This study examined the number of individuals whose IC could be identified on MRI, and the effect of age and sex on the identification rate. In addition, the present study tested whether age and sex could be associated with the muscle CSA in the lower limb, including IC. As a result, IC was identified in 85–95% of individuals on MRI, and its percentage was not associated with age and sex. In addition, sex-related differences were found in the CSA of the IC, while no age-related difference was found in CSA of the IC. Furthermore, age and sex were associated with the CSAs of the IP, RF, and QUAD, with larger areas in men and younger groups than in women and older groups. These results did not change, even after normalizing the CSA by body weight. Additionally, regression analyses have shown significant associations between CSAs of other muscles and age, but not IC. The aforementioned results support our hypothesis. To the best of our knowledge, this study is the first to reveal age- and sex-related differences in IC size.
In general, muscle size is affected by age and sex , which is supported by our results for the IP, RF, and QUAD. On the other hand, this study shows that the CSA of the IC was not different between healthy older and younger individuals, although the CSA of the IC was significantly larger in men than in women. A previous study reported that the thickness and width of the IC were larger in men than in women, and thus supported the sex-related difference in the CSA of the IC in our study . Ikezoe et al. reported that soleus muscle thickness was not affected by age, while the size of other leg muscles, including the psoas, RF, vasti, and triceps surae, decreased with age . Ota et al. showed no age-related decrease in size in the transversus abdominis muscle among the abdominal muscles . These authors mentioned that the muscles that are less susceptible to age-related changes have a high percentage of type I fibers and contribute to joint stability. The low susceptibility to age-related differences was shown in a high insulin-sensitive muscle fiber , and type I muscle fibers were highly insulin-sensitive . The muscle size of the stabilizer may be maintained by low muscle activity during daily activities . To our knowledge, there was no report on muscle fiber types in the IC. According to previous studies, the percentage of type I fibers was 49.2%, 29–43%, and 29–62% in the IP, RF, and QUAD, respectively, and was higher in the deep than the superficial layer within a muscle . Given that the IC can be regarded as a deep fiber of the IP , the IC could have the highest percentage of type I fibers among the four measured muscles. In addition, the IC contributes to hip stability since the origin of the IC is the capsular of anterior hip, and the size of the IC is larger in hip dysplasia than in healthy adults [6, 8, 10]. Therefore, the CSA of the IC was unlikely to decrease with age, at least during healthy aging.
Although the results were similar for the normalized CSA and CSA, the effect size seemed to have decreased in the normalized CSA compared to that in CSA. Because a larger body size would affect the CSA, and because body weight is heavier in men or younger individuals than that in women or older individuals, we presumed that normalization for body weight would eliminate this body size effect on CSA. In particular, the sex-related difference in the CSA would be reduced by normalization for body weight because the difference in body weight is larger between sexes than that between age groups. Our results support previous studies that reported more obvious sex-related differences in the CSA than those in CSA normalized for body weight [27, 28].
This study confirms that the IC was not identified in 8 of the 77 individuals at the measured points on MRI. An anatomical study and studies using MRA reported identification of the IC in all individuals [6, 8, 10]. However, some studies have pointed out that the border between the IC and the IP is unclear, and that these two muscles are often depicted as one muscle group on MRI [11,12,13]. Therefore, no consensus exists regarding the percentage of IC that can be identified on MRI. The discrepancy between the results of the study using MRA and those of the present study using MRI would be due to the individual’s body weight. Heavier individuals have more connective tissue thickness, such as the fascia forming a border between muscles . On average, the individuals in this study had a lower body weight (56.0 ± 8.7 kg) than the previous study measuring the size of the IC (control group: 26 individuals [men, 13]; body weight, 75 ± 20 kg) . It is possible that the thickness of the fascia could be reduced in this study compared to the aforementioned study. Additionally, the thickness of the fascia was 0.5 mm  and 0.3–1.0. mm at the groin and anterior thigh region [30,31,32]. Accordingly, the connective tissue, such as the fascia, might be too thin for the MRI voxel size (0.5 × 0.5 × 4.0 mm) to clearly detect the border between the IC and IP in individuals whose IC could not be identified in the present study. Indeed, the IC was observed around the insertion and/or origin of the IC even in those individuals whose IC was not identified at the measured area (the center of the femoral head). Therefore, it is assumed that the IC was not absent in those individuals.
This study could acquire more detailed MRI images than the previous study since the slice thickness was the same as 4.00 mm; however, the magnetic field strength was stronger (3.0 T) than that in the previous study (1.5 T) [6, 10]. Even though detailed MRI was used, the identification of the IC was difficult in a few individuals. This suggests that the unidentifiable IC could be a limitation of MRI. While MRI is a useful way to evaluate the size of the IC as well as the diagnosis of hip pathologies, additional evaluation by a high-resolution ultrasound machine may allow us to measure the IC in all individuals [11, 33].
This study has a few limitations. First, it is possible that individuals with minor hip pathologies were included in this study. Although individuals with obvious bony deformities on MRI were excluded, individuals with diseases that were difficult to identify on MRI, such as acetabular labral lesions, may have been included. In these individuals, the pathology could affect the CSA of the IC. Second, the muscle size was calculated from one slice of MRI. The result might differ from a study in which the volume or maximal CSA is used in each muscle. In addition, we did not confirm the repeatability of the measurements. However, our method is reasonable since differences in IC size were detected between patients with acetabular dysplasia and healthy individuals in a different study that used the same procedure as in the current study . Future research should focus on identifying the factors associated with the IC size, such as bone and joint morphology, and on determining age-related changes in other properties such as fat infiltration and muscle stiffness in the IC.