA and LM, apart from providing information about their morphology, could be helpful in assessing their function. Since concentric muscle contraction results in the shortening and thickening of the muscle, evaluation of the potential association between muscle's morphometry, assessed by the ultrasound, and electromyography, may be of potential clinical interest. However, this potential relationship is context-dependant. Our aim was to establish the difference in the thickness change of the TrA and LM muscle during activation in subjects with and without low back pain, and to establish the possible correlation between electromyography and the thickness change of these muscles assessed by ultrasound in both groups of subjects.