In this study, we compared the abdominal muscle thicknesses of pregnant women in their second trimesters between those in the LPP and non-LPP groups. The results showed that those in the LPP group had significantly thinner IO muscles compared to those in the non-LPP group. In the non-pregnant population, some literature has reported significant differences in abdominal muscle thickness between those with and without back pain . However, this is the first study to show a significant difference between pregnant women with and without LPP. Therefore, the results may suggest that it is important to focus on the function of IO muscles in pregnant women to understand LPP during pregnancy.
First, the IO muscle may perform an important function of maintaining proper posture. Previous studies have reported, although these results are inconsistent associations between individuals, that postural changes often occur during pregnancy, such as lumbar lordosis , lumbar flattening , and forward tilting of the pelvis . Passive postures, which are commonly reported during pregnancy, rely on the lumbar pelvis structure to maintain an upright posture against gravity , and this may cause fatigue and muscular strain in later stages of LPP . In addition, it has also been clinically reported that these passive postures frequently exacerbate pain [25, 26]. However, in proper posture, bones and joints are in the position to withstand the stress of weight and movement, as they are firmly balanced to hold the body organs in place  and may reduce the strain on the lower back and pelvis. Regarding the effects of the IO muscle on posture, muscle activity decreased in slump sitting and sway standing postures, which are passive postures [25, 27], but increased in proper postures. In addition, another previous study has examined the contribution of abdominal muscles to lumbar stability, using two different abdominal activation strategies, and it was reported that the IO is the most important muscle for stability , indicating a postural stabilizing function of the muscle. Accordingly, when the IO muscle loses the ability to perform its usual function, lumbar stability decreases, making it difficult to maintain proper posture and possibly leading to LPP during pregnancy. Second, it is necessary to focus on the function of IO muscles for pelvic stability. Hormonal loosening of the pelvic ligaments during pregnancy increases the possibility of instability of lumbopelvic joints, such as the sacroiliac joint (SIJ), leading to inflammation in the back and pelvic region and LPP [5, 7]. Protection against shear loading of the pelvic joints can be provided by transversely oriented muscles such as the IO. Based on fiber orientation, IO muscles can be divided anatomically into two parts, and the lower fibers in the pelvis run horizontally. The anteroinferior portion of the IO can produce SIJ compression, which increases the friction between joint surfaces . This is called the self-bracing mechanism, and it stabilizes the position of the sacrum between the iliac bones. Moreover, Snijders et al. reported that IO muscle activity decreased significantly when applying a pelvic belt, while it increased significantly when the belt was taken off . They claimed that the IO muscle has a role like the pelvic belt. These reports have described that IO forces an increase in the stability of the pelvis, particularly that of the SIJ. In contrast to pelvic instability due to laxity of the ligaments and joints during pregnancy, IO muscles may play a role in keeping the pelvis in place. Thus, the non-LPP group might have had optimal pelvic joint stability, more retained IO function, and had thicker muscle thickness than the LPP group in this study. Hence, overstretching of the abdominal muscles during pregnancy might thin the muscle and affect these functions. Furthermore, muscle activities of the IO decrease in passive postures and while using pelvic belts during pregnancy [25, 27], which may contribute to muscle thinning and possibly contributes to LPP. Conversely, there may be a reverse causation with the thinning of IO as a result of LPP onset during pregnancy, involving prolonged passive postural activity, extended pelvic belt using time, and decreased overall physical activity [7, 25, 27].
The results of our study showed that thinner IO muscles might be related to LPP. The most obvious visible change during pregnancy is the expansion of the abdomen, and the influence on abdominal muscle thickness is inevitable. Most biomechanical studies show that TrA is more important in LPP [10, 12, 23, 30], but our results showed that there is no difference in the TrA muscle thickness with or without LPP. In addition to the results of this study, a previous study in women during the postpartum period reported that their IO muscles were thinner than those of nulliparous women, while no significant differences were observed in TrA muscles . Therefore, it is considered that the IO muscle thickness can be more influenced by biomechanical changes during pregnancy than TrA muscle thickness and tends to remain thin even during the postpartum period. This may be a feature in the association between abdominal muscles and perinatal LPP. Although it has been suggested that exercise programs focused on TrA are important for non-pregnant patients with lower back pain , exercises focused on strengthening the IO muscles may be more effective in pregnant women as a countermeasure against LPP.
Consequently, assessing the morphological information of the abdominal muscle, particularly the IO, may be a useful way of screening women with LPP for muscle overstretching and weakness during pregnancy.
This study has several limitations. First, it was a cross-sectional study; therefore, it is difficult to refer to a clear causal relationship between abdominal muscle thickness and LPP during pregnancy. Additionally, the participants of this study were exclusively second trimester pregnant women, so the relationship between abdominal muscle thickness and LPP throughout pregnancy was not revealed. Second, this study was limited to quantitative evaluation using muscle thickness measurement. Qualitative and functional evaluation could not be performed. Therefore, there may be insufficient points for muscle evaluation. Another limitation was the relatively small sample size. Thus, it is necessary to conduct studies with more participants or longitudinal research that follows muscle changes from early pregnancy, to investigate the causal relationship during pregnancy. Further investigations about the relationship between abdominal muscle and LPP during pregnancy may help to better understand chronological muscle changes throughout pregnancy and the risk factors for LPP, and this will lead to the establishment of an effective approach to the abdominal muscles, for evaluation at each period of pregnancy. Consequently, this research may lead to an improvement in quality of life and activity of daily living during pregnancy and postpartum periods.