The effect of therapeutic abdominal drawing-in maneuver using ultrasonography on lateral abdominal muscle thickness and balance

1. Introduction

The lateral wall of the abdomen is composed of the transversus abdominal (TrA) muscle, the abdominal internal oblique (IO) muscle and the abdominal external oblique (EO) muscle. These muscles play an important role in the stabilizing the segmental spine [1]. Especially, TrA is one of the core muscles and it is important in maintaining the stability and proprioceptive sense of the lumbar trunk [2].

The core muscles control the lumbar region’s joint because of its location and the direction of tissue. It manages the stability of the body trunk through co-contraction without a movement of joint. Core muscle system is the total component of a protective mechanism which reduces the stress that harmful forces are applying to spine during the functional movement [3]. These days, core training is on the rise as rehabilitation for the body balance which was described as a program for trunk strength and stability.

Stability is a fundamental skill to maintain the postural alignment for daily task and sport activities [4]. The lack of stability in the core muscles is a risk factor for developing back pain and this problem is associated with postural balance. Postural stability is usually measured by tracking the center of pressure sway [5]. Higher instability conditions increase postural control and balance requirements.

To strengthen these core muscles, various trunk stabilization exercises have been used in clinical practice. Abdominal drawing-in maneuver (ADIM) is the basic exercise for spine stabilization and its goal is to restore proper neuromuscular control as strengthening the deep muscles such as TrA, IO, and EO [6, 7]. ADIM is the exercise method which increases the abdominal pressure by pulling the abdominal walls to the inside that TrA and oblique abdominal are contracted [8]. Because of the increased abdominal pressure, lumbar trunk stability training is effectively accomplished.

In previous studies, the abdominal drawing-in maneuver was effective in increasing the cross-sectional area of the abdominal muscles more than performing the general stabilization exercises only [1, 9].

Many studies have reported that ADIM stimulates deep muscles and some studies reported that core stability is increased while performing the ADIM. The ADIM has been discussed as an exercise regimen for improving trunk stability and it has been widely used in sports and physical therapy fields. However, there is insufficient number of studies that the effect of constant ADIM on the lateral muscles thickness and balance. Thus, the purpose of this study was to investigate the effect of therapeutic abdominal drawing-in maneuver on thickness of abdominal muscle and balance ability.

2. Subjects and methods

A total of 20 healthy students (6 males and 14 females) at Sunmoon University were recruited for this study based on the calculation of sample size in the previous studies [10, 11]. They had no neurological problems or musculoskeletal diseases and mainly lived a sedentary life. All subjects were informed of the study purpose and methods, and signed an informed consent form. All subjects understood the purpose of this study and provided written informed consent prior to participation in accordance with the ethical standards of the Declaration of Helsinki. The subject characteristics are shown in Table 1.

Participants were randomly allocated to two groups: the experimental (ADIM) group ( n = 10) and control group ( n = 10), by using simple randomization (tables of random numbers). A pre-test was performed before the exercise intervention and a post-test was performed after the intervention. Subjects in experimental groups performed a bridging exercise with abdominal drawing-in maneuver, and those in the control group performed only a bridging exercise. According to the command of “pelvis up”, the participant raised the pelvis and maintained the raised pelvic position for 10 seconds. Then, in the “pelvis down” command, the participant lowered the pelvis and rested for 10 seconds. The procedure was repeated 15 times per one set. The intervention was conducted for 4 weeks, and 3 sets per session were performed 3 times a week.

The ultrasonography (eZono 3000, Germany) with linear-array transducer was used to obtain images of the abdominal muscles, TrA, IO and EO. To minimize the difference between the testers, the measurement was carried out by one skilled examiner. Measurements were conducted three times on the right side of the subjects while the subjects were in a hook-lying position. In this position, their hips flexed to almost 30 ∘ and upper limbs were placed along the sides of the trunk. To measure the abdominal muscles thickness, the transducer was placed on the anterolateral abdominal wall, between the 12th rib and the iliac crest, perpendicular to the longitudinal axis of the body [10]. The images were frozen and stored at the end of normal expiration and the thickness of each muscle was determined using image J software program. The measurements were taken three times each before and after exercise intervention. The Tetrax (sunlight, Israel) was used to measure balance ability. The measurements were performed in four conditions; standing with eyes open (NO), standing with eyes closed (NC), standing on pillows with eyes open (PO), standing on pillows with eyes closed (PC). The participants maintained standing position for 32 seconds each. The stability index obtained by evaluating the total amount of sway from the four force plates (right and left heels, toes of right and left foot), which was summed and then divided by the subject’s weight. This value shows that the higher the score, the greater the posture instability.

