The transversus abdominis (TrA) is one of the most important muscles for the stability of the lumbo-pelvic area, and is therefore considered to be of central importance for the treatment of low back pain [
]. There are few methods to assess the activity of the TrA, such as intramuscular electromyography (iEMG), which is invasive, and muscle cross-section measurements from magnetic resonance imaging (MRI), which is expensive. Another method is to derive muscle activity from muscle thickness [
] using ultrasound measurement (USM), which is a relatively cheap, non-invasive, and an easy-to-use method entailing no health risk for the subject [
In the last decades, sling exercise therapy became more popular. Ljunggren
] performed a long-term intervention. At first, the patients underwent general physiotherapy, subsequently they carried out a sling exercise training at home with a compliance of 83 %. This intervention led to a decrease of absenteeism by 75-83 % as compared to the year before. Stuge
] also find a positive effect for the treatment of low back pain after pregnancy using sling exercises. Vasseljen
] focused on the correlation of the increase in TrA thickness and the level of low back pain. The pain reduction was correlated with an increased activation of the TrA and a decreased activation of the
obliquus internus abdominis
] published a systematic review on the effectiveness of sling exercise therapies for the treatment of low back pain, and reported significant differences in the activation of the abdominal muscles, but no significant differences in pain reduction and disability against other forms of treatment. The sling condition used in our study was intended to represent a standardized exercise to activate the TrA, bearing in mind the above evidences concerning the treatment of low back pain.
The objective of the present study was to assess and compare the activation of the TrA during rest, during exercise in a sling system, and during abdominal press, using a non-invasive ultrasound-based measurement method.
The present study aimed to investigate the thickness of the
M. transversus abdominis
(TrA) as a measure for the muscle activation on three measurement conditions: resting condition (RC), sling condition (SC), and abdominal press condition (APC), using an ultrasound-based measurement method (USM). The results demonstrate that the sling exercise is associated with a significant increase of TrA thickness relative to the resting condition, and a significant decrease relative to the abdominal press condition. In line with other studies [
], we derive the activity of the TrA from its thickness. Theoretically, muscle thickness can change independently of muscle activation,
during passive stretching or compression. However, as the subjects remained in a constant position throughout the exercise on all conditions, the TrA can only minimally be stretched or compressed by other abdominal muscles. On the SC, the TrA could additionally have been compressed by the viscera under the influence of gravity, which would have resulted in either a decrease or a reduced increase of TrA thickness relative to the SC. We measured a significant increase of TrA thickness, so that the latter effect, if present, did at least not conceal the increase of thickness due to muscle activation.
Until now there is no standardized measurement method to compare different muscle layers in the abdominal wall, mainly because there are no clearly defined biomarkers for the location and orientation of the transducer. For this study, we used a simple ultrasound-based quantification of muscle thickness for the sectional area enclosed by the superficial and the deep fascia of the TrA. A test-retest reliability analysis based on the first and third measurement day revealed an intra-class correlation coefficient (ICC) of 0.71 and a Cronbach’s α of 0.83 on the RC, which are sufficiently high values to accept the method as reliable on this measurement condition. However, on the other two conditions, the ICC and Cronbach’s α turned out to be relatively low (ICC = 0.54 and α = 0.69 on SC, ICC = 0.29 and α = 0.44 on APC), which indicates rather poor test-retest reliability and might be explained as follows:
First, the poor reliability on the SC and APC may be due to complications during the measurement, afflicting the reproducibility of the procedure. While it is relatively easy to place the transducer at the right spot on the abdomen when the subject is at rest (as on the RC), it is more difficult to place the transducer when the subject is suspended in the sling system (as on the SC), and also when the subject contracts his abdominal muscles with maximum voluntary contraction (as on the APC), so that the abdomen is deformed. Using a supportive device,
e.g. a guiding belt, to fixate the transducer on the abdomen, might reduce these complications.
Second, if subjects contract their muscles in a reactive manner (as on the SC) or in a voluntary manner (as on the APC), it is difficult for them to reproduce the same level of contraction across several days [
]. It is especially difficult to voluntarily reproduce the same level of contraction, so the variability of measured muscle thickness is expected to be largest on the APC, which is what we have found.
Third, the ICC may additionally be impaired by potential neuromuscular adaptations induced by the muscle activation during the measurement sessions [
Despite these impairments on the reproducibility on the SC and APC resulting in low ICC values, we still found a statistically significant difference between TrA thickness values on the different conditions. We conclude that although the absolute value of TrA thickness assessed by the ultrasound-based measurement method used in this study may reliably be used only when the subject is at rest, the method is still reliable enough to differentiate between muscle thickness values on the individual conditions.
The test-retest reliability analysis was based on the first and third measurement day, thus on the largest available time interval (five days), to account for potential fluctuations of the thickness of the TrA over time.
We did not compare thickness values of abdominal muscles other than those of the TrA, because on the APC the intersection points of the OI and of the TrA could not be measured simultaneously using the USM method. It would also in this context be desirable to have a supportive device,
e.g. a guiding belt, to take ultrasound images in a precisely defined transversal plane. We have conducted further studies (manuscripts in preparation) with an ultrasound guiding belt to improve the measurement procedure.
We think that the comparison of TrA activity on different exercise conditions may help to develop specific treatment strategies particularly for the management of low back pain. In this context, the effectiveness of sling exercises, especially the exercise mentioned above, should be clinically tested.
The authors declare that they have no competing interests, and that they also have no financial interest in the company that sells the sling system used in this study.
The authors have no conflict of interest to report. All authors read and approved the final manuscript. JL designed the study, carried out the measurements and the data analysis, and wrote the manuscript. KB contributed to analyzing the data, designing the figures, and writing the manuscript. CP contributed to the interpretation of data and their discussion, helped with the manuscript, and performed the statistics. TS contributed in paper finishing. HW supervised the study and contributed to the writing of the manuscript.