The measured data were analyzed by the statistical package for the social science (SPSS) for Windows version 18.0 (Chicago, IL, USA). And all data were calculated to an average and standard deviation. Nonparametric statistical methods, Wilcoxon signed ranks test for comparison of pre and post values and Mann Whitney U test for comparison between groups, were used. A statistical significance level was set at 0.05.

4. Discussion

Clinically, the selective trunk stabilization exercise like ADIM improves the trunk control ability and the abdominal muscles stabilize the trunk. Moreover, TrA is one of the important muscles in maintaining the stability and proprioceptive sense of the lumbar trunk [2]. Successful performance of ADIM is typically judged by the relative increase in TrA thickness compared with that of the IO and EO muscles, measured using ultrasonography.

Some previous studies reported that there was a strong correlation relationship between muscle activity measured by electromyography and the muscle change in thickness of TrA muscle during ADIM [12, 13]. Hodge et al. [14] concluded that the use of real-time ultrasound imaging would be more effective because a surface EMG feedback is difficult to distinguish signals from each electrode on muscles overlap in the abdomen. In this study, we used the ultrasonography to examine the thickness of abdominal muscles.

For healthy subjects, Hodge et al. [14] reported that the thickness of IO and TrA has increased but the thickness of EO has decreased during ADIM. In other previous study, the TrA has activated up to 109 percent after the lumbar region stabilizing training by ultrasonic imaging [15]. Teyhen et al. [16] examined that the changes in TrA and IO thickness in individuals with and without unilateral lumbopelvic pain while at rest and during the ADIM and found that preferential change in TrA muscle thickness during the ADIM. Cho [17] reported that bridge exercise with ADIM on an unstable surface showed significant increase in abdominal muscles after 6 weeks. In our study, the TrA muscle thickness was significantly increased in the trunk stabilization exercise with ADIM for 4 weeks, which was consistent with previous studies. However, there was no significant difference in IO and EO muscle thickness between experimental group and control group. This shows the improvement in selective TrA muscle recruitment after the ADIM training.

These findings indicate that a bridging exercise with ADIM, selectively stimulated the deep target TrA muscle leading to augmented core stability, is more beneficial for reinforcing the trunk muscles than the bridging exercise alone.

The spinal column and balance are closely related with each other. In a recent study of the correlation between body stabilization and balance, core training is on the rise as rehabilitation for the body balance [18]. Some researchers have reported that core stability training could improve not only trunk function and muscle thickness but also balance [19, 20]. In this study, stability index of NO and PC position in the experimental group showed a significant difference pre- and post-tests. It is considered that ADIM provides effective muscle contraction strategy for core stabilization via the synkinetic activation of the deep abdominal muscles, which in turn reduced the postural sway that typically occurs during stance.

No significant improvements were seen in PO and PC, stability index with eyes closed, between groups. Balance measurements in the closed eye state have the same effect as blocking the visual information, which increases the dependency upon the remaining somatosensory and vestibular systems. Such visual conditions state change could explain our results.

Previous study showed that the effects of trunk stabilization exercise on balance control and gait enhancement [21, 22, 23, 24]. Carpes et al. [24] reported that there was a significant difference in balance after trunk stabilization exercise. Lee et al. [25] suggested that ADIM training with visual feedback has the positive effects on core and postural stability in non-athletes with core instability. Some study examined the effects of ADIM and myofascial release on back pain, flexibility, and balance in the elderly women and stability index of NO, NC, PO, and PC [26]. As a result of Sim’s study [26], they reported that no significant difference between the two groups. In our study, after 4 weeks of experimental intervention, there was a significant difference in the stability index of NO and PC position between the experimental group and the control group. Although this results were not consistent with some study’s findings, the application of the ADIM for 4 weeks seems to have increased spinal stability because of improved neurological adaptation of deep muscles such as TrA. The deep abdominal muscles, TrA and IO, in coordination with the deep multifidus muscle, play a central role in contributing to core stability [27]. Consequently, ADIM, which focus on the deep muscle system, seems to be more effective for improving balance ability than just trunk stabilization training.

There are some limitations to this study. The number of subjects is small and the population is concentrated in the early 20s, so the results of this study cannot be generalized to the general public. In addition, when measuring muscle thickness, only the lateral abdominal muscles were measured, and the effect on the back muscles was not known. Therefore, further studies should be considered whole trunk stabilization muscle thickness and the relationship between muscle thickness and trunk stabilization.

In conclusion, when performing bridge exercises to treat and strengthen the lumbar region, the combined application of ADIM should be used to increase the effectiveness of the exercise.